CONSERVATION AND DESIGN GUIDELINES

THE AGA KHAN TRUST FOR CULTURE

CONSERVATION ANDDESIGN GUIDELINES

FOR ZANZIBARSTONE TOWN

The ‘Conservation and Design Guidelines’ are intended for anyone planning orundertaking building works in the historic Stone Town of Zanzibar. The StoneTown is a unique cultural asset, but badly designed modern buildings and ill-conceived repairs using incorrect techniques or materials are threatening itssurvival. The Guidelines explain how to protect the Stone Town. They includean explanation of how to design new buildings in compliance with the law, ananalysis of traditional stone structures and common causes of failure, detaileddescriptions of traditional building technologies and up-to-date conservationtechniques, and advice on how to plan and execute repairs to traditional buildings.

The Guidelines draw upon the experience gained during the rehabilitation ofthe Old Dispensary building funded by the Aga Khan Trust For Culture(AKTC), a Unesco funded training project at the Old Customs House, andmore recently, the Community-Based Rehabilitation Programme co-funded byAKTC and the Swedish International Development and Co-operation Agency(SIDA) and carried out by AKTC’s Historic Cities Support Programme (HCSP).The Guidelines are part of an on-going effort to provide inhabitants of theStone Town with the tools to maintain their built environment, and to engagethe community as a whole in conservation. In particular, the Conservationand Design Guidelines provides detailed information about appropriateconstruction techniques in Swahili for the first time. Particular thanks are dueUNESCO, who co-funded the production of the Guidelines.

This document was written and prepared by Tony Steel and Stephen Battle.The first section was based upon the Stone Town Planning Regulations,developed under the leadership of Francesco Siravo. The Stone Town Con-servation and Development Authority provided support, and encouragementfor the activities of both AKTC and Unesco. Martin Norvenius contributedillustrations, and Shehin Hirani and Rukia Ali Omar provided invaluableassistance on site. Design and layouts were conceived by Robin Oldacre.The Swahili version was translated from English by Rukia Ali Omar andAhmed Sheikh, and reproduced by Fatu Said Nakashima.

INTRODUCTION

IntroductionMaterialsProtected Architectural FeaturesDoors, Windows, BalconiesColoursStreetscapeListed BuildingsBuilding LinesHeightSignsScale and MassingRecommendationsEnvironmentNew Buildings

IntroductionCommon ProblemsPlanning RepairsRepair Techniques

IntroductionRaking ShoresEmergency RestraintFlying ShoresDead Shores

IntroductionMortarsAggregatesBindersLime Mortar

GOOD PRACTICEGUIDELINES FORALTERATIONS,EXTENSIONS, AND NEWBUILDINGSREPAIR OF

STONE HOUSES

SHORING AND TEMPORARYSUPPORT

LIME AND LIME MORTARS

CONTENTS

1.

2.

3.

4.

1.2

1.41.81.12

1.151.161.18

1.201.221.23

1.241.261.281.30

2.2

2.32.132.17

3.23.4

3.123.163.20

4.24.3

4.44.94.14

IntroductionPlaster and RenderSalts and DesalinationRenderingPatch RepairsGrouting and Pinning

IntroductionReplacing Floor SlabsReplacing Boriti and BanaaNew Openings and Lintels

IntroductionPreparationApplication

IntroductionCauses of DecayPrinciples of Timber RepairJoinery RepairsStructural Repairs

IntroductionBackgroundRepairs

5. REPAIRS TOHISTORIC RENDER

FLOOR SLABS ANDOPENINGS

LIME WASH

TIMBER CONSERVATION

CASE STUDY:HOUSE NO.1287, KIPONDA

6.

7.

8.

9.

CONTENTS

5.2

5.35.65.10

5.135.19

6.26.36.9

6.12

7.27.3

7.6

8.2

8.48.78.11

8.15

9.2

9.39.8

1.1

GOOD PRACTICE GUIDELINESFOR ALTERATIONS, EXTENSIONS,AND NEW BUILDINGS1.


1.2

RESPECT FOR THE HISTORICBUILDINGS OF ZANZIBAR

INTRODUCTION

These Guidelines have been drawn up to protect the traditional character ofthe Stone Town.

The Stone Town is very special. There is nowhere else in the world likeZanzibar Stone Town. Visitors come from from all over the world to see it.There is nothing like the Stone Town in the rest of Tanzania. It is one of thethings that makes Zanzibar different from the Mainland, and special. It is animportant part of the island’s unique cultural identity.Zanzibar should be proud of its Stone Town. The Stone Town is the embodi-ment of Zanzibar’s long and great history. It is proof that the island wasonce the greatest power in Africa, and a great Islamic state.

But the Stone Town is delicate. As times change, people wish to change theirbuildings, or need to carry out repairs. But these changes, unless properlyguided, can destroy the Stone Town’s special character. Like a shell on thebeach, slowly eroded by the waves, each change takes something away, andsoon the Stone Town will lose its beauty and fineness, and become like apebble. If the Stone Town is destroyed, visitors will no longer come toZanzibar, and the economy will suffer.

The best way to preserve the special character of the Stone Town is torepair and maintain buildings using the correct methods, but otherwise toleave them as they are. If changes cannot be avoided, then the changes mustbe influenced by these Guidelines.

The law requires anyone wanting to do building work in the Stone Town tofirst ask permission from the STCDA. The STCDA’s job is to guide peoplewishing to do building work so that the changes they make and the buildingmethods they use do not destroy the special character of the Stone Town.The STCDA will judge the building application according to these Guidelines.

If the building application follows the Good Practice Guidelines, approvalby the STCDA will be quicker and easier.

GOOD PRACTICE GUIDELINES

1.3

1. It is mandatory to get permission from the STCDA beforecarrying out any building work in the Stone Town.

The law says :

« No person shall build, alter, remove or demolish a building, part of abuilding, an architectural or streetscape feature without first making abuilding application to the STCDA for written permission. »

This means ALL building work.

2. The STCDA will judge all applications for building permissionaccording to the Good Practice Guidelines.

All building work in the Stone Town must follow the Good PracticeGuidelines.

When thinking about or planning building work,FIRST look at the Good Practice Guidelines.DO NOT START any building work before obtaining written permissionfrom the STCDA.

DESIGN GUIDELINESFOR ALTERATIONSAND EXTENSIONS

1.4

When carrying out repairs or alterations to existing buildings, always follow these key points :

1. First RestoreWhen repairing existing buildings, always try to restore the existing fabric, rather than replace with new. Thissaves money and protects the special character of the Stone Town.

2. Like with LikeWhen carrying out repairs or alterations, ALWAYS use the same materials as the original for the repairs. The repairis more likely to be successful if the materials used for the repairs are the same as the original. In the long term, thissaves money and buildings.

In particular, NEVER use cement to repair lime plaster.

3. Quality ControlPoor quality materials will not work. The repair will fail and the work will have to be done again. ALWAYS checkthat materials have been prepared and stored properly.

This is particularly important with lime. ALWAYS check :· That the lime is fresh; if it has not been stored as putty, it should be no older than a week.· Store lime under water as putty. The longer it is kept as putty, the better it will be.· Remove unburnt lime particles.

4. Use Skilled CraftsmenAlways check that the craftsmen are skilled. Bad workmanship will fail, costing more money and endangering thebuilding.Always ask for references, proof of experience, or the names of other people for whom the craftsmen have worked.

5. Always Consult the Conservation GuidelinesThe Conservation Guidelines describe repair and restoration techniques. By using this information, the buildingwork will be better, last longer, and cost less.Applications for Building Permission to the STCDA that refer to the Conservation Guidelines will be processedmore easily and quickly.


MATERIALS

THE LAW STATESPart IV, 31

« All repair work to existingbuildings including listedbuildings and architectural andstreetscape features shall usethe same, compatible or similarmaterials and finishes as theoriginal materials and finishes.»

1.5

Original crenalationsDo not change or remove

First restoreOriginal mouldings are protectedDo not change or remove

Original timber windowsFIRST restore. Do not change orremove

Original MouldingsDo not change or remove

Like with LikeRepair original lime plaster withlime plaster

Use skilled craftsmenTo carry out repairs

Carved timber doorDo not change or remove

1.6

DO NOT use cement plaster

DO NOT use textured finishesto repair plaster

DO NOT add modern features tothe front of the building

DO NOT create textured finishes.These are not appropriate to theStone Town

DO NOT add modern windowopenings

Inappropriate alterations

1.7

Inappropriate new building

Perforated concreteblocks

Pre-cast balusterson balconies

Security bars

Textured finishes

1.8

Important Architectural Features both inside and outside are protectedby the law.

Examples of Protected Architectural Features are:

• original doors• covered passageways (wikios)• decorative plaster• decorative tilework• entry porches• fascia boards on roofs and canopies• original windows• niches and arches• balconies and teahouses• timber stairs, etc.

THE LAW STATESPart IV, 33(1),(3)

« Changes to protectedarchitectural features arenot allowed except topreserve and/or restorethe original design. »

ORIGINAL NICHES AND ARCHES ENTRY PORCHES COVERED PASSAGEWAYS


PROTECTED ARCHITECTURALFEATURES

1.9

EXAMPLES OF PROTECTEDARCHITECTURAL FEATURES

Original windows

Decorative timberwork

Decorative tilework

DO NOT change or alter protectedarchitectural features, other than topreserve or restore

Fascia Boards

Decorative plaster

Original doors

1.10

It is not permitted tochange architectural featuresother than to preserve orrestore them.

EXAMPLES OF PROTECTEDARCHITECTURAL FEATURES

Decorative plastermoulding

Niches

Decorative timber Decorative tilework

DO NOT alter or damagedecorative plaster mouldings

DO NOT cut new openings in walls

DO NOT repair lime-plasterwith cement

DO NOT damage original tilework

DO NOT damage or blockoriginal niches

1.11

All materials used to preserveor restore architectural featuresmust be the same as the original.This means using lime to repairlime plaster, and timber torepair timber.

All changes or alterationseither inside or outside mustbe reversible. This means thatchanges can be removed orreversed without causing per-manent damage or alterationsto the original building.

New door opening is not reversibleand can cause structural damage

DO NOT leave block-workunplastered

Modern window does not matchoriginal windows

DO NOT add extra floors.Extra floors can cause structuraldamage

DO NOT use cement to repairoriginal lime plaster

DO NOT alter or destroydecorative plaster mouldings

1.12GOOD PRACTICE GUIDELINES

DOORS, WINDOWS, BALCONIES

The buildings of the Stone Town were built using traditional designs fordoors, windows, balconies, and other architectural features. These fea-tures are an important part of the town’s special character, and must bepreserved.

THE LAW STATES:Part V, 38 (2)

« Details and fittings for newbuildings and additions shall becompatible, in appearance andproportion, with the traditionalcharacter of the Stone Town. »

DO NOT use architectural features that are modern in design or out ofkeeping with traditional buildings.

DO NOT replace original doors and windows with modern ones. First tryand repair the originals. If they are beyond repair, replace with copies of theoriginals made of the same materials.

DO NOT alter, remove or damage balconies or teahouses.

1.13

Traditional windows

Traditional door

The shape and design of themodern window is not compatiblewith the traditional character ofthe Stone Town Proportions and design of new

windows are not like traditionalwindows

DO NOT use pre-cast concreterailings on balconies or teahouses

Proportions and the size of moderndoors are not like traditional doors

The shape and design of amodern door is not compatiblewith the traditional character ofthe Stone Town

1.14

DO NOT replace original timberbalcony with new concrete balcony

DO NOT use pre-cast concretebalcony railings

DO NOT alter, remove ordamage balconies or teahouses

Original balcony

If the original balcony is dam-aged, FIRST try and repair it.If this is not possible, replacewith replica of original.


COLOURS


« Colours : Used on the outsideof the building must blend withthe range of tones and coloursfound in the Stone Town. »

Firstmatch original finishes and colours

Acceptable Colors

Roof paint colourRed oxideWhite

Wall finish coloursLight blueWhiteLight yellow

Joinery finish coloursGreenBlueClear

DO NOT use oil paint on plasteredsurfaces

Use limewash on plaster surfaces,or paints that can breath


STREETSCAPES

THE LAW STATES:Part IV, 34 (2)

« Changes to protectedstreetscape features are notallowed except to preserve and/or restore the original design. »

Streetscape features are protectedby Law.

Streetscape features are bothnatural and constructed. Theycontribute to the characterof the street and urban environment.This category includes specificfeatures such as:

• façades• fountains• gateways• external stairways• tombs• trees and vistas

EXAMPLES OF PROTECTEDSTREETSCAPE FEATURES

FOUNTAINS GATEWAYS TOMBS TREES

1.17

Streetscape features are protectedby Law.

DO NOT alter or destroyStreetscape features.

EXAMPLES OF PROTECTEDSTREETSCAPE FEATURES

Façade Porch

Canon Bollard

Façade

For a full map of protectedstreetscape features refer toSTCDA.


LISTED BUILDINGS

Some of the buildings in the Stone Town are given extra protection by thelaw through ‘listing’. There are Grade 1 listed buildings and Grade 2 listedbuildings in the Stone Town.

Grade 1 listed buildings are shown in the plan opposite.

Grade 2 listed buildings are listed with the STCDA, see the map inside theSTCDA. Allocate a place, best is just inside the door of the STCDA).

The level of protection is different for Grade 1 and 2 buildings. This meansthat building work that might normally be acceptable is not allowed onGrade 1 or 2 listed buildings.

First check whether your building has been listed Grade 1 or 2.

IF YOUR BUILDING ISGRADE 2 LISTED

THE LAW STATES:Part 1V, 32 (3)

Inside1. Changes may be permitted by theSTCDA as long as they conform tothe Good Practice Guidelines.Where alterations are made, theymust be done in such a way that ifremoved, they do not alter theoriginal design of the building in anyway.

Outside2. No alterations or additions to thestructure of the exterior walls will beallowed. In exceptional circumstances,the STCDA may allow minor changes.

3. Any work carried out mustinvolve preserving or renovatingthe original fabric of the building.The Good Practice Guidelines mustbe followed at all times.

IF YOUR BUILDING ISGRADE 1 LISTED

THE LAW STATES:Part 1V, 32 (2)

1. No alterations or additions tothe building are allowed, eitherinside or outside.

2. In very exceptional circum-stances, the STCDA may allowminor changes, but any buildingwork must preserve and/or reno-vate the original.

3. This means using traditionalmaterials and techniques (SeeConstructoin Guidelines). Alwaysfollow the Good Practice Guidelines.

1.19

Grade 1Listed buildings are:

Grade 2

0 200 500

Bharmal Building

Malindi Mnara Mosque

Khoja Ismaili Charitable Musafarkhana

Old DispensaryAga Khan Mosque

Royal TombsPeople’s Palace

Royal BathsBeit al-Ajaib (House of Wonders)

Old FortKilosa House

Former British ConsulateTembo HouseMambo Msiige

Tippu Tib’s House

St. Joseph’s Cathedral

High Court

State House

Peace MemorialMnazi Mmoja Hospital

Hamamni Baths

Anglican Cathedral

Market Building

Darajani Chawl

Hindhu Temple

Minaret of the ShiaIthnasheri Mosque


BUILDING LINES

THE LAW STATES:Part V, 37 (2 a)

« Balconies, canopies, roofoverhangs, and gutters may bebuilt beyond the building lines toa maximum of one third of thewidth of the street, or in any casenot more than one metre.Barazas and steps may be builtbeyond the building line to amaximum of 0.4m and signboards to a maximum of 0.5m. »

0.5

0.40.4

1/3

1.21

THE LAW STATES:Part V, 37 (2 a)

« Any new building or additionshall be sited so that walls andfaçades facing onto streets arein line with adjacent buildinglines. This applies to the groundfloor and all storeys above. »

Modern building breaks the originalbuilding lines in the street

It is not permitted to set thefront of the building back fromthe original street-line

Modern windows and doors arenot compatible with traditionalcharacter of the Stone Town


HEIGHT

Modern building breaks originalbuilding lines in the street


« The height of any permanentaddition to an existing building,or of any new building, may notexceed the maximum height ofthe adjacent buildings, and in anycase, may not exceed three

storeys. The Authority maylimit the height of a building toprotect a Grade I or Grade IIlisted building or a protectedarchitectural streetscapefeature. Penthouses, teahouses,and other such roof-topadditions may be permittedsubject to the discretion of theAuthority. »

DO NOT build higher than adjacentbuildings

Buildings may not exceed three floors

4

3

2

1


SIGNS

THE LAW STATES:Part Vll, 58 (1) (2)

« The use of large projectingsignboards, signs made of plasticmaterials or illuminated signs,as well as any other form ofadvertising considered inappro-priate in character, form orscale for the Stone Town will notbe permitted. »

The erection of signs and otherforms of advertising are subject tothe approval of the STCDA

Signs or advertisements will berejected if they are:

• Too big• In the wrong place, e.g., in a Protected Streetscape• Too bright or dominant• Illuminated

This sign is not acceptable.It is too big and ruins thestreetscape.


SCALE AND MASSING

THE LAW STATES:Part V, 37(5)

« Scale and massing :New buildings, alterations, andadditions shall be compatible inscale and massing with thecharacter of the Stone Town. »

Scale of the new building istoo small

1.25

Massing – the shape of the newbuilding is not compatible with theStone Town and breaks the buildinglines of the street

Scale of the new buildingis too big


RECOMMENDATIONS

PrivacyAlterations, extensions or additions must not intrude on another’s privacy.In particular, the location of new window openings should be carefullyconsidered so as not to affect the visual privacy of another.

VentilationMany houses in the Stone Town rely on the wind for cooling. Alterations,extensions, and additions must not block the wind reaching another.

NoiseAlterations, extensions or additions must not increase the level of noisereaching another, for instance by locating an open restaurant area next toanother’s bedroom.

AccessIt is not permitted to block or impair access to another property.

ViewsAlterations, extensions or additions must not block the views of another, forinstance of the harbour or the street.

ALTERATIONS ANDEXTENSIONS

Note: Alterations and extensionscan harm the comfort of neigboursor other inhabitants of theStone Town.

Always consider the followingpoints when designing :

1.27

New building or extension

New building blocksthe wind reachingnext door building

Noise from newbuilding is disturbingfor neighbours

Windows in newbuilding allow viewsinto next doorbuilding

New buildingblocks views

New buildingblocks breeze

Loss ofprivacy


ENVIRONMENT

The old buildings of the Stone Town were designed to cope with theenvironmental conditions of Zanzibar without resorting to mechanicalsystems of environmental control, such as air conditioning. They do this bymaximising the beneficial aspects of the natural conditions, such as theprevailing wind and night time cool, and minimising those aspects that leadto discomfort, such as direct sunlight.

Old buildings regulate the environment cheaply and without using electricity.There are many lessons that can be learnt from old buildings that canreduce energy consumption and save money.

NATURAL CONDITIONS

1.29

Windows opento the prevailingwind. Throughbreeze cools theinterior

Breeze

Interior isshaded fromdirect sunlight

Breeze ventilatesroof void helpingto keep theinterior cool

High ceilings allowair to circulatemore freely coolingthe interior

Tile or iron-sheet roof over theflat roof protects the buildingfrom hot sunlight

The courtyard is like a cup, fillingwith cool air at night, which slowlyempties into the rest of the buildingby day. The air in the courtyardonly warms up when the sun isdirectly overhead at midday

Thick wallsprotect theinterior fromheat. Sunlight isreflected bywhite limewash

Courtyardincreasesventilation


NEW BUILDINGS

CHECK LIST

When designing new buildingsfor the Stone Town, alwaysfollow these key points.

Respect the SpecialCharacter of the Stone Town

When designing a new building, firstlook at the buildings and the streetaround. Treat these old buildingswith respect. Try to blend in withthe character of the street. Do notbe intrusive.

Fill the Plot

New buildings must always fill theplot and not leave space around. Inparticular, the front of the buildingmust be in line with the front of thebuildings on either side.

Respect Streetscape andBuilding Lines

New buildings must respect thebuilding lines of the original buildingsin the street. New buildings mustfollow the lines of adjacent roofs,mouldings, windows, and canopies.Always treat the old buildingswith respect.

Do Not Exceed the Heightof Adjacent Buildings

New building must not be higherthan the adjacent buildings in thestreet, and in any event, may notexceed three storeys.

Use Traditional Fixturesand Fittings

Always use traditional designs ofwindows, doors, balconies, roofdetails, porches, and canopies, etc.Look in the street around and copy.

Where Possible, use TraditionalMethods of Construction

Use coral-stone walls andlime-plaster finishes. These oftencost less, and the building will useless energy when finished.

Do Not Use TexturedWall Finishes

It is not permitted to use texturedwall finishes. Walls must besmooth.

Use Appropriate Colours

It is not permitted to use brightcolours on walls or joinery. Thecolours on new buildings must bethe same or similar to those onadjacent traditional buildings.

Always Seek the Permissionof the STCDA

The law requires anybody wishing tocarry out building work in the StoneTown to first seek permission fromthe STCDA.

If the Building Application followsthese key points, approval by theSTCDA will be quicker and easier.

2.1

REPAIR OF STONE HOUSES2.


2.3

The foundation is a part of thebuilding that is never seen. It is thejob of the foundation to support theweight of the house, so foundationsare the first thing to be built and areburied underground. In Zanzibar,foundations are the same as wallsexcept they are a little wider andperhaps made of bigger stones. It istheir job to make sure that thebuilding does not sink into theground or that heavier parts of thehouse do not sink faster or furtherthan lighter parts. They work byforming a continuous wall beneaththe thinner walls of the house andspread their weight over a wider

PROBLEMS WITHFOUNDATIONS

area of ground. If you stand onvery wet sand you will sink slowlyinto it. If you stand on a flat pieceof wood only slightly wider thanyour feet you will not. This isbecause your weight has beenSPREAD over a wider area.

Most of Stone Town is built on firmsand. If it remains dry it is strongand will carry a lot of weight but if ithas water running through it, it willsoon wash away, leaving foundationsunsupported and weak. If thishappens the unsupported part of thefoundation will break and the wallabove will sink and perhaps begin tolean. In most cases, the foundationsunder houses in the Stone Town aregood and only cause trouble ifdrains, sewers or water-supply pipesbreak, causing water to wash outsand from around them. Occasion-ally foundations are damaged ifditches are dug too close to them ora dry toilet pit collapses.

In old houses local movementcaused by this type of damage isdangerous. If one part of thebuilding moves it will cause morestrain to be imposed on other oldand tired walls. They too may beginto move and the building maybecome UNSTABLE.

Foundation spreadsweight over awider area.

Broken pipewashes awayground underfoundation, causingit to settle andcrack.

REPAIR OF STONE HOUSES

COMMON PROBLEMS

2.5

With bricks or long stones it ispossible to arrange them so thatthey overlap and hold each otherdown. It is also possible to arrangethem in orderly lines or COURSES.Walls like this are called COURSEDand BONDED and they can resistpulling forces better because of theextra strength provided by thebonding. Bonding also distributesthe weight imposed on the wallmore evenly.

The rounded shape of coral ragprevents efficient bonding and caneasily distribute the imposed loadbadly, leading to localised stress.Walls that are coursed and bondedare also much better at resistingBENDING. Rubble walls are moreBRITTLE and will break if bent.Another important problem inbuilding with rounded corals is seenin corners and in the junctions of

walls. To be strong, buildings musthave good corners with alternatestones from each elevation inter-locking and fastening elevationsfirmly together. In the coral rubblewalls of Zanzibar it is not possible tomake such corners. Walls simplymeet in flat BUTT-joints and there isno interlocking of elevations.Houses built with corners like thisare weak.

Bricks Coursed and Bonded

Brick walls arebonded, and can resistpulling forces

Coral-rubble walls are not bondedand do not resist pulling forces

effectively

Cracks may form at corners

2.7

Arches work because all the stonesin them are in Compression

The arches themselves are alsomade entirely of rubble stone of thesame sizes used in building walls.This makes them weak. Archeswork because all the stones in themare in COMPRESSION. Theweight above squeezes them hardtogether, keeping them in place. Formaximum strength it is better if thestones are big and flat. This gives alarger area for the weight to passthrough. If the stones are small themortar between must be relied onto transmit much of the weight.

In order to stand, arches also needABUTMENT. If the arch is not tocollapse, the weight pushing sidewaysthrough its stones must press onsomething solid, this is clalled ABUT-MENT. In the middle, an arcadeabutment is provided by the columnsand the pushing force from the nextarch. At the ends of an arcade thereis no abutment and because of this apotential weakness occurs.

Large mortar jointsweaken the archFor maximum

strength, it is better touse big, flat, stones

2.9

Roof and floor slabs are made ofboritis (mangrove poles) and limeconcrete. The boritis support theslab and help to keep walls upright.This is very important as rooms aretall and walls relatively thin. Thepoles are needed to stop walls frombending or leaning. This is calledRESTRAINT.

In order to understand how theslabs work and the problems thatcan arise, it is important to knowhow they were made in the firstplace. As walls reached full roomheight, construction would stop.Boritis were placed at close centreacross wall heads and over therooms below. In most cases boritiswere seated on about 3/4 the widthof the wall. The gaps betweenboritis were bridged by placing small

Lime concrete

ROOF AND FLOOR SLABS sections of marine coral betweenthem. On top of this, lime concretewas cast. When the new slab wasfirm enough construction wouldcontinue on top of the floor slabsuntil the next floor was needed, andso on. Constructing floors in thisway means that walls are not con-tinuous from foundation to roof. Ateach floor level the majority ofthe new wall rests on the slabbelow.

Floor slab coverover boritis. Thenext section of thewall is built on topof the slab.

Boriti poles laid ontop of wall.

2.11

Boritis bend under the weight of theslab. As the slab bends it may crackdown the centre of its underside,parallel to the wall. When thishappens, the walls may be pushedslightly apart and, if pushed farenough, they will tear along theholes left by decayed boritis.

All the old houses in the Stone Townsuffer to some extent from thedefects mentioned above. Most ofthem have twisted out of shape, withbent walls, sagging floor slabs, and bigholes eaten into outside render. Theyhave deformed because weaker partsof the buildings have given way to theforces described, and other partshave been required to do more workthan intended. In due course, it maybe that they too will break or at leastbend and crack until they havedistributed the extra load.

EquilibriamWhen buildings DEFORM in this wayand adjust to changed LOADINGS, itis said that they have found EQUI-LIBRIUM. Equilibrium, once estab-lished, will remain until anotherchange in loading imposes newstress in other parts of the buildingand stability is lost. It is possible thatthe extra stress will cause theseparts to break or deform also.When this happens, equilibrium hasbeen lost and damage will continueuntil the load is again shared andequilibrium returns. Dependent onthe quality of the building and thematerials used, there may come atime when there are not enoughstrong parts left to do the workrequired. That is when part of thebuilding will fall down.

Rotten boriti nolonger support theweight of the slab

Cracking occursalong the line ofthe boriti

New boriti leavevoids in the wall

Typical Construction

2.13REPAIR OF STONE HOUSES

PLANNING REPAIRS

The Real Cause

When considering repairs, it is important to first establish the cause of theproblem. If the repair does not deal with the REAL cause or causes of thetrouble, all the money spent will be wasted and your house may still fall down.

Structural and Cosmetic

Repairs can be divided into two types, STRUCTURAL and COSMETIC. Inmost cases both types of repair are needed, the structural repair takes placefirst and SOLVES THE PROBLEM. This is followed by the cosmetic repairthat hides the structural repair and makes the walls look as though nothinghas ever happened.

In the Stone Town, many builders carry out ONLY cosmetic repairs thinkingthat they are solving the problem. Most of these repairs consist of fillingcracks with cement and then rendering over them. If a repair is neededbecause part of a building has started to crack, it is no good simply filling thecrack for a cosmetic repair. The cracks are caused by a change in the workcarried out by different parts of the building. If the repair is to last, it mustbe directed at the cause of the changes. It is sometimes not easy to findwhat the real cause of the problem. In order to do this, the building and itssurroundings must be closely observed in search of clues.

Survey

Looking at the building and site for clues to the problems is called a SURVEY,and must precede ALL repair work. The effectiveness, cost, and quality ofrepair work will only be as good as the survey that preceded it. All damageand symptoms of damage, should be recorded on a scale drawing. Whilstcompiling the survey you will begin to understand the causes of damage andstart to formulate approaches to repair. When complete, the survey will

FINDING THE CAUSE OFTHE PROBLEM

LOOK FOR THE OBVIOUS

1. Has anyone in an adjoining housetaken out window frames orremoved timber lintels?

2. Has anyone in a flat underneathor next to yours removed a wallor cut a new opening or doorthrough a wall?

3. Have any of the neighbours begun to replace boritis or perhapsan entire floor slab?

4. Have any ditches or pits beendug close to walls?

5. Have any big pieces of renderfallen off recently and has itrained heavily, (have the wallsbeen wetted more than usual).

6. Could the drains or mains watersupply be leaking underground?

7. Does your house have a toiletpit? Even if your toilet is canal-ised, find and inspect the old pitif it has not been filled. Collaps-ing pits cause very serious dam-age in the walls above.

All of these things are verycommon causes of damage toold buildings.

2.15

Cracks forminglong horizontallines are causedby some distantpart of thebuilding sinkingor perhapsstarting to lean.

Horizontal crackscan form when wallstear along boritiholes.

When looking for clues indicating the cause of trouble, it is also a good ideato look for parts of the wall or floor that have become miss-shaped or bent.Swellings or bulges within walls indicate the wall is over stressed and beingcrushed, particularly if vertical cracks are to be seen nearby. If a floor slabbends and falls away from a crack, it may show there is a problem with thefoundation and that the wall is sinking or that a part of the wall supportingthe slab is moving down, perhaps caused by a bending lintel. In both of thesecases, the damage will be accompanied by angled cracks in walls nearby.Clues may also be found in the shape of doorways and window openings.They will have been built square and plumb so that if they start to changeshape it is a sign that something is wrong. Only by plotting the location andextent of actual damage and the symptoms of ongoing damage on a drawing,is it possible to truly understand its causes.

WALL AND FLOORPROBLEMS

Angled crackswill form above anew opening witha lintel that is weakand starting to bend.

2.17REPAIR OF STONE HOUSES

REPAIR TECHNIQUES

Structural repairs involve a small number of techniques. Sometimes theseare used on their own, but in many cases some or all may be used together.

Stitching strengthens buildings by connecting elevations and interior crosswalls (remember they are built with butt joints). This repair option isused to repair vertical cracks developing as a result of lack of bondbetween elevations. The form of construction will determine the materi-als used and method of repair involved. If the walls are plain sheetmasonry, as sometimes used in small townhouses and utilitarian rooms inlarger houses, it is possible to make connections by inserting packs ofroofing tiles alternately into each elevation. This technique is not suitablefor arcades, as there is not sufficient mass in the columns to permit therequired cutting away. Stitching involves fastening elevations together byintroducing large corals or packs of tiles alternately into each elevationsuch that they interlock in the way resembling brickwork. Although usingcorals to form stitches is the best option in terms of similarity of mate-rial, they are seldom found in large enough sizes and need to be quarriedespecially for the purpose. Much work is also required to shape theminto fairly regular blocks. An alternative to coral is Mangalore roofingtiles. Ceramic tiles are very similar to corals in expansion and contrac-tion, and are available in town.

STITCHING

2.19

manner described until inter-locking packs of tile are installedup the interior of the junction.

Vertical tile stitches: Tilestitches can also be used verti-cally across horizontal cracks.This process provides gentlesupport to walls fractured alongvoided boriti holes. Although avery effective technique, it mustbe stressed that vertical tilestitches must not be used ifstructural movement remainsactive. In many of the largerhouses featuring interior arcades,there is insufficient bulk in themasonry of the end piers topermit the type of excavationrequired to install effectivestitches safely. In these situa-tions, the cracks must first begrouted as previously but thephysical connection betweenelevations is made by alternatelystitching with steel rod groutedinto pre-drilled holes.

4.

Horizontal tile stiches

Horizontal crackalong line of boriti

2.21

oughly flushed free of debris andsaturated by pouring in water. Itis important that flushing isthorough as small fragments ofmortar can easily block accessto smaller parts of the void.Grout is poured through asimple piece of equipment madefrom a large funnel and piece ofhosepipe.

PouringPouring begins in the second liftof holes and ideally continues inthe one site until grout comesout from the hole beneath. Inreality, void systems can becomplicated such that grout maybegin to flow from a site manymetres from the point of entry.It is essential that entry pointsare kept open and do not be-come obstructed, preventing theingress of further material.

SealingTo keep the grouting hole openit is necessary to pour only smallamounts of grout followedimmediately by an almost equalamount of water. Pouring should

not be continuous, short pausesare taken to allow air to escapeand allow grout to spread. If theweight of liquid grout within thewall becomes too great there isa danger of damage. Groutingshould continue from each siteuntil refusal, at which time thehole is sealed and work begun onthe next. Occasionally a slightlyhigher pressure is required toforce grout into fine or remotecracks. Sufficient pressure willbe generated by erecting ascaffold and pouring the groutfrom a height of four or fivemetres.

Once the wall is grouted itshould be left to cure for twoweeks.

4.

3.

Pour grout using afunnel and hose pipe

Void filled withgrout

Block surfacecracks

500

300

2.23

INSERTING NEEDLES

A ‘needle’ is the name given to anylong thin section of material placedthrough a wall and at 90° to its face.By inserting sections of carefully cutcoral through walls cracked verti-cally within their thickness, it ispossible to tie the parts togetheracross the cracks and void systemsthereby strengthening it.

Mark out a 1m square grid on thewall. Starting at one end of thebottom line in the grid, begin tocut a neat rectangular holeapprox. 300mm square. The holemust pass right through the wall.

If the wall is very thick, needlescan be placed slightly more thanhalfway through from either side.In this case, the excavations oneach side must be off-set slightlyto produce a bonding effect. It isbest to begin cutting the socketby drilling a number of 20mm or25mm holes as close together as

Before placing needles, walls shouldbe relieved of as much weight aspossible. This can be done erectingdead shores to support the floorslab above, and by firmly struttingany openings.

METHOD

1.

2.

possible over the site of theintended socket. Long sharpchisels are then used to finallyexcavate the hole. Before placingthe needle, a bed of 1:3 limemortar is laid along the bottomand the needle firmly beddedinto it with a rocking action.

When in place, a very dry mix of1:3 mortar is tightly packedaround the needle along withsmall stones. Long thin pieces ofwood or steel are used togetherwith a hammer to beat home themortar. It is very important thatthis work is properly done andthat no empty spaces are left.When one site is complete thenext needle to be placed shouldbe as far from the first site aspossible, and so on until thework is complete.

Needles should be approx. 250mm square and cut from thetoughest corals. An electric anglegrinder or disc cutter will help inroughing them out.Precautions

300mm square holefor the needle

Voids in the wallare first grouted

Coral needle 250mmsquare

2.25

Deep tamping is a procedure designed to place mortar at depth within theouter masonry of walls. It is used to repair badly scarred walls severelyeroded by salt activity and abrasion. Although a simple activity if correctlyexecuted, the strength of walls and their resistance to water penetration willbe much improved.

METHOD

The site is flushed free from debris using large amounts of clean water, thiswill also serve to saturate the masonry. A quantity of very dry 1:3 mortar isprepared and a number of small stones in different sizes are placed in abucket of water to soak. Using a trowel, a small amount of mortar shouldbe placed in the void between stones, and a long thin section of timber orsteel used to compact it firmly against the substrate. Compaction must befirm and a hammer used in conjunction with the timber to drive it home.One or two further sections of mortar are laid in the same way and thesmall wet stones beaten into it.

The procedure is repeated until the cavity is filled.

DEEP TAMPING

Begin by compacting mortarand small stones, adding largerstones as appropriate

Wooden Lath

Very dry mortar

Small Hammer

The effectiveness of thisrepair depends on the degreeof compaction and a goodselection of stones

2.27

plumb bob and line is alsorequired.

The batten, plumb bob, andsquare will all be used to main-tain the line of the building. Thisis necessary because each exca-vation is completed and back-filled before the next is begun,and some means are thereforerequired to ensure alignment ofthe sections. Measurementstaken from the suspended plumbbob to the face of the workingsbelow will ensure that whenfinally connected the varioussections will be in alignment.

Excavation work should begin bycarefully opening the first 1msection. It will be necessary tocontinue digging until reliable,undisturbed strata is found. If theexcavation exceeds 1.5m indepth, simple timbering must beinstalled to prevent the possibil-ity of the trench collapsing.Depending on ground conditions,it may also be necessary to installshuttering to the rear of theexcavation. If this is requiredconcrete slabs should be used as

these can be abandoned as workproceeds. Should timber be usedin this location it must be re-moved as work advances, ifabandoned the work will be-come voided as it decays.

As soon as reliable strata isfound, a very dry mix of 1:2concrete is laid as a bed forsuccessive courses of large flatcorals. The stone used must bethe rough, hard variety laid in a1:3 sand/cement mortar. Smallthin stones are introduced intothe bed joint as required to fillcavities. It is important thatcourses are laid with care fromthe rear of the excavation, andthe ground beyond carefullycompacted to exclude thepossibility of voids. Each end ofthe construction is left ‘toothed’to enable bonding with theneighbouring section. The new

3.

4.

5.

foundation is brought up to a point75mm inches below the originalwork. The remaining gap is packedwith a very dry mix of 1:2 cement/sand mortar. This mortar must bebeaten into place with suitable piecesof timber and hammers. No voidsmust be left and the whole must bepacked solid. The site is now backfilled and firmly compacted.

Plumb bob usedto keep newfoundation in line

Coral stones andcement/sandmortar

Bed of concrete

75mm

3.1

SHORING AND TEMPORARYSUPPORT3.


3.2SHORING & TEMPORARY SUPPORT

INTRODUCTION

This section provides information on how to make safe buildings to partsthat are dangerous. Details are also given of the support system neededbefore any attempt is made to cut new openings in existing walls.

Parts of the Raking ShoreRaking Shore for Tall BuildingsSetting up the ShoreLocating the Needle and the Cleat Assay

Parts of the Emergency Restraint

Parts of the Horizontal ShorePositioning Horizontal Shores with Reference to Floor LevelsSpecial Flying Shores

Dead Shoring to Failed Floor SlabsDetails of Floor Slab SupportDead Shores with Needles

RAKING SHORES

EMERGENCY RESTRAINT

HORIZONTAL ORFLYING SHORES

DEAD OR VERTICALSHORES

Note: The procedures described in this section are guidelines only.Before undertaking substantial structural repairs or tackling seriousstructural problems, consult a building professional.

3.3

TYPES OF SHORE Shoring is the name given to the provision of temporary support to buildingsthat are not safe or need to be supported while work is carried out. It isvery important that some types of repair are not attempted without firstsupporting the building.

These repairs are:1. Cutting new openings in walls for doors and windows2. Replacing lintels above openings

If this work is undertaken without the use of suitable support there is adanger that the building may be seriously damaged or even collapse.

There are three main types of shore:Raking Shore, Flying Shore, and Dead Shore or Needles.

Raking Shorefor walls that have begunto lean or bulge

Flying Shorefor buildingsthat are part ofa terrace

Dead Shore or Needlesfor support of roof or floor slabs

3.4

Raking shores consist of inclined boritis called rakers. One end of the rakeris placed against the wall whilst the other sits on the ground. They are usedto support walls that have begun to lean or bulge. The most effectivesupport is given if the raker meets the wall at an angle of 60 to 70 degrees.In tightly packed areas like the Stone Town, this angle will be determined bythe space available, and the width of the footway.

When providing support to an unsafe building, it is often necessary to useboth raking and dead shores together.

SHORING & TEMPORARY SUPPORT

RAKING SHORES

RAKING SHORE

Wall Hooks tosupport wall platesduring erection

Needle topenetrate wall atleast 150mm

Wall Plates

Needle Bracefirmly bolted towall plates

Sole Plate

Needle withintegral cleat(bolted)

Top Raker with flataxed sides to allowfirm location ofsecurity batten

Boriti7m x 200 m o

4

100 x 50mm

Bracing withstrutting nailedthrough pre-drilledholes

Folding wedges

100 x 50mm

Sole Plate or Footfabricated using100 x 50mm

Wall Plates200 x 50mm

1.

3.

2.

3.5

RIDING SHORE On tall buildings raking shores areinstalled in SYSTEMS with one ormore inclined rakers placed againstthe wall in the same vertical plane.Each raker slopes at a different angleso as to support the wall at eachfloor level. To support small town-houses only two rakers will beneeded but to reach the top of thelarger palaces a third raker will beneeded. This need not go rightdown to the ground but can springfrom the back of the top raker.Rakers used in this way are calledriding shores. To support ridingshores properly it is best to use twotop rakers side by side.

Riding Shore

Bottom Raker

Top Rakers

3.6

To prevent the bottom ends of the rakers sinking into the ground they sit ona large wooden SOLE PLATE, this spreads the thrust of the wall over a widearea of soil.

If the top ends of rakers were simply to rest against the wall they wouldslide up it and be useless if it continued to lean. To prevent this, the tops ofrakers are located against short NEEDLES, which are stout lengths of timberset into pockets cut in the wall to provide a firm grip. In addition to seatingaround needles, the heads of the rakers also rest against a timber WALLPLATE, spreading their thrust over a large area of wall.

PARTS OF THERAKING SHORE

1. Detail of the Sole Plate

Rakers are tiedtogether withsteel dogs

Bottoms of rakersare recessed tohouse folding wedges

The plate must befirmly bedded onsolid ground

Rakers aretightened intoposition withfolding wedges

Steel Dog

3.7

2. Details at the Head of Each Raker

Rakers locate around the needle andpress against the wall plate. To keepthe needle rigid it is locked in placewith a cleat

Shoulder to head of rakers mustnot be less than 90mm deep

The needle must measure at least100mm x 100mm and a minimum of200mm must pass into the wall

Cleat 100mm x 100 mm

Cleat

Needle

Raker

3.8

3. Detail of the Wall Plate

To fix the wall plate it should besecured to the face of the buildingby steel wall hooks. The hooks areeasily made from re-bars and aredriven into mortar joints.

If it is necessary to joint planks inorder to obtain a plate of sufficientlength, a bevelled halving joint shouldbe used.

The length of joint must be at leastseven times the width of the plank.

Wall Plate

Wall Hooks

The wall plate should be as wide aspossible but at least 400mm.

Bevelledhalving joint

3.9

RAKING SHORE FORTALLER HOUSES

Riding Shore

Steel Dog

Normal needleand cleat

Top Rakers

Top rakers have to be doubled tosupport the riding shore. This detailshows how to join the two andsupport the needle.

The riding shore must spring from asturdy bracket bolted across bothtop rakers.

Top Rakers

3.10

SETTING UP THE SHORE

It is very important that raking shores are set against the wall in the correctplace. They must only rest against solid masonry and never against the thinsections of in-fill between piers. It is also important that the raker heads andneedles are placed where the wall is strong enough to resist their thrust.These should be inserted slightly below each floor level. If the heads andneedles are placed against plain un-braced walls there is a risk that the wallmay be caused to bulge inwards or crack. If the top raker in the system isplaced too close to the top of the wall, there is a danger that there may notbe sufficient weight above to prevent its thrust pushing off the wall head.

In order to provide maximum support with least danger of damage, needlesand the heads of rakers must be set up so that an imaginary line extendedalong the bottom of the boritis meets both the vertical centre line withinthe wall and the centre lines of the rakers.

Positioning the shore

The wall platesof both raking andflying shores mustbe set up against thesolid columns ofmasonry betweeninterior arcades.

Detail A

3.11

LOCATING THE NEEDLEAND CLEAT ASSAY

Folding wedges must not be driven home with sledge hammers in order totighten rakers into position against the needle. Doing so would createdangerous vibrations that may damage the wall.

Rakers should be jacked intoposition with a crowbar or a carjack and the wedges then tightened.

Car jacks working on steel barinserted through bottom of raker

Folding Wedges

Crow Bar

Floor Slab

Undersideof boriti

Centre line of wall

Centre of raker

Detail A

Boriti

Top Base


EMERGENCY RESTRAINT

Although the raking shore lends the best support to leaning walls, in anEMERGENCY support can be quickly provided by installing temporary wall ties.

This involves tying back leaning walls to stable walls in another part of thebuilding with large diameter nylon rope. In order to do this, it may benecessary to cut holes through other walls in order to provide anchorpoints. This type of support is very useful on the tall palaces.

In an emergency, restraint can beprovided by tying back the wallwith ropes and stout timbers asillustrated.

Holes cut through interior walls

Stable wall to interior of building30mm dia. nylon rope

Timber plateLeaning wall

3.13

Slowing down movement

To provide maximun restraint,horizontal timbers should beattached to long vertical wall platesworking on the outside of piers.Neither this technique or the use ofraking horses will pull walls back tovertical. They are only used toprevent or slow down furtherdevelopment.

Pier

Timber wall plate

Nylon rope

Floor levels

3.14

PARTS OF THE EMERGENCYRESTRAINT

To connect the rope special steelclamps must be made as illustrated

15mm bolts

30 mm dia. Nylon rope

Clamps

12mm steel plate

Saw cuts

Detail 1

3.15

Nylon rope will stretch, so arrangements must be made to tighten it.This can be done by making a special fitting, using a steel rod and washeras illustrated.

Tightening the nylon rope

15mm threaded steel rod

10mm steel washer

Threaded section must be 500mmlong to allow adjustment

Detail 1

Detail 2

Detail 2

3.16

Flying shores are simply flat boriti struts used to provide temporary supportto two parallel walls where one or both show signs of failure. The mostcommon use for this kind of support in the Stone Town is where one housein a terrace has collapsed and some support has been lost to the houses oneither side.

For distances between walls of up to 9m or less, a single shore may be used.For distances up to 15m, a compound or double flying shore is needed.

A single flying shore consists of a horizontal length of boritis set betweenthe walls in need of support. The ends rest on needles set into the wall andare stiffened by inclined struts above and below at either end. The strutsalso provide added support to the walls and must be set to coincide withfloors. As with the raking shore, timbers must be placed with care in ordertoavoid damage.

SHORING & TEMPORARY SUPPORT

FLYING SHORES

PARTS OF THEFLYING SHORE

Joint A

Joint B

Joint C

Wall Plate

Straining Cill

Single Flying Shore

3.17

Joint A Joint B Joint C

Details of a Single Flying Shore

Steel Dogs

Folding Wedges

Needle & Cleat

3.18

POSITION OF FLYINGSHORES WITH REFERENCETO FLOOR LEVELS

Shore

Shore

Shore

C. Runs of boriti joists are different oneach side and the floors are at differentlevels. The wall with joists parallel to it hasless restraint, the shore is placed againstthe wall.

B. Runs of boriti joists are the sameon both sides but floors are atdifferent levels.

A. Runs of boriti joists are thesame on both side and floorsare at same level.

3.19

SPECIAL FLYING SHORES

For distances of more than 15m. usea double flying shore. Spans of thiswidth will require the use of timberrather than boriti.

The Angle ShoreTo provide maximum support wherefloors are at different levels theangled shore should be used.

The Double Flying Shore

Joint C

Floor Slabs

Floor Slabs

Joint C Joint AJoint B

Note: refer back tojoints on previous page.


DEAD SHORES

Stout boritis placed vertically are used to support floor and roof slabsweakened by rotting boritis. They also form part of the support systemneeded before new openings can be cut through existing walls. Additionally,dead shores should be used to relieve damaged walls of much of the weightof floor slabs set into them. When used to support floors whilst newopenings are cut through the walls the props support NEEDLES.

Needles are very strong timbers or sections of steel placed right throughthe wall and at 90° to its face. The needle supports the weight of the workabove and transmits it to the dead shore. When using dead shores tosupport a failing floor or roof slab it is very important that all of the slabs tothe rooms beneath are supported in the same way. This will ensure that theweight of the slab in need of support is carried right through the buildingand down to the ground. Any cellars or rooms below ground level mustalso be strutted.

Shores are placed on sole plates of timber planking laid parallel to the wallsand set about 1/3 of the width of the room in. The purpose of this is tospread the weight transmitted through each of the props over a wider areaof slab and make it easier for the props in the rooms beneath to continuethe load down to the ground.

Directly above the sole plates, heading boards must be nailed to the under-side of the boritis of the slab above.The shores themselves are placed between the two sets of horizontalplanks and firmly tightened into position by driving home sets of foldingwedges placed between the tops of the shores and the heading boards. Theprops, when in position, can be further strengthened by nailing planks diago-nally across them, reducing any tendency to bend.

Dead Shore

Failing Slab

DEAD SHORES SYSTEMFOR SUPPORT OF ROOFAND FLOOR SLABS

Note: Shoring continued through floorsbelow and down to the ground

3.21

DEAD SHORING TO FAILEDFLOOR SLABS

Section through supported slab

shoring systemto slab above

Floor Slab Horizontal Bracing

Boriti Joists

Note thatsupport isprovided clearof rottenboriti ends

Elevation of Support System

Sole Plate Boriti Shore

Heading Board

Diagonal Bracing

Sole Plate

3.22

DEATILS TO FLOOR SLABSUPPORT SYSTEM

Head of Each Shore Foot of Each Shore

Heading board

Locating lathnailed to headingboard

Boriti shore

Slender foldingwedges tighten boritishore against sofit ofslab over

Boriti shore

Foot of boriti shoreis located in simpletimber frame,screwed to soleplate

Sole plate

3.23

Note: To avoid damage to the building it is essential that needles are strongenough to bear the weight above. Always use the thickest timbers or steelbeams that are available. The maximum distance between needles is 1 metre.

DEATILS TO FLOOR SLABSUPPORT SYSTEM

In order to remove a section of a wall, to make a new entrance or windowopening, it is essential that the weight of the wall above is supported untilthe new lintel is fitted.

Providing Support To provide support in these situations it is first necessary to cut a series ofholes about, 1.5m apart, a short distance above the site of the intendedopening. Baulks of timber or steel beams are then passed through the holes,their ends being carried on the dead shores.

The foot of the shore is placed on a timber sole plate. Pairs of hard woodwedges are placed between dead shores and the needles. Before thewedges are driven home securing the needle against the wall above, a bed ofvery dry cement mortar must be placed on top of the needle at the pointwhere it passes through the wall. This will ensure a solid bearing for theneedle through the wall. Once the wedges are finally driven home thewhole must be left for five days to set.

Shores should be further strengthened and prevented from possible over-turning by nailing planks diagonally across them. It is essential that bothneedles and dead shores have an ample margin of strength to avoid anysettlement of the wall through bending of the needle.

1.

2 .

3.

Dead shore

Sole plate

Diagonal bracing

Steel dogs

Needle

3.24

INSTALLING DEAD SHORESAND NEEDLES

Stage 2A short distance above the linerepresenting the TOP of the lintelcut holes to receive the needles

Stage 1Carefully mark onto the wall the fullwidth and height of the opening. Theheight should be measured to thetop of the lintel

Stage 3Insert the needles and shores asdescribed in the text. Ensure thatthe shore firmly braces the needleagainst the top of the hole. Packthe gap between the needle and thewall with a very dry mix of sandand cement.

4.1

LIME AND LIME MORTARS4.


4.2LIME & LIME MORTARS

INTRODUCTION

What are Mortars?

The Shape and Size of ParticlesCleanliness

What Materials Can Be Used as BindersWhich Binder is Best

LimeSlaking & SievingMixing Lime MortarsThe Setting of Lime Mortars

MORTARS

AGGREGATES

BINDERS

LIME MORTAR

4.3

In building, the word MORTAR is applied to any mixture of solid particles thatbonds larger materials such as stone or bricks together, forming a solid mass.Mortars are very important, and are used for many purposes in constructionand restoration work.

Render and PlasterMortar used to hold stones or bricks together, forming walls and arches,coat walls inside and out with a smooth surface, is called render or plaster.

StuccoIn the past, plaster was also used to decorate walls with intricate patternsand designs. When used in this way mortar is called stucco.

GroutA special mortar is also sometimes needed in repair work, to helpstrengthen walls by filling cracks and open spaces that may have developedbetween the stones deep within the wall. This type of mortar is called grout.

In most cases mortars are made of two parts; solid particles orAGGREGATES and the material that holds them together, the BINDER.Sometimes extra pigments are added. Pigment provide colour; set additivesmake mortar more weather resistent and harden more quickly.

WHAT ARE MORTARS ?

Plaster & Render

Stucco

LIME & LIME MORTARS

MORTARS


AGGREGATES

THE SHAPE AND SIZEOF PARTICLES

Aggregates do a number of important jobs. They act as fillers, giving themortar much of its compressive strength and help the wall to breathe byallowing moisture to escape through it. The smallest particles in the aggre-gate will also decide the colour of the mortar. The correct choice of aggre-gate is very important when preparing mortar. The characteristics of aggre-gates used will affect the quality and performance a great deal.

The most important characteristics of aggregates are:

SHAPE

SIZE

CLEANLINESS

4.5

It is best if particles are rough, jagged,and angular. This will help theminterlock and fit together, held firmlyby the binder. Aggregates like this arecalled SHARP.

PARTICLE SHAPES Aggregates that are smooth androunded are called SOFT and willmake weaker mortars because thebinder will not be able to grip themso tightly.

Sharp Aggregates Soft Aggregates

BEST NOT SO GOOD

4.6

SIZE OF PARTICLES

It is very important that the aggregate is made up of particles of differentsizes. Smaller particles should fill the spaces between the larger ones. Try toimagine a pile of footballs; between the balls will be quite big spaces, wecould fill those with tennis balls. Even with tennis balls filling the big spacesthere will be smaller spaces between the footballs and the tennis balls, thesecould be filled with table tennis balls Even with all the spaces filled in thisway there will still be lots of very small spaces left between the balls. It isthese small spaces that the binder will occupy when the aggregate is mixed.

If mortar is made with only big particles it will be very weak and porous. Ifmade with only small particles it will be very difficult to mix with the binderand will crack and fracture easily. The strongest mortars will have a goodrange of particle sizes and an aggregate to binder ratio of approximately1 part binder to 3 parts of aggregate, (1:3). This ratio is called the void ratioand is determined by both the size and shape of particles.

BEST

Good range of particle sizes

4.7

1. Find a deep, flat sided, glass orclear plastic bottle. Certain typesof water bottle will work well, butyou will need to cut off the top.

2. Fill the bottle to about 1/3 with asample of aggregate.

3. Top up the bottle with water,hold a cloth tightly over the top withthe flat of your hand and shake veryhard. Make sure all the aggregate hasbeen taken up and swirled around inthe water.

4. Place the container on a flat levelsurface and leave undisturbed forone day so that the particles cansettle out of the water. As theparticles settle to the bottom of thecontainer, the larger and heavierpieces will settle first. On top ofthese will be the middle-sizedparticles and on top of those will bearranged the finest of the particles.

VOID RATIO OF PARTICLES

1. Find two clear flat-sided contain-ers: water bottles with the top cutoff will do.

2. Fill one to about two thirds withaggregate.

3. Fill the second to the same depthwith water and mark the level.

DISTRIBUTION OF PARTICLES

Follow these steps to determine the size and distributionof particles in aggregates:

5. You should now look verycarefully at the aggregate in thecontainer. In your minds eye dividethe contents into four equal parts. Ifthe bottom 1/4 is more or less allbig pieces, the top 1/4 more or lessall fine pieces and the 1/2 in themiddle mostly medium sized pieces,the aggregate will have a good rangeof sizes.

4. Gently pour water from thesecond container into the first con-tainer containing the aggregate. Takecare not to disturb the contents.

5. Continue to pour until the waterlevel in the second container is levelwith the top of the aggregate.

6. Place the water container on a flatsurface and observe the new level.

7. Ideally, approximately one thirdof the water should have beenabsorbed amongst the aggregate.

If a lot of water was needed to fillthe spaces, the aggregate containstoo many large particles and mortarmixed with it will be prone tocracking and be very porous.If only a little water is required,there may be too many fine particlesand the mortar will be poorly boundand therefore physically weak.

Follow these steps to estimate the void ratio of aggrgates:

Cut off waterbottle

Aggregateseparated accordingto size and heavierpieces settle atthe bottom

4.8

PARTICLE CLEANLINESS It is very important that aggregates, particularly those consisting of soils, are freefrom root and leaf fragments. It is also not good if the soil contains too muchclay. To remove root fragments etc., the soil must be put through a course sieve.

To find out if there is too muchclay in the soil, follow the stepsbelow:

1. Find a flat-sided container likethe one used previously. Place somesoil in it and mix with water just asbefore.

2. When the soil settles to thebottom of the container, if the layerof clay on top is deeper than onetenth of the depth of sand, it is toodirty to use.

If the aggregate stains heavily orforms sticky balls in the fingers itshould also not be used.

To obtain the best possibleresults from an aggregateyou MUST:

1. Select a well-graded material,ranging in size from fine to coarse.(In order to achieve this, it may benecessary to mix aggregates fromdifferent locations. Although poten-tially expensive, such mixing isrecommended for aggregatesintended to form an exterior renderin an exposed position. Rendersexposed to the full heat of the sunwill contract very rapidly in a rainshower. This will produce stresses inthe surface that could lead to earlyfailure.)

2. Avoid a high percentage of clay.

3. Wash the aggregate in cleanwater if there is any danger ofcontamination by seawater.


BINDERS

The binder is the part of the mortar that holds all the different sized parti-cles of aggregate together. Without a binder the aggregate would simply bea pile of sand.

Kimbada cha makutaIn Zanzibar three types of binder are used. In the simple ‘kimbada chamakuta’ of the shamba, heavy clay soil is often used to bind together piecesof coral to make infill between the sticks of the frame. This kind of claybinder and mortar is not very good as it will easily wash out in the rains,although it is simple enough to mix up more and replace the loss. Simpleclay bound soil mortars in Zanzibar are suitable only for crude buildings.

Portland CementAnother kind of binder very widely used is Portland Cement. Cement issupplied as a grey, dry, and very fine powder. It is added to the aggregate dryand then mixed in with water to produce a wet mass, CEMENT MORTAR.These mortars are now used with coral stone, concrete block, bricks and asrenders.

LimeTo build the higher quality coral stone Swahili house, and many years ago tobuild the Stone Town, a third kind of binder was used, lime. Lime is todaysupplied as a fine white powder called HYDRATED LIME or as a thick whitepaste called PUTTY. Hydrated lime is generally only used as a component ofcement/sand/lime mortars and is not of sufficiently high quality to specify foruse as a traditional lime mortar. Lime putty is the binder originally used inthe construction of the Stone Town. Lime in this form has been used tomake mortar for thousands of years in many parts of the world.

WHAT MATERIALS CANBE USED AS BINDERS ?

4.10

WHICH BINDER IS BEST ?

Before looking at the very different mortars produced by cement and limebinders, it is important to understand how crucial mortars and renders are inthe life of a stone building.

When used in a building, stones are encased in mortar. The mortar fills thespaces between the stones and is often applied to their faces as a plaster orrender. Mortar makes the environment in which stones must live. If stones,and the buildings made from them, are to remain healthy, mortars andrenders MUST be as similar as possible to the stones they live with.

Many old houses were built with poor foundations. This often leads tocracks forming in the walls as heavier parts of the house start to sink.Mortar made with a lime binder remains slightly flexible even when set andwill let walls move without cracking too much. Furthermore, lime mortarsrepair fine cracks themselves as rainwater slowly deposits fresh calciumcarbonate taken into solution from the surrounding mortar.

Fine cracks repairthemselves

Lime Plaster

4.11

LIME PLASTER

Old buildings were built without damp proof courses which stop rising dampinside the walls and it is very important that mortars are porous so moisturecan pass through them and evaporate. This helps to keep houses dry andmakes a healthy atmosphere inside.

Cement mortars are very useful to modern builders but potentiallydisasterous when used for traditional construction and restoration work.When cement sets it is strong, hard, waterproof and can be used in almostany circumstances. Cement is also very easy to use and of known qualityand reliability. All of this makes them ideal for many MODERN buildingprocesses but not recommended for use in old buildings.

When preparing a mortar for use with historic masonry, it is important tothink of the building not as a rigid structure, with all of its parts set fast asthough cast in steel, but as a living animal that MUST move and breathe.Cement-based mortars do not have the ELASTICITY to allow the buildingto move without cracking.

CEMENT MORTARS

Moisture risesin the wall

Lime plasterallows moistureto evaporate

4.12

The waterproof nature of the cement mortar inhibits evaporation ofrising moisture and often helps to encourage a damp and humid atmos-phere in the house. Damp walls that have been rendered in waterproofcement in an attempt to cure the problem will become wetter becausemoisture trapped behind the render will naturally rise to new and higherlevels within the wall. This action can also damage and weaken walls as aresult of salt, dissolved in rising moisture, crystallising in the mortarbehind the render and breaking it up.

Cement inhibits moisture evaporation

Salts build up behindthe cement attractingmore moisture

Moisture trapped inthe wall by cement

Moisture

Moisture and saltsare concentrated atthe edge of cement,causing the plasterto fail

4.13

It is common practice in the Stone Town to replace lime renders withcement because it is thought to be stronger and less likely to suffer damagein the future. Areas of cement patching can often be seen surrounded byscars eaten deep into the wall. Cement mortars will also expand and con-tract up to twice as much as limestone or brick. This means that stones andbricks are constantly fighting against the mortar that surrounds them, whicheventually leads to the formation of many small cracks that allow morerainwater into the wall.

Damage caused by cement patch repairs

Moisture and saltsare concentrated atthe edge of cement,causing the plasterto failCement Mortar


LIME MORTAR

Lime mortars are always to be preferred for use with stone or brick andespecially in old houses built with lime mortar originally. If it is possibleduring the course of repair work, it is good to remove as much cementrepair as possible and replace it with lime mortar. This will allow the stonesto breath and move. Cement renders in particular should be removed andreplaced with lime-based plaster. It is always better to use lime basedmortars with coral stone, because lime is made from coral stone and re-mains chemically compatible. Lime and coral stone will bond togetherbetter. In order to understand this, we must look at the way lime is madeand how it works as a binder.

LIME

4.15

THE LIME CYCLE

Carbon Dioxide

Calcium Carbonate

Calcium Oxide

Water

Carbon Hydroxide

Carbon Dioxide

Water

Building

Mixing Mortar

Stone Town Lime Stone

Lime Burning

Quick Lime

Slaking

Lime Putty

4.16

MANUFACTURING LIME

1. The raw material of lime islimestone, which must first be burntin a KILN or CLAMP. In Zanzibar itdoes not matter which kind oflimestone is used, the procedure isthe same.

2. As limestone is burnt, water andCARBON DIOXIDE are driven off,which changes the stone intoQUICKLIME which still looks verymuch the same as limestone but islighter in both colour and weight.

Lime Stone Lime Burning Quick Lime

4.17

6. So lime mortars, whenCARBONATED, are chemically thesame as limestone and will act in avery similar way. Although themanufacture of lime and lime boundmortar is very simple, to get thebest quality product and goodworking results great skill anddetailed knowledge is needed.

3. When cool, the quicklime is putin a tank and covered with water. Assoon as it comes into contact withthe water it will expand very quicklyand get very hot, occasionally thewater may boil. This process is calledSLAKING and the mixture of slakedlime and water is called PUTTY.

4. To make mortar it is necessary tomix the putty, which will become thebinder, with suitable aggregates.

5. Once the mortar is used in thewall or as a render, the water willstart to evaporate and CARBONDIOXIDE, from the air, will beabsorbed. This process is very slowand is called CARBONATION. Asthe lime in the mortar takes incarbon dioxide, it replaces that lostwhen the stone was burnt andslowly turns the putty back into aform limestone.

Slaking Mixing Mortar Building Stone Town

4.18

LIME BURNING ORCALCINATION

The use of lime as a binder is very ancient and has been practiced world-wide. There are many different methods of making quicklime from lime-stone. In Zanzibar the most common way of making quicklime involves theuse of a CLAMP (in Swahili, ‘tanu’). This is the oldest and most basic way ofburning limestone.

Burning LimestoneSeveral layers of coconut logs are arranged on the ground in a large circle. Ontop of these are stacked coral stones, followed by more logs and more stones,the stones becoming smaller towards the top of the dome shaped heap. Thelogs are then set alight and will burn for many hours. Although some of thelime produced in this way will be of excellent quality, there will be much that isUNDER-BURNT and a little that may be OVER-BURNT. This is because agreat deal of the heat produced by the burning logs is lost and many of thecorals never actually get hot enough to be fully converted into quicklime. Forthis change to take place a minimum temperature of 880° C is needed. Toensure that stones are burnt right through a slightly higher temperature isbest. The very best quality and most reactive quicklime is produced in thelower temperature range. The higher the temperature the more the lime isOVERBURNT and the quality decreases. Fortunately, the temperature atwhich coconut burns is just about that which is needed to produce goodquality lime so there is not too much danger of over-burning.

Difficulties using the ClampThe biggest problem with the clamp is that so much fuel is needed toproduce a small amount of quicklime. Under-burnt lime, which has not quitereached the minimum temperature, will contain a core of unchanged lime-stone and only the outer skin will be of any value. Over-burnt lime is oftenwizened and shrunken in appearance and it is also very slow to SLAKE,taking weeks. If lime like this is used to make plaster before it has fullyslaked, the process will continue on the wall, resulting in fragments peelingoff as the particles slowly expand.

4.19

Improving Kilns and Better LimeThe clamp is such an ancient and crude method of production that it is notdifficult to improve. The most important thing is to retain much more of theheat and so change more coral into quicklime and use the fuel more effi-ciently. A very simple way of doing this is to cover the outside of the clampwith a thick layer of wet mud containing palm leaves or reeds as reinforce-ment. Holes should be left in the mud at various points to provide ventila-tion. Although the clay covered clamp is more efficient, eventually the heatof combustion causes the clay to fall away and the advantage is lost.

The Shaft KilnThe next step up from the clamp or the clay-covered clamp is the simpleshaft kiln, The shaft kiln resembles a short fat tower. The outside is oftenmade of concrete, although coralstone and lime mortar will do just as well.To stop the heat inside burning the walls, the kiln is lined with fired claybricks set in earth mortar and, to retain as much heat as possible, an insulat-ing layer of 15mm to 20mm coral aggregate is provided between the brick-lining and stone walls. Ventilation is provided by brick covered channels inthe kiln floor and ports through the walls.

Suitable FuelAlthough timber burns at about the correct temperature to produce firstquality lime, it is expensive and damaging to the environment. It is a goodpolicy to reduce the amount of timber required by adding coconut husksand setting aside areas of land on which to plant new coconut. Coconut logsburn quickly and produce a long flame; this is very useful within the shaft kilnas it helps to produce a good even CALCINATION. Because coconut burnsquickly it can be used to encourage other, slower burning fuels such as dung.

Filling the KilnIt is very important that the stones within the kiln are more or less thesame size, about the size of a fist or a little larger. This ensures that thecharge is filled with small gaps or voids, enabling air to enter the mass andflames to work up through it. If it is necessary to burn two or more sizes ofstone at the same time they should not be mixed. Each size should bepacked together, isolated by fuel layers. The base of the kiln is stacked withlogs so as to form a tunnel, which is filled with brushwood. Above arearranged alternate layers of fuel and stone. To fire the kiln it is necessary toignite the brushwood. If flames come out of the top of the kiln there is toomuch air getting in and the ventilation holes must blocked with clay or brick.Should black smoke come out, more air is needed and ventilation holesshould be enlarged. When the kiln is operating at its most efficient tempera-ture, a very small amount of smoke should be visible. The air above the kilnshould ‘shimmer’ slightly. A rough indication of the temperature inside thekiln can be gained by observing the colour of the combustion through aventilation hole,

500°C slightly visible red glow700°C dark red800°C mellow cherry red1000°C bright red1200°C bright orange

With practice it is possible to judge the working temperature and regulatethe firing conditions.

4.20

SLAKING & SIEVING

In order to obtain the best results it is very important that the quicklime isslaked as soon as it is cool enough to move, no time must be lost in submerg-ing it in water. The longer quicklime is exposed to the air, the more the qualityof mortar made from it will decrease. This is because moisture contained inthe air will very soon begin to slake the lime. The problem with this is that aswell as water, air contains carbon dioxide. If left out the quicklime will bothslake and carbonate at the same time. This is called AIR SLAKING and meansthat the quicklime is turned back into limestone dust before it has been usedin a mortar. This will make it quite useless as a binder.

Maximum 3 days

Slaking

Always put the lime into the water,never pour water over the lime

Concrete Tank

Quick LimeAlways use lime in lumpform not powder

4.21

During slaking there must be sufficient water to completely cover the lime.It is important to remember that as slaking proceeds the lumps will swelland get much bigger, so enough room must be left in the tank or pit to allowfor this. If possible it is best to slake in a large concrete tank, but if this isnot possible slaking can just as easily be carried out in a shallow soil pit. Tomake such a pit it is necessary to select an area of flat ground about 5msquare. It is best if the ground is slightly higher than the land around; this willhelp stop possible contamination of the lime with salts possibly contained inthe soil. A strong earth bank about 1m high is next built around the site.The soil must be well compacted and beaten down with a shovel.

Once the bank is completed the pit is lined with palm leaves and plastic bagsto prevent the lime becoming contaminated with soil. When all of this workis done, the pit should be filled with water to depth of about 500mm and thequicklime shovelled in. Always put the lime into the water, never pourthe water over the lime. As the lime starts to expand, be ready to addmore water should it show above the surface. After two or three hours theheat and bubbling caused by slaking will stop but that does not mean slakingis over. The lime will continue to swell and slake for at least two days. Itmust therefore stay under water, in the pit, for at least four days, duringwhich time it must be hoed and turned frequently.

Making Lime Putty

Make sure that the lime putty stays inthe pit for a minimum of 4 days,or as long as possible

It is best to slake lime in a largeconcrete-block tank

Water must alwayscover limestone Water

Hoe and turn puttyfrequently

4.22

SIEVING

After at least four days under water the QUICKLIME has become LIMEPUTTY and is now ready for REFINING. This is a simple task involvingpicking out all the remaining unburned limestone and any other large impuri-ties, followed by sieving. It is best if the sieve has a mesh size of between 1mm and 2 mm. As with every other part of lime technology, refining issimple but if it is not done well the quality of the putty will suffer. Afterrefining, the putty must again be stored under water until it is required. It ispossible to store putty in a number of small pits dug into the ground. Thepits must, of course, be lined as before and their tops covered with palmleaves to reduce evaporation and exclude unwanted material.

FOR WELL-SLAKED PUTTY FOLLOW THE STEPS BELOW:

Quicklime must be slaked assoon as it is cool enough tomove and must never be left forlong periods in the open air.

It must be slaked in clean pits ortanks free of soil or vegetablematter.

The quicklime must be com-pletely covered by clean waterduring slaking and sufficientwater must be kept on hand totop up the tank when necessary.

The quicklime must remain in thetank and fully covered by waterfor a period of at least 48 hours.This will ensure that all the limeis slaked.

Whilst in the slaking tank themass must be stirred and agitatedat regular intervals. This willmake sure that water penetratesthrough the mass and the wholeis completely slaked.

When slaking is complete theputty must be transferred toclean storage tanks where itmust remain under water for aslong as possible before use andfor a minimum period of sixweeks.

During in the transfer fromslaking to storage tanks, theputty must be refined andunburned fragments of stone andlarger pieces of fuel debrisremoved. The putty must thenbe forced through a sieve with amesh size of 1mm to 2mm.

1.

2.

3.

4.

5.

6.

7.

Lime Putty Press puttythrough sieve

Mesh size between1 and 2 mm

4.23

MIXING LIME MORTARS It is not possible to mix lime mortar in the same way as cement mortars,which are simply turned over and over, with occasional chopping.To make a strong lime mortar it is essential to coat each particle of aggre-gate with lime paste, this means that the pile of mortar, after initially mixingin the aggregates with a shovel, must be beaten with pick axe handles inaddition to chopping and turning. The longer mixing and beating can beprolonged the better and more efficient the mortar will be.

It is also important that only the smallest amount of extra water is addedduring mixing. The temptation to pour on lots of water in order to makemixing easier must be resisted. Additional water will severely weaken themix and cause a great deal of shrinkage and cracking on drying.

In addition to all of the above it is also very important that the correct ratioof aggregate to binder is maintained. Once mixed, the mortar should bestored for as long as possible before use. This enables the lime to thor-oughly coat every particle of aggregate, ensuring an efficient and well-boundmortar. Lime mortars that have not matured for long enough will not bondwell with the aggregate. Lime particles become smaller as they mature anddevelop closer contact with the aggregate. If lime mortars are used soonafter mixing, the lime does not have time to develop a close contact withaggregates that may already be coated with a film of water.

4.24

FOR THE BEST QUALITY MORTAR FOLLOW THESE STEPS:

1. Metering is easy to achieve byregarding a suitable bucket as onewhole unit of measure. A one tothree mix would consist of onebucket of lime to three of aggregate.

3. Using a shovel, divide the limeinto a number of small parts spreadevenly over the mixing area. Usingthe same bucket measure out thecorrect amount of aggregate, addingsmall quantities to each of the pilesof lime.

Note: Throughout the wholemixing process, no extra watershould be added. However, if someof the aggregate is composed of avery dry and porous stone dust itmay be necessary to add a smallamount. If this is required a handheld spray should be used in orderto avoid swamping the mix.

2. Using the same bucket, measureout the correct amount of aggregatewhich should be as dry as possible.

Lime(1) Aggregates (3)1:3 ratio

Mixing Board

Do not add extra water

Lime Putty

Aggregate

4.25

4. Using a shovel, fold and shop theaggregate into each of the piles of lime.

6. When each of the small piles isthoroughly mixed they should beshovelled together to form a singlelarge pile which should again bechopped and beaten.

7. Once mixed, the mortar shouldbe stored for as long as possiblebefore use and for a minimumperiod of three months. It should bestored in a clean pit or in a numberof plastic or galvanised steel bins.Whilst in storage air must beexcluded from the bins or pit. Thiscan be done by covering them withdamp sacking and palm leaves.

5. Each pile must next be firmlybeaten with timber paddles andoccasionally chopped with a shovel.The longer this process can bemaintained the better the quality ofmortar will be.

Chop

Beat Small piles shovelled togther Storage Bins

Damp Sacking

4.26

CarbonationLime mortars set very slowly, as the water required for mixing and prepara-tion evaporates and carbon dioxide from the atmosphere is absorbed. Carbondioxide works with the lime producing a growth of tiny interlocking limestonecrystals, which bond with the aggregates binding them together. Although thisreaction may take place quite quickly on the surface, it can take years tospread through the mortar in a thick wall. This slow process of setting as aresult of contact with carbon dioxide contained in the atmosphere alone iscalled CARBONATION.

Carbonation is helped if the mortar is encouraged to dry very slowly, makingit much easier for carbon dioxide to enter if it is slightly moist. This can bedone by gently spraying the surface with a garden spray. A rendered wallMUST also be protected from direct sunlight by hanging sheets of wet sackingin front of it. If renders are allowed to dry very quickly they will crack andwill not bond properly with the masonry.

Setting with AdditivesAdding other materials to lime mortar that react chemically, can speed up thesetting processs. Set additives are essential if the mortar is to be used in asituation that is constantly wet, as carbonation will not take place if the mortarremains saturated. Lime mortar alone will NOT set under water, to do so, itmust have an effective set additive mixed with it. The materials that will work inthis way are called POZZOLANAS and contain SILICATES or ALUMINATES orboth. A very common building material that makes an excellent pozzolana isbrick. To be used in this way, the bricks must first be broken into small piecesand then reduced to dust by sieving through a fine flour screen. The brick dustis added to the mix of lime and aggregate just before use. To one part of limeand three parts of aggregate should be added 1/4 of a part of brick dust. Al-though it is not best practice, if no other pozzolana is available, a small amount ofcement can be added to the mix. This should not exceed 1/2 of a part.

THE SETTING OFLIME MORTARS

Shade work from the sun

5.1

REPAIRS TO HISTORIC RENDER5.


5.2

Why Repair ?The Causes of Failure

Rising DampDesalinationIsolating Salts and Masonry

Mix RatiosWall PreparationDubbing OutRender CoatTop Coat or Setting Coat

Removal of Rusting IronPatch RepairsRepairing Mouldings

GroutingPinning


PLASTER AND RENDER

SALTS AND DESALINATION

RENDERING

PATCH REPAIRS

GROUTING AND PINNING

REPAIRS TO HISTORIC RENDER

INTRODUCTION

5.3REPAIRS TO HISTORIC RENDER

PLASTER AND RENDER

An undamaged and well maintained lime render is essential if walls are toremain strong and the atmosphere inside houses is not to grow dank andhumid. Repairing broken or missing rendering should not be seen as a luxurybut as a vital step in the repair process that can be guaranteed to deliverimportant benefits and offer real value for money.

Renders, both interior and exterior, are very easily damaged by:

• salt action• abrasion• water penetration• neglect• rusting nails• iron• steel fittings

If the roof of a house is compared to an umbrella, the render is an overcoatkeeping out the rain. Lime renders are not waterproof like cement, butporous. They work by ABSORBING rain and holding it in the surface. Thisprevents the rain from soaking right through and wetting the masonrybeneath. When the rain stops and the sun begins to shine again, therainwater soon evaporates and the render dries.

WHY REPAIR ?

5.4

Cement RenderCement renders work in a differentway. They ARE waterproof and donot allow any water to enter. Thissounds like a good idea but if therender is cracked or damaged andrain gets in it cannot escape. The wallwill remain wet, even if the sun shinesall day, water will be trapped inside,evaporating only very slowly. Eventu-ally this causes damage in the form ofdeep scarring and pitting of the

Lime RenderSalts will damage lime render in thesame way as a cement render but inthis case it is in the face of theRENDER not the wall. So limerenders protect walls by becomingdamaged themselves. To safeguardthe wall, all that need be done isremove the damaged render fromthe lower section of wall and replaceit with a fresh one and so on.

The porous nature of lime renderallows water to evaporate freelythrough it.

Because cement render iswaterproof, escaping water cannotpass through it and all the damagetakes place in the wall BEHINDthe render.

surface. The damage is caused bysalt taken up in rainwater as it movesover and through contaminatedmasonry. As it slowly evaporates outof the small cracks and holes throughwhich it entered, salt crystals are leftbehind. These grow and begin topush against the stone or mortararound them. Eventually this actionwill break the mortar and pittingwill develop.

Rain

Cement Render

Salts buildup behindcement render

Moisture

Rain

Moisture

Moisture evaporatesfrom lime renderedsurfaces

5.5

It is not sensible to begin a large programme of rendering without firstfinding out why the render failed in the first place.

If the cause of the damage is simple abrasion, the result of years of scratch-ing and rubbing by carts and cars, etc., rerendering is strait forward.However, it is likely that the wall will be contaminated with salts of one typeor another, in which case rerendering without other supporting treatmentswould be a terrible waste of money as the new work would quickly fail. Tore-render externally without first renewing or repairing gutters and down-pipes etc., would also be a mistake. Just as render protects the wall, guttersand down pipes protect the render.

In the same way as structural repairs, re-rendering and repairs to renderingrequire careful planning and thought. The first task is to test the surface of theremaining render as it may be that a lot can be stabilised and kept, reducingthe amount of new work needed, this has advantages from a conservationpoint of view also as more valuable original material will remain.

THE CAUSES OF FAILURE

Testing Existing Plasterand Render

Testing is a very simple matter. Thesurface is lightly tapped with a smallsection of iron rod. If the soundproduced is hollow the bondingbetween coats is poor. If there areonly isolated areas that sound likethis, the render is in good conditionand can be kept. If, on the otherhand it sounds hollow all over, itwould be best to replace it.

If the wall surface is very badlyeaten away, it will need more thanjust re-rendering; some attemptmust be made to DESALINATE it asit will be contaminated with salt.


SALTS & DESALINATION

When looking for the cause of plaster failure on the ground floor, the firstthing to look for is RISING DAMP. This simply means are the bottoms ofwalls and ground floors wet or damp? If they are it is necessary to find outWHY. If your house is in the Stone Town, unless it is very close to the sea, itis unlikely that rising ground-water will be the cause of the problem.Ground-water is water that is naturally contained in the earth and in theStone Town will result from rain fall. If you live close to the sea, wall thereis a chance that sea water that has permeated through holes in it whilst thetide was high is soaking through the soil and up into your house.

The depth at which ground-water is found below the surface is determined bythe type of soil and strata. If the soil is not very porous, like clay, ground-water can be close to the surface and cause walls to be damp. The StoneTown is built on sand which is very porous so ground-water should not be aproblem. However, walls can seem to suffer from serious rising moistureproblems caused not by ground-water but by LOCALISED PONDING of rainwater. This is caused by water draining through broken street covers andcollecting against walls, saturating the wall below the street. Before re-rendering, this type of problem must be cured by carefully filling holes in thestreet covers and, if possible, making gentle gradients AWAY from the buildingand into the street.

Desalination is the name given tothe removal of salts from stone,bricks, and renders, etc.

RISING DAMP

Rising damp patch

5.7

2. The surface of the wall is nextsaturated with clean drinking waterand apply a ROUGH RENDER whilstthe wall is still wet. A rough renderconsists of a coarsely broken porousaggregate mixed with a lime binder.The traditional mix of 1:3 is nor-mally used. It is not necessary to useexpensive good quality lime, ordi-nary HYDRATED LIME from thebuilding merchants will do. Theaggregate can be any broken upcoral with a good range of particlesizes. The mix is quite wet andshould be THROWN against thewall to ensure contact.

DESALINATION A great deal of the salt contaminating buildings in the Stone Town comesfrom the use of beach-sand and other aggregates with high salt contents. Itis also possible that sea water was used in construction and for slaking lime.Masonry walls are therefore often contaminated to their full height andthickness, a situation making complete desalination impossible.

However, if salt content is reduced or eliminated in the outer 150mm of thewall surface before new render is applied, it is likely that the new render willsurvive. This is because if the salt is removed from deeper than from wherethe rain penetrates, the salts within the wall will not turn into a solution andmigrate, damaging the surface.

The only sure way to remove salt isto remove as much as possible ofthe material that contains it.

1. The first step is to cut away allthe damaged render right back tothe large corals forming the walls. Ifthe mortar between the corals isnot firm and solid, this should alsobe removed.

Rough render

Remove existing renderSaturate withclean drinkingwater

5.8

3. It is the job of the rough renderto draw salts out of the masonry.The water applied to the wall willsoak into it and on its way, dissolvemuch of the salt in its path. Eventu-ally, it will begin to return to thesurface for evaporation. As it doesso, it will carry its charge of saltsalong with it. The rough render willact as an extension to the wall and

the water and salt will move onthrough it to the surface. There, thesalts will be concentrated.

4. When quite dry, the salt infestedrender is removed and cleared awayfrom the site. It is important that therough render does not get wet afterit has dried and before it is removed.If rainfall is allowed to saturate it,

there is a danger that salts containedin it will be washed back into the wall.

5. When the process is completedthe salt content within the masonryshould be much reduced. If risingmoisture is not present the treatmentshould be a permanent cure, assumingthe rerendering is well done and whencomplete is maintained! However, if

the house is close to the sea wall orstill suffers from rising damp due toponding, the new render may bedamaged again quite quickly.

Salts migrate tothe surface

Salts in thewall substrate

Salts removed inrender

Salt content ofwall reduced

5.9

If after desalination the wall is still obviously contaminated, it will be necessaryto isolate the salts from the new work. This simply means that a WATER-PROOF BARRIER has to be set up between the wall and the new render.

The work should begin by vigorously brushing the wall with wire brushes toremove more of the contaminated surface material without digging toodeep into the mortar between corals. The wall is then brushed with softbrushes to remove dust. The edges of adjoining intact render should bepointed and secured. In order to make the isolating barrier between walland render, the entire affected area is painted with a layer of bituminouspaint. Once dry, re-rendering may proceed.

For the new render to adhere to the paint, a number of short hardwoodskewers approx.100 mm long should be prepared and hammered throughthe bitumen and into convenient mortar joints beneath at about 300 mmcentred vertically and horizontally. Rough coconut string is tied around andbetween each of the skewers to make a strong net effect. The net will holdthe render firmly against the wall.

ISOLATING SALTSWITHIN MASONRY

bituminous paint

hardwood skewers

rough cocunut string


RENDERING

WALL PREPARATION

The wall is first thoroughly wettedwith clean water, this helps the newrender to stick. If the wall is notwetted, the dry stones and mortarquickly SUCK much of the moistureout of the render and it will notbond with the wall. In all plasterwork, it is important to rememberthat the various coats are not

DUBBING OUT

This is the name given to filling thedeeper holes and scars. Dubbingout brings the wall surface to arough average flatness. Followingwetting, a 1:3 mix of lime and redlaterite soil is THROWN off the endof a trowel into the hole. It shouldbe about 40mm thick.

Small stones are then pressed into it.The stones are first immersed inwater to saturate them. Using stonesin this way will reduce shrinkage andthe water contained in them will helpcontrol drying and reduce cracking.If the work is in direct sunlight, itmust be protected by hanging sec-tions of wet Asian cloth in front of it.Direct sunlight will cause it to drytoo quickly and crack.

If the holes are very deep, dubbing outshould take place in two or threecoats. Each must be allowed to dry,and all cracking and shrinkage musthave ceased before the next is applied.

GLUED together. ALL that causesthem to stick is friction and, for thisto work, the new render mustcontact every part of the face of thewall or render. If the wall is notwetted and the render dries veryquickly, it does not have time to takehold of the rough surface and thebond will be weak.

1:3 mix of limeand red lateritesoil, 40mm thick

Wet wall

MIX RATIOS

SOIL SAND DUST 1* DUST 2** LIME

Filling deep holes 3 0 0 0 1& dubbing out

Render coat 0 0 3 0 1

Setting coat 0 0 0 1 1

* Dust 1 is crusher dust** Dust 2 is finely crushed soft white coralstone.

5.11

This is the name given to the first coat (closest to the wall). It is veryimportant that the way it is applied reflects the character of the room.REMEMBER, WE ARE NOT BUILDING NEW HOUSES, WE ARE CONSERV-ING OLD ONES. THIS MEANS THE WORK CARRIED OUT MUST NOTBE BETTER OR WORSE THAN THE ORIGINAL. IT HAS TO BE THE SAME.So the rendering must be carried out with the old work in mind.

Remember that although the render coat is not the top coat, it DICTATESthe look of the finished work. The top coat is only 3 to 5mm thick so itsimply follows the line of the render beneath. If the render coat is notcorrect the top coat will also be incorrect. This is especially true of mouldingsand arches.

1. The wall and dubbing out aresaturated. A line is stretched alongthe top of one of the walls a littlebelow the boritis. It is stretchedbetween hardwood pins driven intomortar joints. The line is about 40mmabove the wall surface and representsthe thickness of the plaster.

2. Pieces of line are then cut toabout the same length as the heightof the room. A bent nail or smallstone is tied to one end of each line.The lines are now suspended atabout 1.5m centrd from the hori-zontal line already set up. The linesact as plumb lines and control theverticality of the work.

RENDER COAT

Straight edgeused to controllevels betweendabs

Saturate the wallwith water

3. Small dabs of plaster with a stonepressed into each are formed be-neath the vertical lines at about 1,5mor 2m centres. When the dabs areset, the lines are removed, the wallsaturated and plaster applied be-tween dabs. A straight edge is usedto control the level between dabs.

4. It is very important that thesurface of the render is well keyedbefore it sets. To do this the surfacehas to be scored. Steel trowels mustnot be used for this purpose as thescoring is too thin, it is better to usea stick about 5mm or 10mm wide.

1.

2 .

3. 4.

5.12

When the render coat is fully dried and all shrinkage and cracking hasceased, the wall can be dampened and setting coat applied. It is essentialthat the mortar is applied quickly, whilst still plastic.

It is best to apply setting coat with a flat wooden or plastic float. Plaster isplaced on the float and very quickly smeared with an upwards action ontothe wall. A small rounded steel trowel is used to ‘iron’ it firmly down ontothe render coat. During this process the plaster is wetted as necessary soan average flat but undulating surface results.

Note: Examination of patination and texture on much of Zanzibar’s historicrender indicates that setting coat was originally applied with small rounded steeltrowels. In order to maintain this texture similar tools or should be used.

Keying must be taken seriouslyand carried out with thought, it isparticularly important that areasaround doors and windows are wellkeyed as these are subject to vibra-tion. REMEMBER IT IS ONLYFRICTION THAT HOLDS THERENDERS IN PLACE.

TOP COAT ORSETTING COAT

Top coat

Saturate wall

Stick for scouringto produce key


PATCH REPAIRS

This type of repair is used for small areas of damage within larger areas ofintact render. As the surrounding render is not damaged, salt is not likely tobe the cause of localised damage of this kind. Patches are often required toimprove the quality of surface, following the removal of cement mortar thatmay have been used to fill in following the introduction of an electric wire.or to make good places damaged by rusting iron fittings, etc.

REMOVAL OF RUSTING IRON, ETC.The size and extent of the metalwork will dictate the precise details ofremoval but regardless of size, the principles will remain the same.

As far as possible, the fitting should be drilled out using a non-percussiondrill so as to avoid vibration damage to the delicate bond between coats ofrender and masonry substrate. Holes are drilled at close centres around theoutside of the metal, it is then gently tapped with a light hammer and gradu-ally worked free. Should the feature be set deeply, it may be necessary tocontinue excavating the holes begun with the drill by hand. For this purposea special chisel called a QUIRK is useful. The special ‘fish tail’ shape of thetool will permit very deep excavations with no damage to surrounding work.Once the metalwork is removed, the cavity in the wall is filled with mortarand small stones to the wall surface. When all shrinkage has ceased, thepatch repair may proceed.

Damaged surfaces

5.14

Before placing a patch of new render the site must be prepared.It is important that the joint between new work and old is crisp and regular,the joint should not be jagged and crude. A very sharp scalpel or craft knife isused to clean up ripped and broken edges around the site. The patch followsthe line of the break but, avoid sharp and jagged angles. The edge is slightlyundercut so as to help make a good key between old and new work.

Flush the siteThe site is flushed free of all debris and dust. Flushing continues until thesite is saturated and will accept no more water. Particular attention must bepaid to wetting the various joints between layers of render and the masonry.It is important to pay attention to these joints, as capillary attraction alongthem is very strong and will rapidly suck water from the new render, leadingto fine cracks parallel to the joint and a very poor bond.

Applying new renderDuring application, it is important that the work is compacted and thatslightly more plaster is applied, ensuring that the patch does not becomelower than surrounding work. When applying small patches within areas ofvigorously textured original work, it is important that the repair shouldreplicate the rhythm of the existing. If required, craftsmen should practicethe required surface before final application.

PATCH REPAIRS

5.15

Note: To ensure good bondingbetween new work and old, therepair site must be painted with aslurry of lime paste and boiledseaweed water. This mixture worksas a binder and will secure the newwork. If seaweed is not available, adilute mixture of PVA and water canbe used.

To prepare the slurry:

1. Take a small amount PVA wood-working adhesive and dissolve it atthe ratio of 1:20 in clean warm water.

2. Take a small amount of settingcoat and add 1/10 of a part of thePVA water, mixing the lime into athin paste.

3. Take the slurry and using a smallfirm brush, vigorously work it wellinto the wetted substrate, ensuringas far as possible that some hasbeen drawn up along the joint lines.

4. The new base coat should beapplied immediately onto the slurry,ensuring that it is very firmly ironeddown and compacted with somepressure against both substrate andall joint surfaces.

5. When fully dry and all crackingand shrinkage has ceased, the settingcoat may be applied. Before applyingthe setting coat the entire site mustonce again be saturated and treatedwith slurry as described below.

The site must beflushed free of alldebris

Sharp edges

Apply PVA/limeslurry to helpbond betweennew and old

New basecoat firmlyironed downand compacted

Top coat appliedslightly proud ofsurroundingplaster

1. 2.

5.16

A slight variation on the patching procedure is required to repair thepointed cusps to mirhabs. These features are damaged very easily and arefrequently broken off. Because they are pointed, it is not possible to mendthem in the same way as holes in flat plaster.

To make the repair, and assist the plaster to stick, it is necessary to make anARMATURE. This is simply a frame to hold plaster in place.

1. The first step is to remove all the damaged plaster and dust. A wheelbrace and 5mm tungsten tipped bit are used to drill a number of holes in thewidest parts of the repair site. The holes are flushed free of dust and thesite saturated.

2. Small hardwood pegs are cut and set into the holes with sieved limemortar (1:3) with the addition of a little PVA water or boiled seaweed water.Rough local coconut string is bound around and between the pegs forming aframework. The length of peg and general form of the frame is dictated bythe nature of the individual repair and judgement will be required to makesure the frame is buried below the plaster. At no point should the framecome closer than 15mm to the surface.

3. The framework and site are again wetted and the lime slurry workedwell into the surface. This is followed immediately by render coat which isfirmly ironed down against the surface and around the peg and string frame-work. Once set the render coat is rubbed down with sand paper so that itmore accurately follows the shape of the cusp. When all shrinkage hasstopped the render coat is again wetted and setting coat applied in thenormal way.

REPAIRS TO MIHRABS

5.17

The method described above can also be used to repair mouldings. If thedamage is severe and the mouldings are heavy, wooden pegs can be replacedwith sections of ceramic tile.

REPAIRING MOULDINGS

To strike a vertical line acrossa moulding

1. Place a flat boardagainst the moulding.

2. Ensure the flat face ofthe board is at 90° to thewall by holding a squarebetween both.

3. Ensure the boardintersects the moulding at90° by placing a spirit levelagainst it.

4. To strike the line, tie apencil to a piece of doweland holding it tight aginstthe board, strike the line.

To record the section

1. Cut carefully alongthe line with a sharp hack-saw blade.

2. Insert card into the slotand draw the section.

5.18

To construct sectionsof mouldings

1. Place full-size plywood sectionsat regular intervals along the lengthof the repair.

2. Reform the stones backing themoulding. Those that remain wellembedded in the wall should be left.Loose stones must be removed andrepalced with tile. Missing stonesshould also be replaced with tileinserts.

3. Acurate placing of the base coatis essential, the setting coat simplyfollows its line.

Original stone support withtile inserts

Plywwod template

Base coat

Setting coat


GROUTING AND PINNING

Areas of original render that have lost adhesion and are in danger of fallingfrom the wall, may be reattached by injecting a liquid grout into the spacebetween render and wall. If the area of detachment is large, pinning mayalso be needed to secure the plaster. Grouting is an expensive and labour-intensive procedure and should be reserved for the conservation of valuableand interesting examples of plaster. In the Stone Town grouting is of value inthe conservation of column heads, mirhabs, mouldings and stucco ornament.

WORKS SHOULD PROCEED AS FOLLOWS

1. The first requirement is to locate the full extent of the areas of detach-ment, some of which will be obvious while others will not. To find them,simply tap the surface with the blunt end of a pencil. Detached areas willsound hollow. As these are located, a soft pencil is used to mark theiroutline on the wall.

2. A number of 2mm holes are drilled at the lowest point along thebottom of each area of detachment so as to form a continuous slot about10 to 15mm long. A single 3mm hole is drilled at the highest point in eachdetachment.

3. Using a large veterinary hypodermic syringe (with a blunt needle) injectclean water into the top hole in the detachment. As it enters, the water willflush out debris and plaster fragments from within the void and dampenthe interior.

When fully wetted, a mixture of PVA wood-working adhesive and watermust be rinsed through the detachment, causing the inside to become veryslightly sticky and helps in re-attaching the render. The flush should beprepared in the following manner. A small amount of PVA adhesive must bedissolved in clean warm water in the ratio of 1:20.

GROUTING

Areas of originalrender havedetached fromthe wall, leavingvoids

Locate areas ofdetachment bytapping surface

Clean waterinjected into void

To counteract the weight of groutcausing the detachment to bulge, apiece of wood may be leant againstthe wall

Steps 1 & 2 Step 3

5.20

4. As soon as flushing is complete, the lower slit in the detachment isplugged with cotton wool and grout introduced through the top hole. Tocounteract the weight of grout causing the detachment to bulge, a piece ofwood may be leant against the wall.

Grout should consist of the following:

Lime 2 partsVery fine brick dust 8 partsPVA water as required

It is very important that grout is well mixed. Mixing should continue for atleast 20 minutes prior to pouring. Thorough mixing can only be achievedwith the use of an electric drill fitted with a bent wire mixing device. Whengrouting is complete the cotton wool plugs should be removed and replacedwith a little setting coat.

PINNING

If the areas of detachment are large, it may be necessary to pin through theplaster layers and into the masonry beneath, in addition to grouting. Pins willhelp support the weight of the plaster and make the hole more stable.

Before pinning, it is best to allow one week after grouting. 3mm holes aredrilled with a non-percussion drill in the middle of the larger areas of de-tachment. The pins themselves do not require great strength and it is moreimportant that they are COMPATIBLE with stone and plaster. In Zanzibar, avery suitable material from which to make pins is well dried bone. Beforeinserting the pin into the hole, it is filled with a mixture of setting coat and alittle PVA water. The pin is then pressed home and the hole sealed withmore setting coat.

Grout introducedthrough top hole

Plug lower holewith cotton

ImportantGrout must bewell mixed usingelectric drill

Pins must becompatible withstone and plaster

Dried bone can be usedto fabricate pins

6.1

FLOOR SLABS & OPENINGS6.


6.2FLOOR SLABS & OPENINGS

INTRODUCTION

This section provides guidelines for replacing traditional boriti-pole and lime-concrete floor slabs, replacing individual boriti-poles in a floor-slab, and forforming new openings in walls. It also provides guidelines for the constructionof brick relieving arches.

Floor SlabsProcedure for Replacing Floor SlabsExceptional Circumstances

Boriti and BanaaReplacement Procedure

New OpeningsProcedure for Installing a New Reinforced Concrete LintelReplacing Boriti LintelsRelieving Arches


REPLACING FLOOR SLABS

REPLACING BORITIAND BANAA

NEW OPENINGSAND LINTELS


REPLACING FLOOR SLABS

In the buildings of the Stone Town, floor slabs were traditionally made of boriti-poles with a lime-concrete cover laid over to form the floor above.Roof and floor slabs should be replaced if they begin to sag to the extent thatcracks develop along the underside, parallel to the walls. If this situation devel-ops the slab will be entirely dependent on the boriti for support. Even if all ofthe boriti are renewed this type of loading will rapidly cause them to bend.

Where most of the boritis in a floor slab have decayed and are due to bereplaced, the lime-concrete floor should be carefully inspected for signs offailure (e.g., cracks parallel to the wall as described above). If the floorshows signs of failure, the slab should be replaced along with the boriti.

FLOOR SLABS

Failure of slab in wall Floor slab is sagging

Cracks Boriti polescheck for rot

6.4

It is not safe to remove a complete slab at one time. If an entire slab isremoved, the walls in that area will lose significant restraint and may begin tobend or bulge under the weight.

Slabs must be removed in short sections of about 2.5m. Each new sectionshould have a male or female dovetailed key cast into it so that it locks intothe next section when it is cast. It is best if sections are not removed andreplaced one after another all along the floor, instead replace them on arotational basis. This will allow freshly cast sections to cure slightly beforethe one next to it is removed. This approach will avoid areas of weaknessdeveloping.

It is a good idea to replace all of the boriti along with the slab, since remov-ing the slab makes it possible to fit much longer poles. If this is too expen-sive, at least one long pole should be inserted into each section of repair.This will improve restraint.

PROCEDURE FORREPLACING FLOOR SLABS

Section of newly cast slab

2.5

2.5

Replace slab in sections

max. 2.5 m

Dovetail cast into form key

6.5

Removal of existing floor slab

1. Work should begin by dividing the floor into a number of sections about2.5m wide. Divisions should be drawn on the floor with chalk.

2. Next, decide in which order they will be cut out and replaced. Cuttingout must start against an end wall with the opening of a channel about300mm wide to separate the slab from the wall. This work must NOT bedone with a mechanical demolition hammer as the high frequency vibrationmade will travel a long way through the wall and may cause damage, even if itis not seen at once. It is best to use a 2lb hammer and sharp punch, al-though an electric drill fitted with a small diameter bit may be used to help.The section of slab should then be un-picked on a piecemeal basis. Largesledge hammers MUST NOT be used to break up the slab!!!

3. If the slab is very thick or the room is very long, it may pay to usespecially made wedges as illustrated. They can easily be made by a localsmith and consist of three parts. To use them, it is first necessary to drill aseries of holes about 300mm apart along the front edge of the section to beremoved. The three parts of the wedges are assembled in the holes withthe middle part sticking up.

4. A 2lb hammer is then used to gently drive the wedge home. Eachwedge must be tightened in turn, a little at a time until the sections fractureoff. At least five sets of wedges should be used together. When cuttingaway the small pieces of light coral used to bridge the spaces betweenboritis, remember to save the coral for reuse in the new construction.

Centre part driven inusing 21b hammer

3 part wedge

Holes drilled about300 mm apart

300300

6.6

Construction of new floor slab

Reconstruction should begin by replacing the boriti with new longer poles.If banaa were originally used, they should again be used in the new construc-tion. If this is too expensive and you can only afford plain boriti, try at leastto fit one or two banaa to the original pattern as an indication of the originalappearance. If you cannot afford to replace all the boriti, try at least to fitone good long pole every 1.5m.

Once the boriti are fitted, coral salvaged during demolition must be replacedto close the gaps between poles. 25mm thick boards must be arranged asshuttering to contain the lime concrete when it is cast. If the walls are ingood condition and the slab is not thicker than 400mm, the replacement slabshould be the same thickness. However, if the slab is very thick, or the wallis showing signs of over stressing, the slab must be reduced to 300mm.

A suitable mix for the concrete is:¼ part cement2 parts lime2 parts sand3 parts broken brick, fines up to 18mm (¾’’ gauge)

Lime and aggregates are mixed as long as possible before needed and thecement added immediately prior to use. The mix should be dry and vigor-ously compacted on laying.

When casting is complete and at least four weeks allowed for curing, a thincement screed should be run over the surface to produce a flat, smoothfinish. To do this, it is necessary to set up levels.

25mm thick boards used asshultering

After laying the lime concrete, themix should be vigorously compacted

Thin cementscreed finish

Coral placed to fill gapsbetween boriti

Banaa

6.7

Water level

A simple way of making a water level is to use two water bottles and alength of hose pipe as illustrated.

1. To use the level, make two simple tripods from sticks or small boritis tiedwith string and suspend one of the bottles from each. Position one tripodand bottle (bottle 1) over the highest point in the newly cast slab. This siteis the DATUM and must not be moved.

2. The second tripod, with bottle 2, can be moved freely about the slab.Bottle 2 should be positioned at the furthest point from the datum.In order to transfer the datum to this point, bottle 2 should be laid on theslab and water poured into bottle 1 (at the datum). When a clear stream ofwater, free from air bubbles, comes out of bottle 2, it is slowly lifted until thewater level in both bottles is steady and the same.

3. The bottle should then be tied in place and accurate measurementstaken from the top of the water in the bottle at the datum to a point, about15mm.above the slab and a dab of concrete laid to that height.

4. The same measurement must be repeated from the top of the water inthe second bottle and another dab of concrete placed. This process shouldbe repeated at regular intervals over the face of the slab. It is then possibleto float a fine, flat layer of concrete over the slab.

Bottle 1at the highest pointin newly cast slab

Bottle 2can be movedacross slab

dab of concrete

6.8

EXCEPTIONAL CIRCUMSTANCES

In some situations, walls may be so weakened and damaged that an especiallylight slab should be used.

To make such a slab, boritis are laid in the normal manner but the coral usedto bridge the gaps between the boritis are replaced with two layers of roughhessian (sack cloth). The cloth is purchased in long wide rolls and spreadover the top surface of the boritis. It is secured by nailing along the back ofeach boriti at 300mm centres. Small sections of aluminium are cut fromdrink cans and used as washers to prevent the cloth pulling through the nails.The cloth must be stretched as tightly as possible. Soaking in water before itis fixed can assist this.

A 100mm thick layer of concrete, mixed as previously specified, is laid overthe cloth. Empty aluminium drinks cans, orientated vertically, are pressed intoits surface at centres of approximately 150mm. This allows the concrete tobe firmly packed between.

Cast another100mm layer of concrete over the cans. The finished slab willprovide good insulation and be 40 % lighter than a conventional slab ofsimilar thickness.

Light weight slab

Lime concrete

rough hessian orsack cloth

Boriti

Drink cans

6.9

Boriti are the round mangrove poles on the underside of floor slabs. Inlarger houses or in important rooms, the mangrove is often square andthese are called banaa.

Both boriti an banaa have the same functions; they stop the slab above frombending, tie walls together, and during construction, they work as ashuttering for casting the slab.

Tying walls together requires poles to be as long as possible becausefriction is required to keep them in place. The deeper poles are embeddedin the wall, the greater the friction. Embedding much of the pole in the wallalso helps to distribute the weight of the slab more evenly throughout thewall. During construction boriti should be placed to more than half thethickness of the wall.

Unfortunately embedded boriti rot, and it is occasionally necessary toreplace them.

FLOOR SLABS & OPENINGS

REPLACING BORITI & BANAA

BORITI AND BANAA

Boriti or banaa

6.10

Shorter poles = loss of structural strength

The task of replacement is a difficult one as it is necessary to fit the polebetween its two neighbours. To obtain the maximum length, the pole isoffered up diagonally between the ones either side. It is then straightenedinto a position parallel with its neighbours and a small amount at each endlocated in the wall. Replacing poles in this way inevitably causes the newpoles to be significantly shorter than the originals. As more poles arereplaced so the quality of available restraint diminishes.

It is very important that as more boriti are replaced restraint is not lost.Although it is not possible to fit poles any longer than the diagonal betweenthe ones on each side, it is possible to artificially increase the quality ofrestraint afforded by short poles. This is done by fitting specially manufac-tured brackets to the ends of replacement boriti.AB

A

B

6.11

REPLACEMENT PROCEDURE

1. Following removal of the old boriti, work carefully on the emptysocket with a sharp chisel and wooden mallet to form a slight cone shapeextending the full depth of the space left by the original poles. The widerbase of the cone is to the back of the excavation. The mouth of thesocket, where the new pole will rest, is also made a little deeper and aflat base cut, to allow a pad stone to be fitted below the pole, spreadingits weight and avoiding point loadings.

2. Before fitting replacement poles, drill one small hole in the side about100mm from each end to fix the bracket as illustrated.

3. Lay a suitable coral pad stone on a bed of 1:3 lime mortar. Apply abed of the same mortar to the top of the pad stones and lift the pole intoposition. Working around the pole and in the socket, slide the bracketinto position and hammer the end into the hole in the boriti.

4. The hole is now carefully packed with a very dry mortar and smallstones. This must be done with great care, making sure the socket andbracket are tightly packed. The mortar is compacted with a hammer andlong thin piece of wood or steel. To complete the repair, re-render the area.

Boriti pole

Drill small hole 9cmfrom eachend and fix bracket

Cone shapeexcavation

Coral pad stone layed on abed of 1:3 lime mortar


NEW OPENINGS & LINTELS

Openings not formed under arches need long stones or timbers placed overthem to support the wall above. These are called lintels. It is common intraditional construction to use boriti poles as lintels, and boriti lintels aresometimes put inside the wall and hidden, or left exposed. In modernconstruction or alterations, it is common to use reinforced concrete lintelsto form new openings. Whilst this may have some advantages, the workman-ship is often poor which can have potentially serious consequences. Thissection provides guidelines for the construction of reinforced concretelintels and other more appropriate forms of support over openings.

Important note:Although the method of cutting new openings is described in the text, itis potentially dangerous. In general, it is NOT recommended to formnew openings in historic buildings. When replacing lintels or cuttingnew openings, it is essential that the masonry above the work site is wellsupported BEFORE the work begins. If this is not done, very serious damagecan result. (See section 3 of the Guidelines, Shoring & Temporary Support.)

NEW OPENINGS

6.13

Cutting a new opening

1. Begin the work by carefully installing a needle and dead shore system tosupport the building during construction work, as described in Section 3 ofthe Guidelines (Shoring and Temporary Support).

2. If a new opening is to be cut, it is important to make as little vibration aspossible and it is ALWAYS good to work as neatly and cleanly as possible. Itis best to begin by drilling a few holes in the centre of the area to beremoved with a big electric drill (not a percussion drill).

3. Stones can then be quickly unpicked with a good sharp punch and ham-mer. As far as possible, cut away the mortar between stones as this willcause less vibration. The sides should be unpicked as cleanly as possible toform neat reveals. An electric angle grinder can help in this. It is importantthat the top of the opening is cut to form slots wider than the intendedopening so as to provide support for the lintel.

PROCEDURE FOR INSTALLING A NEWREINFORCED CONCRETE LINTEL

Area of opening

Drill a few holes in thecentre area

Unpick the stoneswith a hammer

6.14

PADSTONES

If a pre-cast concrete lintel is to be used it is first necessary to castconcrete padstones on top of the slots to either side. This ties the top ofthe wall together and makes a good firm base for the lintel to sit on.

A suitable mix for padstones is:1 cement to 4 coral aggregate up to 7mm (¼inch).

After a period of 28 days the padstones will have cured and a bed of mortarconsisting of 1 part cement, 2 parts lime, and 8 parts of aggregate is laid ontop of each padstone and the lintel firmly bedded into it.

Following a period of two to three days, to allow the mortar to become firm,a very dry mix of 1 part cement, and 3 parts of aggregate is beaten into thegap between the top of the lintel and the wall above. It is very importantthat the work is carried out well and that the mortar is firmly packed intothe space. To do this, hammers and short pieces of wood are used to drivethe mortar home, two people must work together, one each side of the wall.

If the concrete lintel is to be cast in place (in situ), there is no need to castpadstones on the wall as these can be a part of the lintel but slots must stillbe cut to support it.

Padstonescast in situ

Lintel

Firmly beat1part cementand 3 parts ofaggregate intothe gap

6.15

PRE-CAST CONCRETE LINTELS

Lintels must never be less than 75mm in thickness. As a rough guide for largeropenings the lintel should not be less than one twelfth (1/12) of the span.

Although concrete is very tough it is surprisingly weak in tension. When usedas a lintel, concrete will easily bend and may snap. To prevent this 18mm (¾inch) steel bars must be cast into the bottom. The bars bend up at the end toprovide more support and are about 25mm up from the bottom bed. To makesure the lintel is fixed the correct way up (with the bars to the bottom) theword TOP is clearly scratched into the wet concrete.

To make the lintel, it is first necessary to make a mould into which to pourthe concrete. This must be robust and made from 18mm (¾ inch) planking.It is a good idea to make it so that it bolts together. This will make it mucheasier to remove the lintel once it is set. Before pouring the concrete intothe mould, the inside is lubricated with soap to help in removing it.

A suitable concrete mix for lintels is:

1 part Portland cement2 parts sand4 parts broken brick or coral aggregate up to 18mm (¾inch) gauge.

Note: These specifications are guidelines only. ALWAYS consult astructural engineer.

Casting lintels in-situ

Although this is often done in theStone Town it is not recommendedto cast lintels in-situ. Lintels mustbe very strong and to make concretestrong it must be well compacted andcured. Curing takes 28 days, if thesupports are removed before thistime the lintel will not be at full

strength and will easily bend under theweight of the wall.

Badly cast lintels have causedbuildings in the Stone Town tocollapse. It is MUCH safer to use aPRE CAST lintel, which can be fullycured before it is fixed and theopening used almost as soon as thework is done.

Lubricate mould with soap to helpwhen removing lintel

Use bolts to makeremoving the lintel easier

Steel bars must becast 25mm fromthe bottom bed

Mould made from35mm planking

6.16

REPLACING BORITI LINTELS Boriti lintels are weak and prone to rot and bend. As this occurs, the ma-sonry in the wall above subsides and cracks. This type of action can give riseto knock-on effects in neighbouring parts of the building. It is important thatrotted boriti lintels are replaced as soon as possible.In the smaller houses of the Stone Town, boriti lintels over doors and win-dows are often left exposed. These features are very important and mustbe preserved. They must NOT be replaced in concrete because they add agreat deal of character to rooms and are part of the historic structuralsystem. Like must replace like and new boriti MUST be used.

Cracks where lintelno longer provides support

Boriti lintels areprone to rot

6.17

RELIEVING ARCHES To protect the replacement boriti from bending due to over-stressing, it ispossible to construct very simple arches over them. This is called a reliev-ing arch and it will carry much of the weight of the wall above the opening.Relieving arches can also be inserted to protect door and window caseswhere lintels were not employed in the first place.

Work should begin with the introduction of needles and dead shores tosupport the building during construction, as described in Section 3 of theGuidelines (Shoring and Temporary Support). However, in many cases labourand expense can be reduced by utilising the boriti in the soffit of the slababove as support for the masonry. This will only work if boritis enter thewall at 90° degrees to the opening and they are in good condition. If theyare parallel to the wall or badly rotted they will not provide support of anykind and needles should be used. When using boriti for support in this wayit is STILL necessary to install dead shores beneath them. The boriti sup-port the wall above but the boriti must be supported by shoring.

In the majority of cases it will be necessary to remove and replace notonly rotted lintels but also the masonry above, as it will be shattered andmuch weakened.

Boriti in floor above supportthe masonry

Check conditionof Boriti

It is still necessary tointroduce dead shores tosupport the boriti

Dead shore

6.18

PLACING BORITI AND BUILDING THE ARCH

With the wall supported, and defective masonry and rotted boriti removed,reconstruction and installation of the arch can begin.

1. New boriti should be selected and fitted in the same location as theoriginals. They should match as closely as possible in terms of size andgeneral shape. New boritis should be firmly bedded on a 1:3 lime mortarand the spaces between bridged with small pieces of coral and mortar in thetraditional way. Allow two or three days for the mortar to harden andconstruct a simple centring immediately on top of the boriti.

2. The centring is the formwork around which the arch is constructed. Itis required because the arch will not stand until it is in compression and atleast one ring or layer of stones or bricks is completed.

3. The cheapest and simplest way to form the centre is to use pieces ofvery light white coral to form a segmental shape. This should resemble thetop of a very large circle. For an opening 2m wide the hump should beabout 400mm tall in the middle. The centring is constructed along the fulllength of the lintels. It is important that the stones or bricks in the arch arenot seated on the boritis. The arch, when complete must rest at either endon masonry. If it sits on the timber it will fail if the timber rots.

Centring

New boritisLight white coral piecesform the segment shape

6.19

4. Once the centre is in place the arch is built around it. Although it ispossible to build the arch in coral it is easier and will involve less labour touse fired brick. The bricks or coral, should be arranged around the centre byadding a course to each side alternately. They are laid in a bed of 1:3 limemortar and the last bricks to go in should be in the middle and the jointvertical.

If bricks are used they should be arranged as headers set on their sides, asthis will minimise joint thickness. A minimum of four rings is required toprotect a two-metre opening.

When complete, the arch must be left for as long as is practical beforecontinuing construction above it. The centring remains in place and forms anin-fill between the timber lintel and arch. When the arch is complete it willcarry all of the weight of the wall above the opening.

Bricks or coral are arrangedaround the centre adding acourse to each side alternately

Bricks are laid ina bed of 1:3 limemortar. The lastbricks should bein the middle ofthe vertical joint

Bricks should bearranged asheaders set ontheir sides

Brick arch mustnot bear onboriti ends

7.1

LIME WASH7.


7.2LIMEWASH

INTRODUCTION

Limewash is the material traditionally applied to walls as a decorative andprotective coating. Originally all the buildings in the Stone Town werelimewashed regularly as part of their maintenance programme. This sectionprovides guidelines for preparation and application of limewash.

MixingPigmentsPreventing mould growth

Preparation of wall surfacesPainting


PREPARATION

APPLICATION

7.3LIMEWASH

PREPARATION

At its simplest, limewash it is very finely sieved lime putty in an excess ofwater. The basic wash, however, is not durable and will dust off easily. Toprevent this, it is possible to add a variety of materials or binders. Unfortu-nately, most of them are proteins and very quickly encourage mould growth inthe wash, especially on exterior walls. In colder climates, it is possible to addanimal and milk fats to the mix, both of which work very well and also help inwaterproofing. In tropical climates like Zanzibar, it is not easy to recommendan additive that will prevent dusting and not encourage mould growth.

BindersA very effective binder for use indoors is water in which seaweed has beenboiled. The liquid is allowed to cool and form a sticky gelatinous mass.Before use, it is thinned with boiling water and put through a fine sieve.Approximately half a litre of liquid is added to 20 litres of wash.

For use outside, common salt is perhaps the best choice although this mustnot be used if the render is of significant historic value. Approximately half akilo of salt is added to four gallons of wash.

MIXING

Eucheuma Denticulatumcommonly grows along the coastof Zanzibar

7.4

Lime must matureLike other aspects of lime technology, limewash will be at its most effectiveif it is carefully prepared. The lime should be well matured putty made fromgood quality quicklime.

Before use, the putty is sieved through a very fine muslin. The finer theparticulate size the better the protection it will give. This is because thebinding power within limewash is weak and large particles are simply tooheavy to be held in place. Very often coarse unsieved putty is used. Thismakes a rough surface which will quickly attract dust and brush off easily.

Matured limeputty

Sieve with muslinused as mesh

Press putty throughthe sieve

7.5

Limewash can carry a small amount of pigmentation, but the more that isadded the weaker and more prone to dusting the wash will become. Vividcolours cannot be achieved with a limewash and only delicate shades arepossible. The best pigments are called earth pigments and they are metaloxides, prepared from natural earths by firing. Copper sulphate can also beadded to make a delicate blue.

Many artificial pigments are damaged by lime and the combination of lime,sunlight, and oxygen can cause them to loose their colour. These pigmentsare not lime fast.

Yellow ochre, golden ochre, burnt ochre, raw sienna, raw umber, and burntumber are all lime fast earth pigments.

To put the colour into the wash it is first necessary to mix the pigment intoa thin paste with a little water. The paste is then stirred into the wash.

It is very difficult to prevent the development of unsightly black algal slimes.The best solution is to scrub or sand-paper off the slime and re-applylimewash. Commercially manufactured mould inhibitors are available (e.g.,Robbialac/Berger Fungicide Wash, available in Dar es Salaam), and theseshould be liberally applied after the old mould has been removed and justbefore the application of the new coat of limewash. Mould inhibitors willnot last for more than about a year and must be re-applied.

The cheapest and most convenient method is to rinse walls with Carbolicacid. The effect of this will not last more than one year and re-applicationshould be made after the monsoon rains.

PIGMENTS

PREVENTINGMOULD GROWTH

7.6LIMEWASH

APPLICATION

It is important that wall surfaces are well prepared before application.

1. Remove loose flaky material with a stiff brush and dampen the wall.

2. Dampening is best achieved by puncturing a hole through the top of amineral water bottle. This can then be used to thoroughly wet the wall anddirect water accurately into cracks and crevices. Before application thesurface is saturated but not ‘running wet’.

3. Painting

• For exterior work, wash is applied with a soft wide brush. Work quicklyand apply very thin coats. Each should be no thicker than milk.

If plain wash is being applied the first coat should be invisible until it is dry.As subsequent coats are applied the previous layers may also becometransparent. This tendency will lessen as more coats are added. Each coat isallowed to dry before the next is applied.

Do not apply limewash to exterior walls whilst it is raining.

• For interior work application should be carried out with a soft widebrush, but a much better quality finish will result if the wash is sprayed on. Aten litre hand pumped spray is adequate for this work. Several very thincoats are applied in the same way as described above.

PREPARATION OF WALLSURFACES

Remove flakymaterialwith a stiff brush

Water bottle todampen the wall

Application ofwashwith a widesoft brush

8.1

TIMBER CONSERVATION8.


8.2TIMBER CONSERVATION

INTRODUCTION

This section provides guidelines on recommended procedures of repair andmaintenance of both structural and non-structural timber elements.

Diagnosis

Key PointsGlue

Plug RepairRepair to Mortice and TenonRepair Around a HingeReplacement of Broken Louver Without Opening the FrameRepair of Rat Damage

Scarf JointsHalved ScarfRepair of Carpentry JointsBeam End Repair – Steel Flitch SystemBase of Timber Post


CAUSES OF DECAY

IMPORTANT PRINCIPLESOF TIMBER REPAIR

REPAIRS TO JOINERY

REPAIRS TO STRUCTURALMEMBERS

8.3

Timber elements of buildingsin the Stone Town

All buildings in the Stone Town include elements made from timber. Thesemight be doors, windows, balconies, teahouses, verge boards, roofs, canopies,etc. The timber elements perform important jobs: doors and windowsprovide security and privacy, roofs keep out the rain, and balconies increaseventilation. In order to continue functioning correctly, it is important thatthey are maintained and repaired properly.

But, apart from the practical tasks they perform, the timber parts of buildingsare an important aspect of the historical character of the Stone Town. Manyof the timber elements such as doors and balconies are very old, and areoften elaborately carved. These pieces are valuable because they weremade by skilled craftsmen and demonstrate the richness of Zanzibar’shistory. When they are allowed to disintegrate through neglect, or aredestroyed or removed by bad repair work, the Stone Town is diminished.

Balconies

Windows

Canopies

Tea Houses

Verge Boards

Doors

Stairs


CAUSES OF DECAY

Most problems with timber in the Stone Town can be traced back to thepresence of water. Timber elements were originally made from hardwoodsuch as teak, mninga or mvule. As the name implies, hardwoods are fardenser and heavier than softwoods such as cypress, and are therefore muchmore durable and resistant to the harsh environment of the tropics.

But even hardwoods become vulnerable to decay caused by insect attack ormould growth when saturated for long periods. Occasional saturation willnot normally cause problems, so long as the timber dries in between, butwhen moisture remains in the timber for long periods, it breaks down thenatural resistance of the hardwoods, quickly leading to decay.

Well-detailed and maintained timber should not become saturated. Wateringress is often caused by the failure of other building parts immediatelyadjacent to the timber or even some distance from it. For instance, acracked plaster window sill can allow water to collect around the bottom ofa window frame causing it to rot; or, a damaged roof can allow water tocollect around the buried ends of timber beams leading to decay.

When considering a repair, it is important that the cause of the damage isdiagnosed and dealt with, as well as the actual damage to the timber part. Ifthe causes are ignored, the repaired piece will quickly become like theoriginal damaged section. In most cases, it should be enough to repair theoriginal detail (e.g., re-plaster the sill or repair the roof), but in some cases,the original detail may have been at fault (e.g., the sill may have slopedinwards instead of outwards allowing water to collect under the sill). Inthese cases, the fault must be corrected or the timber given extra protec-tion to help it cope with the effects.

DIAGNOSES

8.5

Common problem areas foundin timber elements

Decay caused by waterFirst check joint between roof andwall. If this is open, it may be a signof serious structural subsidence.

Signs of rot in beam setinto wallFirst check for serious structuralsubsidence, using a spirit level andplumb bob. Check extent of damageby tapping with a metal object ordrill small holes with a hand-helddrill. Decaying timber sounds hollowand will be soft.If deck or balcony are damaged,consider how to prevent water fromcollecting here. Repair with boardsand plugs.

Water in the ceilingCheck the roof for leaks and repairthe roof.

Open jointsCheck for structural subsidence.First check if the balcony is leaningwith a spirit level and plumb bob. Ifthere is a problem, support thebalcony with temporary support,( see Section 3 of Design Guidelines).

Decorative Verge BoardCheck for rot and possible causefrom roof damage.

8.6

Important: Check all structuraltimber for signs of decay, e.g., beams,posts, joints, etc. Where possible,repair timber, or if decayed, replacewith identical piece.

Broken planksUse a plug to repair.

Water collectingaround timberFirst consider howto prevent waterdamage and decaycaused when timberis set into the wall.

Open jointsCheck that the structure is notsubsiding or falling apart.

Posts set in the groundCheck for decay. Ensure the base isclear of debris and check timber inthe base, frequently very vulnerable,and replace bottom part if required.

Decaying window frameCheck plaster. Cracks may allowwater into the frame.

Open jointCheck for signs of movement in thewall above. Subsidence can createstructural load on the window frame.

Cracks aroundhingesUse hook and eyeto fix shutters backin the wind.

Damaged plasterAllows water intowindow frame.

Decayed sill topCheck joint between metal rails andsill. Rust and decay may mean wateris collecting in the joint.

Decayed sill bottomCheck plaster adjacent to sillfor damage.

Holes in frameRats gnaw throughthe timber frame.

Broken louverShear wear andtear cause damage.


PRINCIPLES OF TIMBERREPAIRIn historic buildings, it is always better to retain as much of the originalhistoric timber as possible. Where the original is covered in fine carvings,the reason for this is obvious. But even if the timber piece is quite plain, itwill have a character that comes from age, which is impossible to reproduce.It is this character that makes the Stone Town unique.

Always try to repair a timber element in a building rather thanreplace it.

Often decay or structural damage affects only a part of a timber element.For instance, timber beams may only be rotten where the timber is buried inthe wall; the rest of the beam may be perfectly sound. In such cases, it issometimes enough to cut away and repair the section of timber that hasfailed or is rotten, whilst retaining most of the original piece.

When carrying out repairs to timber elements, always considerthese key points:

1. Assess the condition: Fitness for the jobAs timber ages, it often becomes rough in texture as the grain is exposed,and wear and tear leaves marks and scars across its surface. But this textureof age is not in itself a reason to replace old timber. In fact, the texture or‘patina’ of age is what gives old timber its character.

When considering repairs to timber, it is important to try and keep as much ofthe original timber as possible. The judgement about whether to retain orreplace a timber section should not be made on the basis of appearance alone,but rather on the fitness of the timber piece or element to do the job it wasdesigned for. In other words, a rough grained louver on a window does notneed to be replaced if it is strong enough to hinge properly, will continue tostand-up to daily use and keeps the elements out, whereas a badly decayedbeam that can no longer support any weight, must be replaced in its entirety.

KEY POINTS

Louvers are weathered but do notneed to be replaced. Repair whererequired, treat and finish.

Bottom sill of window frame isdecayed and no longer protects theinterior from outside elements.Replace with new timber sill.

8.8

2. Like with LikeAs a living tree, wood is composed of millions of small cells filled withmoisture. When a tree is made into timber, it is dried and most of themoisture in the cells evaporates. The timber shrinks, but the cellular struc-ture remains, and because of this, timber will continue to respond to itsenvironment, expanding or shrinking according to the dampness of the air.

It is important to bear this in mind when carrying out repairs to old timber.For the repair to be effective, the damaged part must be cut out and a newpiece of timber fixed in its place with glue or a joint. The new section isfixed tightly against the old, but both the new timber and the old will con-tinue to move according to the environment. If the new timber is verydifferent from the old in density or age, it will move at a different rate. Thejoint between the two is put under pressure and may eventually fail.

When carrying out repairs, always use timber identical or similarto the original!

Moisture content: Typically, newhardwoods will have a moisturecontent of between 15% and 18%,whereas old hardwood may have amoisture content as low as 7%. Forthis reason, whenever possible,patching repairs to old joinery itemsshould be made using old timber ofapproximately the same age, takenfrom another old timber piece thathas been discarded. Where, forinstance, an entire member of a frameis to be replaced, new hardwood canbe used, but the joint between old andnew must allow for movement.

Similar timbers: It is best todetermine the species of the timberto be repaired and to use the samespecies for the replacement piece.Never use softwoods such ascypress to repair hardwoods. Therepair will always fail.

Appearance: If the timber elementis to be finished with a clear finishsuch as teak or linseed oil, an effortshould be made to match the ap-pearance of the new timber piece(e.g. colour, grain pattern, etc.) withthe original.

8.9

3. GrainWhen carrying out repairs by plugging, or repairs that in any other wayinvolve letting-in and mating pieces of new timber to original joinery items, itis essential that the direction of the grain of the repair is the same as thedirection of the grain of the surrounding original timber. When making newmembers for joinery items, the grain direction of the new member must bethe same as the original member.

4. StrengthThe strength or rigidity of a joint must not be altered in any way as a resultof a repair. Where joints have originally been left open to allow for move-ment, or fixed without glue or mechanical fastening, these must not be‘strengthened’ during the course of the repair by the addition of glue, me-chanical fixings such as screws, or by any other method. Generally, careshould be taken to make joints that are not too rigid or strong, as movementis inevitable, and if a joint is inflexible, the stresses will be transferred to thetimber, which, if old and brittle, may crack.

5. Repair in-situIt is generally better to carry out repairs to timber in situ, or in other words,whilst the timber element remains in its original position. This is becauseremoval usually results in more unnecessary damage both to the timber andto the buildings around, and if an element is disassembled for repair,reassembly once the repair is complete is very difficult and usually shows.

When carrying out structural repairs, the same principle applies, althoughgreat care must be taken to ensure that the structure is properly supportedand prevented from collapse before repairs begin (see Guidelines, Section 3,Shoring and Temporary Support).

A tusk tenon joint allows the frameto be slowly tightened in response tomovement. Do not repair with glue.

8.10

GLUES

During the course of the restoration project on the Old Dispensary, severaldifferent glues or adhesives were tested. Whilst the results of this researchis presented below, it should be noted that the manufacturer’s instructionson use should be followed at all times.

The choice of adhesives will be determined by the expected structuralperformance of the glue joint, the location of the piece to be glued(e.g., whether or not it is exposed to rain and moisture), and cost.

Urea-formaldehyde glues are both strong and highly resistant to moisture,but they are also expensive, and so it is recommended that they be onlyused where a joint is load bearing or is likely to be in contact with moisture,and lower specification glue used elsewhere.• It should be noted that Urea-formaldehyde glues are usually two-pack,which means that the two ingredients must be mixed up on site in order tomake the adhesive. Once mixed, two-pack adhesives must be used immedi-ately, and cannot be stored in its mixed state.

Note:We do not recommend the use of organic wood glues (e.g., casein gluewhich is milky-white in colour), as these tend to quickly deteriorate whenexposed to moisture. A wood glue called ‘Woodfix’ is commonly used inZanzibar; however, our tests showed that this glue failed when saturated. Inparticular, NEVER use casein glues such as Woodfix for structural repairs.

1. ADHESIVES FOR JOINERY WORK

External(Joinery exposed to moisture)

Internal

External & Internal

Aerolite 306, manufactured byThe Kenya Swiss Chemical Company,E. Africa (Nairobi).

Ponal-3, manufactured byHeinkel, E. Africa (Nairobi).

All adhesives used for load-bearingstructural timber work, to beUrea-formaldehyde type, to BS 1204

PART 1;Aerolite 306, manufactured byThe Kenya Swiss Chemical Company,E. Africa (Nairobi)

Resorcincol, available in Nairobi

Aerodux 185 by Ciba-Geigy (Novartis)

2. ADHESIVES FOR STRUCTURAL TIMBER


JOINERY REPAIRS

PLUG REPAIR

Note: the grain of the timber of theplug must run in the same directionas the surrounding timber, and themoisture content and species mustbe the same as the piece beingrepaired.

damaged face of timber

alternative plug shape

1. Cut rebate with a chisel. The plugshould be proud of the surroundingtimber.

2. Glue and plain to finish.

8.12

REPAIR OFMORTICE & TENON

Detail 1

3.

1.

2.4.

Patch incorporating tenon

Face patch

Slip tenon

Half scarf incorporating slip tenon

8.13

REPAIR AROUND A HINGE

5. Use saw cutsin dowel to assistpositioning ofdowel

Damage aroundhinge

Dovetailed plugrepair

Note:ensure graindirection is thesame

Detail 2 Detail 3

REPLACEMENT OF BROKENLOUVERS WITHOUTOPENING THE FRAME

1. Cut off oldpivot to removelouver

New shutter

2. Drill hole inshutter end forpivot dowel

Saw cuts

Note:glue on louverside of dowel

4. Push theloose pivot inthe deep holeuntil it is flushthen positionlouver

3. Deepen pivothole

dowel

8.14

REPAIR OF RAT DAMAGE

Detail 4

Principle:maximise surface area forgluing

Problem:rat damage at baseof doors

1. Cut insidedamage to depthof 10mm

2. Originaltimber under-neath

3. Original timbermay be tenon

4. 5.


STRUCTURAL REPAIRS

SCARF JOINTS

Detail 1

1,4.

1.

1,4.

2,5.

3.

Spayed scarfeg., for memberssubject to bendingstresses such asbeams

Halved scarfeg., in wall plate

8.16

HALVED SCARF

Detail 2 Detail 3

REPAIRS OF CARPENTRYJOINTS

halved scarfincorporatingslip tenon

Use in compressionmembers such asposts.

Repair of carpentry joints

patch incorporatingtenon

face patch

slip tenon

8.17

Detail 4

BEAM END REPAIRWITH STEELFLITCH SYSTEM

Bearing plate offlitch rests in wall

Timber insertsconcealing flitch

Steel Flitch detail

Elevation

Plan

Decayed end ofbeam cut away

Top plate

Original beam

Bottom plate

Bearing plate

Steel flitch

Timber insertsconceal bottomand edge of flitchon soffit of beam New timber

‘cheeks’ boltedthrough flitch

Bolts recessed andconceled by pellets

8.18

BASE OF TIMBER POST

Detail 5

Concrete pierfootingwidth and depth dependson pier load and bearingcapacity of soil.

Wood post

Steel anchor

Concrete footingfor small loads

Wood post repairedwith halved scarf joint

Steel anchor

Minimum 150 mmfrom base of timberpost to ground level

Concrete pier

Steel anchor.Galvanise toprotect fromcorrosion.

9.1

CASE STUDY:HOUSE NO. 1287, KIPONDA9.


9.2CASE STUDY NO. 1287

INTRODUCTION

This section describes the major works undertaken as part of the conserva-tion of a badly decayed house in the Kiponda district of Zanzibar Stone Town(House 1287, or the so-called ‘Indian Ladies House’). The works werecarried out as part of a training exercise for the Conservation BuildingBrigade and others involved in construction in the Stone Town, betweenJanuary and April 1999. On top of the structural and masonry repairsdescribed in this chapter, other works were undertaken at the house; muchof the joinery was restored or replaced, a new toilet was installed and thebuilding was completely re-wired. Detailed information about constructioncosts is available upon request.

DescriptionHistoryMaterialsCondition Assessment

The Repair ProcessStabilisationGeneral Principles of the Structural RepairsFoundationsWallsPlacing Boriti and Building the ArchesThe Provision of RestraintFloor Slabs and Roof

BACKGROUND

REPAIRS

9.3

House No. 1287 is a very simple two-story building. Accommodation onthe ground floor consists of three interconnected rectangular rooms. Princi-pal access is via a simple small door set in the west elevation. This providesdirect access to a narrow entrance room, which at its far end contains acrude dog-leg staircase leading to the first floor. The entrance room pro-vides access in the west to a small kitchen and wash room. This is the onlyroom in the house with a water supply. A manhole in the east gives accessto a toilet-pit beneath. A door in the north of the entrance room givesaccess to a bed/sitting room, a corner of which is crudely partitioned toform a changing and washing area. To the north of this room is a pair ofhandsome double doors providing access to a small rear lane. Currently thedoors are sealed closed. The floor plan established on the ground floor isrepeated in the first floor. The head of the dog-leg staircase gives access toa narrow landing. To the south is a toilet room and store. Sewage disposalis via a vertical cast iron pipe feeding directly into the toilet pit under theroom below. North of the landing is a bedroom. Access to the flat roof isvia a simple open stair rising from the west of the landing.

CASE STUDY NO. 1287

BACKGROUND

DESCRIPTION

North

9.4

HISTORY

The building dates from the end of the 19th. Century, C1886/90 and is partof a small terrace, originally constructed as shops or workshops with livingaccommodation attached. The shop sold food stuffs and is known to havedealt in cooking oil. The area of Kiponda in which the building is located, isone of the older districts of the Stone Town and became noted for a rangeof small-scale commercial activities. House1287 is a fine example of autilitarian form of architecture motivated by low profit margin trade.

The current occupants and their relatives have lived in the house for 80 years,during which time they have witnessed minor but significant changes. Thehouse retains many original features including much of the original fenestra-tion, and a dry earth closet, which is still in regular use. The most significantinterventions the building has witnessed comprise the removal of a smallinterior wall that once divided the commercial premises from living space, theintroduction of electricity and plumbing and the complete re-rendering of theinterior in cement mortar. The floor to the toilet/store room on the first floorwas also removed and replaced in cast concrete. It is likely that the ad hocpitched roof and parapet walls were also added at this time. MATERIALS

Roof and First Floor SlabsOriginally a slab of lime concrete approx. 600mm thick acted as a roof forthe building, supported on undressed mangrove poles set at 400mm centres(boriti). The mangrove acts both as shuttering to allow construction of theslab, and as reinforcement to the soffit of the completed slab, resisting thedevelopment of tensile forces. The spaces between boriti are bridged bysmall sections of coral. In an attempt to increase the water resistance andgeneral durability of the slab, this construction was modified by the additionof cement based render. Finally, a simple pitched roof of corrugated iron andcrude mangrove joinery was constructed some distance above the slab, analteration seen throughout the Stone Town.

9.5

WallsIn common with the vast majority of buildings in the Stone Town all the wallsare constructed from rounded nodules of coral rag laid in the form of randomrubble, bound by a mortar composed of yellow laterite soil, and locally burntlime. The proportion of lime to aggregate is typically 1:3. If bound by goodquality mortar, such walls can have a high compressive strength.

Regardless of the quality of mortar, walls composed of rounded coral rubbleare weak in tension, a characteristic resulting from the inability of roundedbuilding units to overlap and create a bond. Such walls will easily tearvertically if subjected to modest tensile forces. The absence of bonding alsoreduces the elasticity of the wall, reducing its ability to bend and deform inresponse to high compressive loading, point loading and minimal restraint.Walls are therefore brittle and prone to rupture horizontally or developvertical cracks within their thickness together with bulges.

A further problem encountered in walls composed of rounded rag is seen inthe junctions of elevations and cross walls, where the nature of the materialprevents effective interpenetration and tying in. As a result of these factors,walls and buildings are very sensitive.

The exterior and interior of the masonry structure were originally renderedin a two-coat lime plaster. The first or render coat typically consists of a 1:3mix of yellow laterite soil and lime. The top or setting coat in almost allcases consists of a 1:1 mix of lime and finely ground soft white coral stonedust. Currently, the entire interior of the building is re-rendered in a cementand sand composition.

Rubble walls cannot withstandtensile forces and crack

Rounded coral rubble Render

9.6

CONDITION ASSESSMENT

General ConditionThe exterior lime render is heavily patched with unsightly cement mortar. Itis extensively employed in all elevations and gives the building a shabby andderelict appearance. Many open cracks can be seen in all the elevations. Saltcrystallisation and abrasion scar the lower 3m to 4m of exterior elevations.Inside the building, numerous open cracks are visible. The southerly sectionsof the ground floor, first floor, and roof slab are all badly cracked and inclinedtowards the south east corner. The south wall leans out significantly.

Roof SlabThe roof is divided by a large crack running diagonally from the mid point ofthe west elevation to the east wall. Boritis in the soffit are rotted above thetoilet room and landing. Above the bedroom, 50% of the boriti are rotten.

First Floor SlabIn the first floor slab, significant cracks can be seen in all rooms. Above thekitchen/store the cast concrete slab has cracked diagonally from the door tothe south east corner and exhibits a significant deflection to the south east.In the landing, a crack extends diagonally in continuation of that in the toilet.The crack continues diagonally across the bedroom floor. In both thebedroom and landing, a slight deflection to the south east can be observed.

Synopsis of problems

Failure of openings

East elevationsinking

Failure if part ofsouth wall

Roof

First floor

Ground floor

Large verticalcracks

9.7

Ground Floor SlabsThe floor to the kitchen and store area is badly damaged and scarred bymany attempts at repair. Many large cracks divide it diagonally from west tosouth east. In the south east corner, the concrete sounds hollow and isclearly unsupported. A deflection is noted to the south east. In the entranceroom and bed/sitting room, a single large diagonal crack is evident in replica-tion of the pattern in the rooms above.

West ElevationFrom the middle of the elevation to its junction with the south wall, threedistinct crack systems are visible, running vertically down the full height of thebuilding. They range in width from 3mm to 35mm. In all cases they have beenfilled with cement render and have subsequently reopened, indicating an activeand ongoing settlement. The wider cracks in the systems extend through thefull thickness of the wall and a number of smaller cracks appear to be activelydeveloping in their vicinity. This is particularly in evidence in the brittle cementrender of the interior. Salt damage is much in evidence throughout the lowerpart of the entire elevation. The inner cross wall in the south of the entranceroom, is badly cracked above the door into the kitchen. The opening appearsto have been constructed without an effective lintel. The full weight of thework above is carried on the door case, which has developed a significantdeflection. In the south wall of the landing (immediately above), a similarcondition exists above the door into the toilet room.

South ElevationFrom ground level to the first floor the entire elevation exhibits a rotationof about 70mm. From first floor level to the roof, the elevation is 40mm

out of plumb. A scale plot of the distortion, in relationship to the thicknessof the wall, shows that the centre of gravity of the wall, remains within thecentral 1/3. It can therefore be assumed that the wall is currently stable butlacks effective restraint, and is subject to ongoing progressive rotation. Thewall is further weakened by a number of horizontal and shear cracks, effec-tively dividing it into three sheets of rubble masonry. The lower south eastcorner is particularly badly shattered.

North ElevationThe elevation is scarred with cement repairs and numerous crack patterns,probably the result of long-term settlement and knock on effects generatedby the developing instability in the south elevation.

Analysis of Damage and ConclusionsExamination of crack patterns, deflections in the floor slabs, and deformeddoor jambs and lintels, etc., suggest an active differential settlement in thesouth east corner of the building. The most likely cause of this is a collapseof the pit associated with the earth closet, which is located in the extremesouth east corner of the building. It is also probable that traffic vibration hasplayed a part in inducing the collapse and furthering ongoing damage.Although the collapse of the toilet pit is responsible for the bulk of thedamage observed, it must be borne in mind that the failure has disturbed thedelicate equilibrium existing between various elements of the structure andhas given rise to significant ‘knock-on’ effects that will exploit intrinsicstructural and material weaknesses elsewhere in the building. If the currentrate of rotation in the south elevation and growing instability is maintained,the south elevation will shortly be in danger of collapse.

9.8CASE STUDY NO. 1287

REPAIRS

By far the easiest solution to the structural problems of house 1287 wouldhave been to demolish the south elevation and a section of the west eleva-tion, back fill the toilet pit, dig new foundations, and rebuild.

However, the nature of the exercise was not simply to produce a restoredhouse, but to train local craftsmen and building professionals in the method-ology of sensitive structural repair. With this goal in mind, works to thehouse were divided into the following phases.

Works began with the installation of temporary support systems to preventfurther rotation of the south elevation and relieve weak walls, and the failingfoundation in the south. The dead shoring systems required to secure thefloor and roof slabs transmitted the load from floor to floor, directly to theground. For maximum safety and efficiency, ranks of shores were erectedone above the other. Shores were positioned with some care to avoiddenying access to walls and sections of the ground floor required for repairs.All windows and door openings were also firmly strutted and cross braced.

THE REPAIR PROCESS

STABILISATION

Raking Shore

Needle topenetrate wall

Wall hooks tosupport wallplates duringerection

Needle brace

Sole plate

Wall plate

Boriti

Strutting nailed throughpre-drilled holes

Folding wedges

Base plate or foot

Needle withintegral cleat

9.9

Local tradesmen fabricated the timber components and steel fittings that arenot stock items. Shores were erected as follows:

1. Needle holes were marked on the wall and cut. This work was carriedout as neatly as possible and with as little vibration as possible.

2. The two wall plates were positioned either side of the socket holes. Thewall hooks supporting these were driven into convenient mortar joints.

9.10

3. The position of the foot was established. To do this, a length of string wasprepared the same length as the long boriti rakers.

Holding one end of the string in the top needle hole, an assistant was askedto take the other and move away from the wall until the string became tightand in contact with the ground. A pick and shovel were then used to cut anaccurately shaped rectangle to accommodate the foot.

4. The lower needle, needle-brace, and raker were installed by inserting theneedle into its socket and held hard against the top of the cavity.

9.11

The needle brace was then positioned across the wall plates so as to fitfirmly above and behind the needle. The position of the needle brace wasthen marked with a scribed line along its lower edge, across the wall plates.

The needle was then lifted away and the brace securely bolted into position.A dry mix of 1:2 cement, sand was placed in the needle cavity, and a mediumhammer used to drive the needle into its socket and against the brace,compacting the mortar around it and consolidating its position against thetop of the cavity and the brace.

9.12

The raker was then lifted into position beneath the needle, with its lower endresting on the foot. Using a stout steel crowbar between the foot and thebottom of the raker, the raker was levered up until hard against the needle.It was secured in this position by driving home folding wedges.

Rakers must never be tightened into position by beating home the wedgeswith sledge-hammers, as the vibration created will damage the wall.

Note: Should it have proved difficult to lever rakers sufficiently tight againstthe needle, two hydraulic lorry jacks would be used in the following way:

9.13

Dead ShoresIt is important that all loads are transmitted through the building and to theground as efficiently as possible. To do this, shores must make close contactwith the floor surfaces and the boritis of the slabs overhead. This requiresthe use of sole and heading boards.

To Fit the Heading Board

• The boards must consist of 20mm x 200mm soft wood planks nailed to theunderside of the boritis.

Initially, attachment should only be made to boritis of common height.Folding wedges must be used to pack the spaces between poles of smallerradius. To secure the wedges, holes should be drilled through boards andwedges, into boritis, and suitable nails introduced.

´ 40mm square battens were then nailed down the centre of each headingboard to locate positively the heads of boriti used as shores.

• To locate these accurately a plumb line was used. The boards must be atleast 200mm x 20mm.

• Dead shores between sole and heading boards consisted of boriti with aminimum diameter of 150mm set at 1.5m centres. The head of each shorewas accurately notched, so as to locate around the batten nailed to theheading boards. The shores were tightened by inserting folding wedgesbetween notch and batten.

Drill a 35mm hole through the base of the raker and introduce a shortsection of 30mm steel rod. Holes must be sufficiently far above the foot toallow the introduction of one lorry jack under each side of the bar. When inposition gently jack the raker into position.

5. The above procedure was repeated to position the lower raker, and whenin place the side bracing was added.

9.14

General Principles of the Structural Repairs

The objective of repair work is to remove both the CAUSE of damage andto stabilise its RESULTS, preventing further damage developing.

As the CAUSE of the problem was the collapse of the toilet pit, an impor-tant part of the repair was the provision of new foundations to support thesouth and east of the building (UNDERPINNING). Following this, thedamage in the remainder of the building was stabilised.

This meant that walls were to remain bent but would not move any further.Stabilisation would ‘freeze’ them where they were. To achieve this the southwall was strengthened (increased in load bearing capacity) and restrainedwith the provision of wall ties.

Repairs began with the demolition of the southern section of the roof slaband parapet to the head of the south wall. This action was taken to reducefurther the loading on the weak south wall. Although much of the load wasnow carried on the dead shore system, the slab was badly cracked and all ofthe boritis in that section required renewing. Removal of the slab wouldenable long new boritis to be installed, enhancing restraint of the south wall.

FOUNDATIONS

Works began with the demolition of the concrete floor to the kitchen.Once removed, the full extent of the failure in the toilet pit was visible.

The pit was found to be extensive and shared with the house next door. Itsfailure had undermined and left entirely unsupported the whole of the southeast section of the party wall between the two buildings. Provision of newfoundations under this side of the building would require the toilet-pit to befilled with concrete and hardcore.

A suitable mix for this kind of work is:

1 part Portland cement2 parts sand4 parts broken brick or coral aggregate up to 18mm (¾inch) gauge.

Before pouring the concrete, the contents of the toilet pit were removed.Most of the old sewage was found to have broken down into a spongy blackmass which proved to be very compressible.

In addition to filling the pit, the dead mass of concrete was used to support a300x500mm ground beam and needle, penetrating and supporting the east ofthe south exterior wall and the east of the entrance room wall. Concretewas shot into the pit by the bucket-full and roughly compacted with timberrams. In order to pre-stress the wall and avoid residual settlement, theconcrete was cast to within one foot of the underside of the existing foun-dation. It was then allowed to set for one day, and three lorry jacks placedon top of it, directly below the original foundation. The jacks were tightenedto stress the wall. Concrete pouring continued to a slightly higher level thanthe bottom of the original foundation in order that it could be compactedunder it. The jacks were lost.

In preparation of the works to strengthen the south wall, a strip foundationwas also constructed parallel to that of the interior south wall. This workextended the same depth below ground as the original construction and wasattached to it at numerous points by coral ties.

9.15

WALLS

The severely weakened south wall was strengthened by ‘grafting’ onto it asecond wall, constructed to the interior and to the full height of the kitchen.In the east of the elevation, the new wall entirely assimilated the existing work.To the more stable centre and west, a large blind arch was formed in the newwork to maximise the space available in the kitchen. The two walls werefirmly married by the insertion of wall ties. Below 1.5m above ground levelcoral ties were employed. Above that level, 17mm steel reinforcement barsprotected by two coats of Alkyd paint were used. In addition to the steel ties,coral needles were also used to help transfer and share the load of theexisting wall. In the extreme south of the elevation the two walls merge.

The construction used large flat coral in a mortar of the following composition:

Cement Lime Stone Dust ½ 2 8

All cracks and lacunae were grouted with a well-matured mix of:

Cement Lime Silica Sand ¼ 2 14

All shattered masonry above the entrances to the kitchen and toilet wasremoved and fired brick relieving arches inserted. Traditional boriti lintolswere then installed.

Existing damaged wall

New wall

17mm steel reinforcement

Ground beamFilled toilet pitsupports beam

Foundation and Wall Repairs

New stripfoundation tosupport secondwall

New & oldwalls merge

North

9.16

Placing Boriti and Building the Arch

Following removal of the defective masonry and rotted boriti, reconstructionof the openings and installation of the arches began. New boriti were selectedand fitted in the same location as the originals. They matched the originals asclosely as possible in size and general shape. New boriti are firmly bedded ona 1:3 lime mortar and the spaces between bridged with small pieces of coraland mortar in the traditional way. Following two or three days for the mortarto harden, a simple centring was constructed immediately on top of them.

The centring is the formwork around which the arch is built. It is requiredbecause the arch will not stand until it is in compression and at least one RINGor layer of stones or bricks is completed. The cheapest and simplest way tomake a centre is to arrange empty drinks cans so as to form a segmental shape.This should resemble the top of a circle. For an opening 2m wide, the hump ofcans should be about 350mm tall in the middle. The cans should be set acrossthe full width of the boritis and bedded in lime mortar or gypsum plaster. Oncethe centre is in place, it is possible to build the arch around it.

It is much easier and will involve less labour to do this in fired brick, althoughit can easily be made in coral if preferred. The bricks or coral should bearranged around the centre by adding a course to each side alternately. Theyshould be separated by a bed of 1:3 lime mortar and the last bricks to go inshould be in the middle and the joint vertical. If bricks are being used theyshould be arranged as stretchers, set on their sides. This will minimise jointthickness. A minimum of four rings is required. When all the bricks are inplace the arch must be left for as long as is practical for the mortar to harden.

The wall above is next rebuilt in ordinary 1:3 mortar, care being taken to packstones tightly around the boriti. To complete the job, all that remains isremove the drink cans and pack the underside of the arch with light coralstone followed by re-rendering.

Centring

Bricks make arch

Boriti poles

Drink cansform centring

9.17

THE PROVISION OF RESTRAINT

Repair of the foundations solved the problem of ongoing rotation in thesouth elevation, but the weakness of the wall and the extent of rotationsuggested the need for additional restraint. The need was met by installingfive steel wall ties and restraining bars. The ties were secured to variouspoints on the exterior of the south wall and anchored to points up tosixmetres away, in the north (stable) part of the house. Additionally, the newmasonry to the ground floor interior will apply an eccentric loading to theelevation, which will also serve in stabilisation.

Face plate

Fracture

Tie rod

Anchor plate

Floor slab

Boriti pole

South wall

Face plate Floor slabs

Anchor plateTie rod

9.18

FLOOR SLABS AND ROOF

The southerly section of the roof slab was rebuilt to the traditional designbut to a depth of only 300mm. Cracks in the first floor slabs were groutedwith the mix specified above. Brick relieving arches

protect openings

Steel ties restrain wall

New wall strengthensoriginal and providesadditional restraint

Synopsis of Repairs

Documents

CONSERVATION AND DESIGN GUIDELINES