Raised timber deck structures on new homes - desired service life … · 2013-09-26 · Load bearing assumptions The guidance in this document provides for a uniformly distributed

Code of Practice:

Raised timber deck structures on new homes

- desired service life 60 years

This Code of Practice has been produced by the Timber Decking Association to meet the quality and performance

requirements expected by NHBC and other organisationscommitted to improving home building standards.

Code of Practice:TDA/RD 08/01

© Timber Decking Association November 2008

Code of Practice: TDA/RD 08/01

Raised timber deck structures on new homes - desired service life 60 years

1st edition November 2008

© Timber Decking Association

The information contained in this document hasbeen given in good faith. Every effort has beenmade to ensure the guidance given is accurate.The Timber Decking Association cannot acceptany liability for loss or damage arising from theinformation contained herein or from any designeror contractor not complying with this guidance.

© Timber Decking Association Ltd (TDA) All rightsreserved. No part of this document may bephotocopied, scanned, reported or transmitted inany form without the prior permission of the TDA.

The TDA is an independent technical and advisoryorganisation founded in 1998 to establish andpromote the materials, design and installationpractices required to create high performancedecks, boardwalks and associated landscapestructures in wood.

The TDA operates DeckMark™ -a quality assurance andperformance scheme formaterials and contractors.

A list of approved products andcontractors is available on request.

Page 1

© Photographs - Acknowledgments

Page 5: Five Degree PiersPage 9, Fig 8a: Hoppings Softwood Products; Fig 8b: Richard Burbidge; Fig 8c: Cheshire Mouldings

IntroductionTimber has a long history of use as a structuralmaterial. Unlike man-made materials it is asustainable construction material with widelyrecognised environmental credentials.Increasingly, designers are turning to timber asthe “green” building and engineering material ofchoice. The use of wood for external structureslike decks, boardwalks and bridges has grownsignificantly in recent years. Where suchstructures are intended to be permanent, materialselection, design, installation good practice andmaintenance are key factors in ensuring fitnessfor purpose and long service life.

House builders are increasingly choosing toinclude timber decks on new properties to addcustomer appeal. On sloping land, raised decksare a practical solution to providing externalleisure space where a traditional patio or gardenis not possible.

Timber decks can be designed to meet a varietyof service life requirements. For qualityinstallations, 15 years is considered to be theminimum standard by the Timber DeckingAssociation (TDA). Longer service lives, typically30 and 60 years are also readily achievable. ThisCode of Practice (CP) has been developed toassist homebuilders meet the quality andperformance requirements expected by NHBCand other organisations concerned with improvingstandards.

Leading designers, manufacturers and the WoodProtection Association have been consulted in therecommendations contained in this CP.References have also been drawn from theBuilding Research Establishment (BRE), TRADAand British and European Standards whererelevant.

Those responsible for commissioning raiseddomestic decks should only use designers andinstallers of assessed capability with theexperience and expertise required to carry out thework.

Definition of a raised deck For the purposes of this CP, a “raised deck” isdefined as a timber structure:

a) having any part of its deck platform more than60 cms from the ground;

b) that is attached to a new residential property,no higher than the first storey level;

c) that was included on the original design of theproperty and which is;

d) installed prior to the issue of a new homecompletion certificate.

Any design having a deck platform under 60 cmsin height is regarded by the TDA as garden or lowlevel “decking”. Because of the lateral and verticalforces that a raised deck may be exposed toduring its service life they are a lot more complexto build. The safety of those using the deck is theprincipal structural consideration.

Scope of this Code of Practice (CP)This CP applies to raised decks for which adesired service life of 60 years is required. Itincludes guidance on:

• Timber selection

• Timber treatments

• Column specification and foundations

• Beam size and fixing

• Ledger board and fixing

• Joist size and fixing

• Deck board specification and fixing

• Parapet design and fixing

• Stairs

• Aftercare and maintenance

• Relevant Standards and References

Note:a) Residential decks on which hot tubs or other

excessively heavy loads are to be placed arebeyond the scope of this CP.

b) Raised decks should not be used or occupieduntil final inspection and approval has beengiven by the new home insurance provider orbuilding control officer.

c) All dimensions quoted in this CP are targetsizes unless otherwise stated.

Code of Practice TDA/RD 08/01: Raised timber decks on new homes

© Timber Decking Association November 2008 Page 2

Load bearing assumptions The guidance in this document provides for auniformly distributed load of 3.0 kN/m2 andconcentrated load of 1.4 kN for the main deckplatform and its support. The TDA recommendsthat this is the minimum standard that should beused for a raised timber deck on a residentialproperty.

Parapets, whether a modular system or custom-built, must be capable of withstanding ahorizontal, uniformly distributed load of 0.74 kNper metre and a single point load to the infill of0.50 kN. Refer to Section 3.10: Parapet design &construction for details.

Metal fixingsAll metal fixings shall be made from corrosionresistant materials such as stainless steel (Grade316 minimum), silicone bronze, hot dippedgalvanised (BS 7371-6) or other high performancecoated steel. The suitability of a metal fixing forthe type of timber being used should be verifiedby the manufacturer prior to use. The same typeof metal should be used for fixings andconnectors on the same assembly to preventgalvanic corrosion.

Electroplated, brass and standard ferrous metalsshould not be used because of their potential tocorrode when used for exterior applications.Aluminium should not be used in direct contactwith treated wood.

Consult TDA Technical Bulletin 08:Metal Fixingsfor detailed guidance.



Safety Parapet. Custombuilt or strength approvedmodular system

Newel postconnection

Hand rail

Balusters

Base rail

Newel post

Edge joist

Deck boards

BracingColumn shall be out ofground contact, mountedonto pre-cast concrete piersor saddlestones

Column

Blocking

Joists

Wall platefasteners

Wall plateattached toproperty

Beam to columnconnection

Raised deck - general arrangement

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Top rail

Section 1. Timber selectionWhilst good design and installation practice havean important bearing on the long-termperformance of a raised deck the first step toachieving a desired service life of 60 years istimber selection. Such an extended service lifecan only be satisfied by specifying timber that:

a) has appropriate natural durability or

b) has been made suitably durable by anappropriate wood protection process.

Table 1 lists those softwoods and hardwoodsconsidered most suitable for structural use inextended service raised decks.

Designers and builders should only use timberfrom certificated, legal and sustainable sources.Components manufactured in accordance withthe DeckMark™ quality assurance schemeprovide independent confirmation of quality andfitness for purpose.

All timber for outdoor applications must have amoisture content below 20% at the time ofinstallation to minimise the potential formovement – see note 1.4.

Table 1: Timbers suitable for decks requiring a60 year service life

1.1 Notes about softwoodsComponents made from softwood must be givenadequate durability by an industrial woodpreservation process in accordance with WoodProtection Association specifications and BS ENstandards. Modified wood processes alsoproduce timber suitable for external applications.Guidance on wood protection is given in section2.0 of this document.

European Whitewood (spruce) is not consideredsuitable for a structural use in decks orboardwalks where a service life in excess of 15years is required.

1.2 Notes about hardwoods:Hardwoods with well-documented performanceproperties and a history of external structural useare set out in the timber design code BS 5268: 2.Those considered to be the most suitable fordecks are set out in Table 1. All the species listedin Table 1 have a BS EN 350-2 class 1 (verydurable) or class 2 (durable) rating and arecapable of a long service life for structures suchas decks, boardwalks and bridges.

There are also a number of other hardwoods,from certificated sources, that are nowcommercially available for deck construction.Examples include: Cumaru, Garapa, Ipe, Keruing,Kempas, Massaranduba and Tatajuba. The majorsuppliers, such as the manufacturing members ofthe TDA, may be able to provide technicalinformation about their performance properties ifrequired.

Only hardwoods from which all sapwood hasbeen excluded should be used.

1.3 Timber strength classTimber used for structural purposes must bestrong enough to support the loads placed uponit. This is a safety critical requirement of UKBuilding Regulations. Every structural componentin a raised deck should be made from strength-graded timber. Graded timber is grouped into anumber of Strength Classes as defined in BS EN 338: Structural Timber. The strengthclasses for softwoods are prefixed with the letterC and hardwoods with the letter D. In thesoftwood strength classes, C16 is the minimumstandard for raised deck construction, provided itsspan and load capabilities are not exceeded. C24is the strength class more typically recommendedbecause it allows longer spans, slightly smallercomponent sections and may have a better visualappearance. D30 is the minimum strength classfor hardwoods.

The same species and strength class must beused for all the support structure elements of thedeck.

Graded timber that does not carry the “gradestamp” of an approved accreditation body shouldnot be used. Where, for aesthetic reasons, agrade stamp is not applied to the timber then thestrength class must be capable of identificationby other means such as supplier documentation.



Softwoods (see note 1.1) Hardwoods (see note 1.2)

British Pine/European Redwood Balau (yellow)

Corsican Pine Iroko

Radiata Pine Jarrah

Southern Pine Karri

Douglas Fir – European OakBritish & North American Opepe

Larch Teak

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1.4 Installation moisture content The moisture content of wood is directly relatedto the humidity and temperature of thesurrounding air. The equilibrium moisture content(EMC) occurs when the wood has reached anequilibrium with its environment and is no longergaining or losing moisture. In the UK, EMC rangesfrom around 19% in winter to 13% in summer. Tominimise the potential for defects such ascupping, warping and cracking, timbercomponents should have a moisture contentclose to the prevailing EMC at the time of itsinstallation. As a minimum standard, the moisturecontent of timber components shall be under20% at time of installation.

Deck construction components delivered to siteshall be given adequate, ventilated, weatherprotection until required for installation.

Section 2.0 Timber protectionFor domestic decks, two methods of providingsoftwood with adequate durability to satisfy a 60year service life are considered in this document.They are: a) high pressure impregnation with wood

preservative (see 2.1) and,

b) wood modification (see 2.2)

2.1 High pressure impregnation withwood preservativeBS 8417 sets out penetration/retentionrequirements for different timber types, end usesand service life requirements. This Standard isused as the basis for the treatment specificationsset out on this page in conjunction with BS EN335-1, which classifies end useapplications, Wood Protection Association (WPA)commodity specifications and major preservativesuppliers. BS EN599-1 defines the test criteriathat supports the long-term performance ofcomponents treated with the preservativesdefined in BS 8417.

It is the responsibility of the treater to use theappropriate schedule to achieve the values andquality required in the specifications set out inthis CP. As such, designers and builders arerecommended to use treaters accredited to theWPA.

Notes about high pressure wood preservation

a) Components must be machined to their finaldimensions and have a moisture content priorto treatment not exceeding 20%.

b) During installation, surfaces exposed by crosscutting, notching or boring shall be given twoliberal brush coats of a suitable end grainpreservative. Contact the TDA, WPA or majorpreservative manufacturers forrecommendations.

c) If it is necessary to cross cut columns to sizethen the cut end should always be at the topand never placed close to or in contact withthe ground or concrete foundation.



Treatment specification for a 60year desired service lifea) Columns (support posts) installed out of

ground contact:

Columns shall be treated in accordance with BS 8147: 30 years extended life specification forBS EN335-1 Use Class 4: direct ground orfreshwater applications. Sapwood penetrationshall be in accordance with EN351-1,penetration class P8 to an acceptable qualitylevel (AQL) of 10% (see Note:e below). Inaddition, the preservative retention shall bederived from 10 years ground contact field testdata.

For glue-laminated posts, untreated zones notexceeding 10% of the sapwood for whichpenetration would be expected in an individualcomponent of the lamination may be ignored inthe assessment of penetration.

b) Stair stringers:

Stringers close to ground contact shall betreated as columns as 2.1(a). All other stair partsshould use the specification set out in 2.1(c)below.

c) Beams, wall plates, joists, deck boards andparapets/balustrades:

Treatment shall be in accordance with BS 841730 year extended specification for BS EN335-1Use Class 3 (uncoated applications) or better. Inthis specification, sapwood penetration shall bein accordance with EN351-1, P8 rating to anacceptable quality level of 10%.

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d) Machining or regularising treated componentsis prohibited unless they are to be re-treated bythe timber supplier. Deck platforms shouldalways be designed to avoid the need to cut adeck board along its length.

e) Acceptable quality level (AQL): This is thenumber of components in a treatment chargewhich can be below the required penetrationlevel before the entire charge is deemed noncompliant with the specification.e.g 10% formost common pines but 25% for morenaturally durable species like Douglas Fir.

2.2 Modified woodModified wood is a term used to describepermeable softwoods that have been modifiedchemically, biologically or by a physical agentsuch as heat to provide enhanced performanceproperties - for example acetylated wood.

Because “modified wood” is still a relatively newconstruction material it is not, as yet, covered byBritish or European Standards. However, BREDigest DG504 sets out the suitability of modifiedwood for construction purposes and decking isidentified as a potential application.

TRADA has carried out an independent evaluationof Accoya™ (acetylated radiata pine) for deckstructures for which 60 years service life isdesired.

Section 3.0: Principles ofraised deck design and goodpracticeThe durability of the structure and its ability toachieve the service life required depends not onlyon the correct selection of materials but also ondesign detailing and installation good practices toprevent moisture retention and facilitate goodventilation and drying. Section 3.0 of this CPcovers the basic principles for a deck where thedesired service life is 60 years. Guidance on deckstructures where 15 or 30 years service life isrequired is available from the TDA.

3.1 ColumnsRaised deck support columns are usually squarein section but round poles may be used ifappropriate to the design. Traditionally columnsare solid wood but increasingly designers areturning to laminated columns.

Laminated columns comprise multiple sections ofstrength-graded timber bonded together with glueor mechanically. Laminated columns are moredimensionally stable than solid ones of equivalentsize and are less likely to develop surface defects.This gives them high aesthetic appeal for raiseddecks where the columns are visible.

The recommended target size of support columnsis 150 x 150 mm. The actual finished size shall beno less than 140 mm x 140 mm.

The sole purpose of a column is to support theentire structure. It is recommended that supportcolumns are not carried through the deck to serveas newel posts for a parapet. Parapets for raiseddecks should always be designed as separatefeatures – see section 3.10 for details.

3.2 Column footingsColumns embedded in the ground are suitable fordecks with a service life of up to 30 years. Todeliver a 60 year service life, timber columns shallbe installed clear of the ground on a concretefooting, pier or saddlestone – see Figure 1.



Figure 1:Post Footings

Metal bracket

Pre-cast drainage ring orother type of former

Pouredconcrete fill

150 mm minimum

Haunched or re-inforcing barsto tie both parts of footingtogether

150 mm minimum

Pre-cast concrete pierPoured concrete footing. Size and depth to suit siteground conditions

Column

Haunches or re-inforcingbars to tie both parts offooting together

Pre-cast concretesaddlestone and stainlesssteel connector.

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Stainless steelor polypropyleneflange

3.3 Column spacingThe correct spacing of support columns isdetermined by a combination of:a) the area of the deck to be supported

b) beam position and frequency

c) the dimension of the beam

d) the nature of the ground and the 3 kN/m2

loading of the deck.

3.4 Beam sizeThe main support beams may be made fromeither solid timber or from double sections of thesame size and strength component mechanicallyjoined together.

The minimum target size for a single solid beamis 75 mm x 195 mm (actual size 69 x 187 mm).

The minimum target size for each element in adouble beam assembly is 175 mm x 47 mm(actual size 170 x 44 mm).

Beam spans for various recommended sizes andstrength classes are set out in Table 2. Beams canbe extended beyond a column centreline by up to30% of its clear span.

Table 2: Double member beams – maximumclear spans between columns

Actual size mm C16 C22 C24 D30

170 x 44 1.5 1.55 1.65 1.7

195 x 44 1.8 1.95 2.1 2.2

220 x 44 2.4 2.5 2.7 2.8

245 x 44 3.0 2.15 3.3 3.4

Single member beam - maximum clear spansbetween columns

Actual size mm C16 C22 C24 D30

187 x 69 1.8 2.0 2.1 2.2

Note: The above spans are indicative only andassume stable ground conditions.

All raised deck designs shall be verified by astructural engineer or suitably qualified designerprior to construction.

3.5 Beam to column connectionBeams are attached to support columns bymeans of notching the column and using 12 mmthrough bolts at 100 mm centres - see Figure 2.All through bolts shall have washers at the bolthead and nut. These washers shall be of a sizethat is the equivalent of 3 times the diameter ofthe bolt.

3.6 Wall plate The deck is secured to the wall of the property bymeans of a horizontal joist or beam known as awall plate or ledger board. This supports one endof the deck joists. The other end is supported bythe column and beam assembly.

Wall plates shall be equal to or greater than thesize of the joist that is to be used.

The wall plate shall be attached to the house wallusing masonry anchor bolts or chemically securedbolts. The wall needs to be smooth, structurallysound and capable of withstanding the lateral andpull out loads that will be placed on it by thedeck.

A gap of no less than 10 mm shall be left betweenthe wall plate and the wall to allow any rainwaterrunning down the wall to drain away freely. Wallplate fixing details are shown in Figure 3.

Note: Wall plates on timber frame properties

On timber frame properties decks should not befixed to the outer skin of the property unless thedeck wall plate/ ledger board fixing point hasbeen designed to be an integral part of theproperties construction.



Figure 2: Beam to post assembly

2 x 12 mm through bolts with washers

Beam must sitfully on notch

Notched post toaccommodate beam

Solid beam Double beam Prohibited beam

Figure 3: Wall plate

600 mm

Masonry bolts

Add spacers to createa 10 mm (min) gap

Fixing points 25% in from topand bottom of wall plate50 mm typical

Masonry orchemicallycured fixings

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3.7 JoistsThe recommended processed joist size is 170 mm x 44 mm or larger, installed at 300 mm to 600 mm centres depending upon the size andgrade of deck board being used – see Table 4 insection 3.9 for details. As a general rule, it is goodpractice to increase the frequency of the supportjoists rather than the thickness of the deck board.

The table below gives details of the maximumspans for the most popular joist sizes.

It is preferable to use processed joists with easededges to assist water shedding.

Table 3: Maximum joist clear span distance at400 mm centres

Actual size C16 C22 C24 D30

170 x 45 mm 2.7m 2.8m 3.0m 3.1m

195 x 45 mm 3.0m 3.1m 3.3m 3.4m

Joists may be mounted on top of the wall plate orthe outer beam, but not both. One end shall beface fixed to prevent turning – see Figures 4 to 5.

Joists may be cantilevered over a beam by up to30% of their permissible clear span.

3.8 Blocking and bracing3.8.1 Blocking, (also known as strutting) should beinstalled between joists to further reinforce thestrength of the structure and prevent joists frommoving in service. As a minimum requirement, alljoists with a span of more than 2.5 m shall beblocked at mid-span – see Figure 6a. Deeperjoists will need blocking at more frequentintervals.

Joists adjacent to other load-bearing componentssuch as a newel post fixing point should alwaysbe blocked - see Figure 9 on page 11.



Figure 4: Joist over beam deck design

Joist hanger

Clear joist span

Optional cantilever max30% of clear span

Beam

Joist

Deckboards Wall plateEdge joist

Column

Precast concrete pier/saddlestone

Joists secured to beam by diagonalscrewing or metal fastener

Figure 5: Beam span and joist arrangement Figure 4a: Wall plate fixingJoists mounted on beam

Beam

Column

Clear beam spanClear beam span

Optionalcantilevermax 30%of clear

spanJoists can be face fixed to

wall plates and beams usinga suitable joist hanger

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Figure 6a: Blocking joists

Blocking (strutting) isrecommended tostrengthen the structure

3.8.2 Diagonal bracing provides lateral stability totall columns and should be used on all decks thatexceed 1.5 m in height, whether they are freestanding or attached to a property – see Figure 6b.

3.9 Deck boardsThe decked surface is the most visible part of adeck and provides the structural integrity for theload bearing capability of the platform. It is alsoimportant that boards are of good visualappearance. As a consequence the specificationof unsorted or good 5th quality boards isrecommended as the minimum standard forsoftwood and that they are visually checked forquality before installation. Hardwood deck boardsshould be visually checked prior to installation.Any sections in which sloping grain affects thesurface appearance of the board should berejected.

Deck boards come in a range of finished sizesand styles from around 90 mm in width upwards.To minimise the effect of changes in deck boardmoisture content and improve drainage andunderfoot grip, the maximum actual width shallnot exceed 145 mm.

Table 4 details the maximum span capabilities oftypically available deck boards for a platform witha design load of 3.0 kN/m2. Where diagonallyboarded platforms are concerned, joist centreswill need to be closer to meet the span distancesdetailed in Table 4. Hardwood boards should beused in preference to softwood versions where aboard under 28 mm in thickness is required.

A 10% tolerance has been built in to theserecommendations to allow for grooved deckboards on the assumption that the grooves are nodeeper than 5 mm and that the grooves do notcover more than 50% of the machined surface.

Table 4: Deck board maximum support centresfor a 3.0 kN/m2 platform

Actual size mm C16 C22 C24 D30 (OakTH1)

95 x 22 - - - 300

95 x 34 450 500 550 550

120 x 22 - - - 350

120 x 27 400 425 450 450

120 x 34 500 550 600 600

145 x 22 - - - 400

145 x 27 400 425 450 450

145 x 34 500 550 600 600

Deck board style - plain, ribbed or grooved is amatter of aesthetics – surface drainage is themost important factor in how they are laid andperform long term. Complex patterns should beavoided.

3.9.1 Surface drainage

All boards shall have radiused or chamfered outeredges to assist water shedding and minimise thelikelihood of edges splintering if subject toimpact.

To further assist drainage, ventilation of thestructure and accommodate any seasonalmovement of the wood, a gap – 5 mm minimum,8 mm maximum - shall be left between eachboard. This gap shall be consistent in size acrossthe entire surface. A minimum gap of 5 mmshould be left where a board abuts a balustradeposts.

Boards that are designed to help channel surfacewater should ideally be laid with the groovesrunning on a line of positive fall, preferably acrossthe short dimension of the deck. Plain boards maybe laid in any direction.



Figure 6b: Bracing

10 mm dia.through bolt andwashers

45°

Side BraceFront Brace

Edge joist

Column

Beam

Joist at columnlocation

Beam

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3.9.2 Board fixing

Ideally, only full lengths of boards should be used.Where it is necessary to abut two boards thenthis junction shall be over a joist and an additionalbatten or section of joist shall be used to supportthe join (see Figure 7a for details). A small gap,maximum 2 mm should be left between abuttingboards to allow for any lateral movement of theboard throughout the seasons.

Nails and proprietary clips are not consideredappropriate for installing boards on decks wherethe service life requirement is 60 years. Screwsshould be used because they are more secure;are unlikely to lift during service; can be removedto permit maintenance if required; are less likelyto be damaged during installation and dents in thesurface caused by hammers or high pressureguns are avoided.

Screws shall be at least twice the depth of theboard size. All joist crossing points shall besecured by two screws positioned at the quarterpoints of the board i.e. 25% in from the side - seeFigure 7b.

Screw heads should be countersunk level withthe surface of the board or just slightly below thesurface if the board has a moisture content higherthan the equilibrium moisture content at the site –see section 1.4.

On grooved boards the fixing point should alwaysbe at the bottom of a groove, flush with thesurface of the wood.

Fixing points at board ends shall be no closer than25 mm to the board end and should always bepredrilled to prevent splitting.

For hardwoods, every fixing point shall be predrilled and countersunk level with the surface. The drill hole should be 2 mm larger than thediameter of the screw shank being used to allowfor any movement that may occur as the timberadjusts its moisture content with the seasons.

3.10 Parapets and balustradesBuilding regulations require that decks more than600 mm from the ground be fitted with a safetyparapet or balustrade system. These shall be atleast 1100 mm high and must be capable ofwithstanding a horizontal load of 0.74 kN permetre and a single concentrated load of 0.50 kN.In addition the parapet design shall discourageclimbing and the spaces between individualelements shall measure less than 100 mm.Parapet newel posts must be completely separateto the principal deck platform support columns.

To meet the above specification raised deckparapets are mostly custom made on-site andguidance can be found on the next page of thisCP. Modular balustrade systems may be used ifthey have been independently tested inaccordance with BS 6180 and 6399:1 for high-level applications and have quality andperformance certification under schemes such asDeckMark operated by the TDA and Q –Mark byTRADA. Modular systems must always beinstalled in accordance with the manufacturersinstructions.



Figure 7b: Board fixingScrews at least twicedepth of deckboard

Screws position 25% in from side

Figure 7a: Abutting deck board connection

Joist

Batten lengthequivalent to three

deck boards in width

Deckboard

2 mm gap maxScrews at board ends 25 mm infrom edge. Pre-drill before fixing

Figure 8a: Strength assessed raised level parapet system.

Figures 8b and 8c: Contemporary design modular systemsfor raised decks

Figure 8a Figure 8b

Figure 8c

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3.11 Custom built parapetsFor custom-built parapets only timber from theC24 strength class (or hardwood equivalent)should be used unless all the components arepart of a strength assessed parapet system.

The minimum actual size for a raised deck newelpost shall be 90 mm. These shall be spaced at nomore than 1500 mm centres unless thecomponent is part of a system that has beenstrength assessed for use at greater centres.

Figure 9 shows a typical custom-built parapetspecification. It should be noted that newel postsshould be fixed directly to the edge joists and thefront “header” joist. All joists must be at least 45 mm wide and suitably reinforced. Typically,newel posts are secured using two 12mmthrough bolts with washers at both the bolt headand nut. These washers shall be three times thediameter of the bolt in size.

Deck support columns must never be carriedthrough the deck to serve as a newel post onraised decks. Comprehensive design details canbe found in TDA Technical Bulletin 04: ParapetDesign & Construction.

All newel posts and vertical members should becapped to prevent end grain exposure to moisture.

3.12 Access stairsWhere raised decks have stairways, theinstallation shall be inline with Part M of buildingregulations. Additionally, the following guidelinesare recommended.

Stairs for decks up to 2.5 m in height can beaccommodated in a single run. There should beno more than 14 stair treads in a single run. Fordecks higher than this or where there is a desireto make stairs a less imposing feature then a halflanding or change of direction should beincorporated.

For steps to be safe and comfortable the riserheight and step (going) must maintain aconsistent relationship. The minimum step goingis 250 mm. For outdoor stairways, risers arenormally left open - but any openings must beless than 100mm. It is good practice to only usesolid stringers on raised decks. A typical stairwaystyle is shown in Figure 10.

Stairs that are less than 1.22 m wide must have ahandrail meeting the same requirements as thedeck parapet (Section 3.10). Handrail newel postsmust always be bolted to the stair stringer, neverto stair treads.

Specialist advice is available from the TDA forproperties that require a timber ramp for wheelchair access.



Figure 9: Custom built parapet

Plan view

Side view

90 x 90 mm (min) Newelpost with infix balustrade

Deckboard

Blocking

JoistEdge joist

2 x 12 mm bolts

Hand railTop rail

Base rail

Newel post

Internaljoist

Edgejoist

Blocking

Figure 10: Typical stair design

Joist

Stringer

All openings to beless than 100 mm

Separate riser/treadsscrew fixed to stringer

Step going250 mm min

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Section 4.0 Aftercare andmaintenance If the guidance on material selection, design andinstallation contained in this CP are followed thenminimal housekeeping maintenance and cleaningwill be required.

The principal structural timbers recommended donot require any further treatment or protectivecoating and can be allowed to bleach out andweather naturally. Timber decks and boardwalksthat have weathered for 60 years and more haveconsiderable character that many people findattractive. However, where there is a desire tomaintain the colour and appearance of the newlyinstalled deck, or to decorate with another colour,then clear water repellent sealers, oils orpigmented stains and paints may be used. Suchproducts may be used at any time during the lifeof the deck. Surface preparation, application andmaintenance shall be in accordance with theproduct manufacturers recommendations.

Components that are supplied prefinished with afactory applied decorative coating should belinked to an appropriate manufacturersmaintenance and aftercare scheme.

During the weathering process small surfacesplits or checks are inevitable. They are caused aswood expands and contracts with the seasonsand should not be regarded as defects. They donot have any effect on the structural integrity of adeck. If however major cracks and through splitsdo appear within the first few years followingcompletion of the deck then the homebuildershould be contacted immediately to carry out astructural assessment.

Section 5.0 Specificationverification“We consider that a raised timber deck that isdesigned by a suitably qualified person inaccordance with this Code of Practice usingappropriate quality materials installed by acompetent contractor will produce a structure ofoutstanding durability and stability capable ofmeeting a 60 year service life with minimummaintenance.”

Timber Decking AssociationJanuary 2008



Section 6.0 Relevant standardsand referencesBS 5268-2: Structural use of Timber. Code ofpractice for permissible stress design, materialsand workmanship.

BS 4978: Specification for visual strength gradingof softwood.

BS EN 335:1 Use classes of wood and wood-based products against biological attack –Classification of Use classes.

BS EN 335:2 Use classes of wood and wood-based products against biological attack – Guideto the application of use classes to solid wood.

BS EN 350-1 Durability of wood and wood-basedproducts – Natural durability of solid wood – Part1: Guide to the principles of testing andclassification of the natural durability of wood.

BS EN 350-2 Durability of wood and wood-basedproducts – Natural durability of solid wood – Part2: Guide to natural durability and treatability ofselected wood species of importance in Europe.

BS EN 351-1 Durability of wood and wood-basedproducts. Preservative-treated solid wood – Part1: Classification of preservative penetration andretention.

BS EN 351-2 Durability of wood and wood-basedproducts. Preservative-treated solid wood – Part2: Guidance on sampling for the analysis ofpreservative-treated wood.

BS EN 460 Durability of wood and wood-basedproducts Natural durability of solid wood: Guideto the durability requirements for wood to beused in hazard classes.

BS EN 599-1Durability of wood and wood-basedproducts - Performance of wood preservativesdetermined by biological tests Part 1:Specification according to use class.

BS 8417 Preservation of timber –Recommendations. Guidance for specifiers on thetreatment of timber drawing on relevant sectionsof BS EN Standards.

BS 5756: Specification for visual strength gradingof hardwood.

BS 6105: Specification for corrosion resistantstainless steel fasteners.

BS 6180: Barriers in and about buildings – Codeof Practice

BS 6399-1: Loading for buildings. Code ofPractice for dead and imposed loads.

BS 7359: Nomenclature of commercial timbers,including sources of supply.

ReferencesBRE Digest DG503: External Timber Structures –preservation and durability

BRE Digest DG504: Modified Wood

TRADA – Timber Decking “The Professional’sManual”

TDA – Technical bulletin 04:Parapet design &Construction

TDA – Technical bulletin 08 : Metal fixings

TFT – Information sheet: Strength GradedTimber – the basics

Wood Protection Association – Industrial woodpreservation specification & practice

Part M. Building Regulations England and Wales:Access to buildings.






Timber Decking Association5 Flemming CourtCastlefordWest YorkshireWF10 5HW01977 558147www.tda.org.uk

Documents

Raised timber deck structures on new homes - desired service life … · 2013-09-26 · Load bearing assumptions The guidance in this document provides for a uniformly distributed