Re Pointing Rubble Stonework

8/4/2019 Re Pointing Rubble Stonework

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RepointingRubbleStonework


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In traditionally built rubble stone walling it should be expected thatthe mortar that was used in its original construction will deteriorateover time and a need or repointing will eventually occur. I thewall was correctly built in the rst place this is only liable to be

required some time ater the rst 100 years. I repointed properly aurther century o lie can readily be expected rom the wall withoutmuch additional maintenance. Over the last ew decades theprocesses involved in doing this work efectively have not been ullyunderstood. As a result, much supercial and inappropriate workhas been carried out that will not last as long and will soon have to

be redone. Tis INFORM aims to guide building owners throughthe steps that are required or contractors to carry out repointingwork properly.


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The need for pinning stonesWhen traditional rubble stone masonry walls

were originally constructed it was common

practice to use a variety o small stones, called

pinnings, to make the larger stones secure

in the wall. Both the large and small stoneswere bedded, or set, in lime mortar (although,

occasionally, clay was used as a mortar).

Troughout the country a variety o materials

were used in constructing walls, and to make

them as structurally sound and weather-tight

as possible. Tis gave rubble walls distinctively

varied appearances across the country

depending upon what local practices and

materials were used.

Depending upon the region, the lime mortar

nish around the stones could be fush or

slightly recessed so that all o the stone aces

could be obviously seen. Elsewhere the mortar

can be ound to be progressively smeared over

the aces o the rough stones until the nal

nish almost emerges as a ull lime render.

In other areas additional lines o small stones

were pushed into the wet mortar or joint lines

were drawn onto the wall ace beore it set to

give the impression o squared stonework.

Irrespective o the nish, the underlying

rubble stones were generally originally built so

that the stones and smaller pinnings touched

each other. In that way the structural load o

the wall could be satisactorily carried to its

oundation whilst the mortar still had to set.

Tis building process is an important point

to bear in mind when carrying out repointing

work because, i it is ignored, the visual

appearance o the wall can be signicantly

altered.


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When repointing is requiredRepointing will be necessary when the

bedding or jointing mortar becomes washed

out rom the wall and the stones start to

become loose and risk alling out o place.

I let unattended to rainwater can readilypenetrate the heart o the wall and encourage

urther deterioration. I this is not repaired

the wall can ultimately collapse.

In recent times cement mortars have been

commonly used or repointing work but this

is not recommended as it can also damage

the stonework and interior nishes through

keeping water trapped inside the wall. Limebased mortars are much more appropriate and

eective or masonry repointing work as they

let the walls breathe.

Te successul use o lime mortars depends

to a great degree on good site practice.

Provision should be made at an early stage or

appropriate scaolding to be built so that the

work can progress in an appropriate manner.

Beore work starts it is always advisable to

take some detailed photographs o the area tobe repointed. Tis will give a visual reerence

o what the wall looked like beore any

changes are made to it, and the photographs

can be regularly used to check the quality o

the work against the original appearance as it

progresses.


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Site preparationLime mortar repointing work should not be

carried out during periods o heavy rainall or

in rosty conditions. Adequate protection will

need to be considered and this could require a

ully protected scaold. It is also necessary toensure that appropriate access, mortar mixing

areas, site protection, the correct equipment

and all health and saety requirements are in

place at all times.

Raking outo provide a sound base or the replacement

mortar it is necessary to remove any decayed

mortar rom the ace o the wall and romaround the stones. I, at some time in the

buildings past cement mortars have been used

to patch the wall it may be necessary to use

a hammer and chisel to careully break away

the cement and re-expose the underlying lime

mortar. Tis needs to be done with care to

avoid damaging the edges o the underlying

stones. Otherwise, raking out should be done

with a variety o thin metal tools which can

easily penetrate between the stones to remove

the loose or deteriorated old mortar.

Te aim o this approach is to reach theposition where sound mortar remains in

the depth o the wall. In the process it is

inevitable that some o the small pinning

stones will become displaced and all out

o the wall. Occasionally some o the larger

stones might also become displaced, especially

i the wall is in a poor state beore work starts.

Any displaced stones should be careully setaside or re-use in their original position when

the repointing work takes place.

o ensure all o the loose and decayed mortar

is removed the raked out areas should be

careully fushed out with water, taking care

to also fush the loose debris down the entire

wall ace rather than let it accumulate on

the aces o lower stonework. Te resultingdepth o the space into which new mortar

needs to be placed should be in the region o

two to three times the width o the original

joint. Anything less will not give the new

mortar sucient space to set and perorm

appropriately. In the raking out process,

power tools should not be used as they can

be dicult to control and can badly damage

or mark the remaining stonework. During

the raking out work it is also important to

ensure that all vegetation and any penetrating

roots are completely removed rom the wall

structure. Tis is necessary to avoid the risk outure damage to the repointed work should

the growth re-establish itsel.


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In some cases it may be necessary to

dismantle a particularly loose section o wall

so that it can be rebuilt during the repointing

work. Where this is required it is appropriateto paint a square grid onto the ace o the

stones beore dismantling work starts. Using

water-soluble emulsion paint, the loose

stonework should be marked with a square

grid and this area should be specically

photographed or uture reerence beore

work starts. Dismantling should occur so

that the stones are careully laid out beside

each other in the manner by which theywere taken apart rom the wall. Tat way,

using the painted grid lines and pre-work

photographs as a reerence, they can be

readily identied when the time comes to

rebuild them back into the original position

- careully removing the painted grid ater

rebuilding.

Preparation of mortarIn preparing the repointing mortar it is

important to batch the volume o the

lime and sand materials accurately so

that successive mixes can ollow the same

proportions. It is also necessary to understand

the behaviour o lime mortar and how it

sets to use it appropriately. In gaining this

understanding it is important to recognisethat regional variations in the mortar mix can

exist and that these should be copied so that

the appearance o the nished work matches

the original. o do this properly it may be

necessary to obtain some specialist support to

analyse what the original mix consisted o to

ensure that the replacement material will look

right and perorm equally well.

TampingTe process o repointing starts with the need

to push new mortar back into the heart, or

core, o the wall in a process called tamping.

Using appropriate tools the mortar should

be pressed well into the spaces created by

raking out so that it comes into direct contact

with the original mortar thereby eliminatingany voids. Te mortar mix used or tamping

should be the same as that used or the nal

repointing.

PointingDuring this process it is important to replace

any missing pinnings or loose stones back

into their original position and to ensure that

all the replaced stones make physical contact

once more. In doing so, this gives the wall

back its ull structural strength and original

appearance. Any missing pinning stones will

generally be revealed by the remaining widejoints, and these should also be replaced as

work proceeds. o ensure that the pinnings


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are correctly positioned in the wall the

mason will probably have to drive them into

the spaces between the large stones using

the wooden shat o the hammer. Tis is

preerable to using the metal hammer head as

there is a real risk that the metal could shatter

the small stones and render them useless.

As work progresses care also needs to beexercised to nish o the appearance o

the wall in such a way as to match the

original. Te surace nish and texture o

the lime mortar is also important to the nal

appearance. When the mortar has started to

set, dabbing the surace with a wet sponge,

or gentle stroking with a bristle brush, can

be eective in avoiding an overly smooth

nish. Reerence to the pre-work photographs

can assist in creating the right appearance,

the nal joint size and nish on the rough

stonework. Tis can be dicult to control

as sometimes only a ew extra millimetres o

mortar thickness can lead to the creation o a

wider surace nish as the mortar smears on

to the ace o the rough rubble stonework so

particular care is required at this stage o thework to get the nish right.

Lost pinning stonesTe appearance o the wall can be

dramatically altered i pinning stones are

missed out during the tamping stage o

the work. Tis can oten happen as recent

repointing practices have tended to ignorethe time consuming eort that is required

to replace the small stones. Consequently, the

volume o stone that was originally used in

the wall is much reduced, only to be replaced

by a similar volume o mortar.

Tis can give the alse impression that only

large stones were used to build the wall in the

rst place, and that they were surrounded bywide mortar joints. Tis approach not only

creates the wrong appearance but it is also

a alse economy. Because o the increased

amount o mortar that has to be used and will

be exposed to the weather, there is a greater

risk that it will decay aster and the expected

lie o the repointed work will be much

reduced.


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Principal author: Ingval Maxwell

Published by Technical Conservation, Research and Education Group, January 2007Historic Scotland, Longmore House, Salisbury Place, Edinburgh EH91SH

Tel: 0131 668 8638 Fax: 0131 668 8669

www.historic-scotland.gov.uk email: [email protected]

TeHouse1/07Producedfromsustainablematerial

Weather conditionsAs noted , tamping and repointing work

should not be carried out in extreme

weather conditions, and particular care

needs to be exercised i work is being

carried out when there is a risk o rost.In such cases, some orm o insulation

should to be provided to protect the

wall ace that has been worked on.

Usually this is provided by hessian

sheets. Equally, care needs to be exercised

during repointing works when heavy

rain is expected. Tis is to avoid the

resh mortar rom being washed out o

the wall ace and appropriate protectionmay need to be put in place to prevent

that rom happening. In extremely hot

weather intermittent gentle spaying with

clean water, or covering the work with

dampened hessian, will help prevent too

rapid drying.

PerformanceMany lime mortared original rubble walls

have stood in good condition or over

a hundred years and in numerous cases

or much longer, whilst only requiring

minimal maintenance. By recognising

how these walls have perormed in the

past, and understanding the original

techniques that were used in building

them, appropriate repointing workshould ensure their continued survival or

another century or more.

Useful contactsHistoric ScotlandLongmore House, Salisbury Place, Edinburgh,EH9 1SH0131 668 8600

www.historic-scotland.gov.uk

Historic Scotland TCRE GroupPublications Department0131 668 8638

Historic Scotland Conservation Bureau &Technical Enquiry Service0131 668 [email protected]

Historic Scotland Investment andProjects Team0131 668 [email protected]

Historic Scotland InspectorateListed buildings 0131 668 [email protected]

Further readingAN 1 Preparation and Use of Lime MortarsHistoric Scotland, Reprint 2005,ISBN 1 903570 42 5

Mortars in Historic Buildings Literature ReviewHistoric Scotland, 2003, ISBN 1 903570 82 4Te Repair of Historic Buildings in ScotlandHistoric Scotland, 1995, ISBN 0 9517989 2 8Te Conservation of Architectural Ancient Monuments

in Scotland Guidance on PrinciplesHistoric Scotland, 2001, ISBN 1 903570 32 8

All photos I Maxwell

Documents

Re Pointing Rubble Stonework