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Aesthetic Advantages• creates a lighter and smoother appearance

compared to traditional brick masonry,

• range of profiles, textures and coatingssuitable for most contexts,

• can be constructed to suit non-rectangularshapes with relative ease,

• blends well with natural surroundings.

Energy Efficiency Advantages• can be designed and detailed to meet

regulatory requirements in a variety ofclimates,

• clad buildings are less reliant on thermalmass and are therefore less sensitive tobuilding orientation and solar access, thusproviding siting flexibility,

SCOPETimber cladding provides an envelope thatprotects and beautifies a building. This guidecovers design and construction advantages,popular cladding systems, detailingrequirements, and regulatory issues.

ADVANTAGES OF TIMBER

Structural Advantages• can be used on a wide variety of foundation

conditions because of its flexibility,

• light weight requires smaller footings,

• clad walls can be supported by posts,columns or piers, and therefore continuoussub-floor wall support is unnecessary – thusreducing costs.

External Cladding

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• reacts quickly to both heating and cooling,taking little time to warm a cool room, or tocool a hot one. Insulation will also add tothis by helping to reduce life-cycle energyconsumption and green house gasemissions.

Construction Advantages• rapid construction,

• reduced material handling and scaffolding -especially on steep sites,

• no wet trades involved.

GOOD DESIGN PRACTICE Good design practices help to ensure the bestperformance from timber cladding. Forexample, wide eaves and verandas help toprovide protection.

Flashings at corners, doors, windows and wallintersections must be detailed to hold a headof water in harsh weather environments.

Cladding should finish at least 150mm abovefinished ground/paving level to avoid moistureup-take as shown in Figure 1. In conjunctionwith this, the bottom edge should be cut toslope upwards and inwards (i.e. at an angle of15 degrees) to prevent water tracking back intothe building – as shown in Figure 2.

SARKING Sarking is used to direct water that may havepenetrated the cladding back to the outside. Italso provides a barrier to prevent drafts, winddriven rain and dust from entering the wallcavity. Sarking must be weather proof butvapour permeable, and is often made fromaluminium foil or bitumen bonded insulation.This is fixed directly behind the timbercladding – as shown in Figure 3. Additionaldetailing may also be necessary wherecondensation is a concern. This occurs wherethere is a large temperature difference betweenthe indoor and outdoor environments – as invery cold climates. Cavity insulation can helpby reducing cold outer air from meeting warminner air. In extreme conditions a devotedvapour barrier on the warm side of theinsulation may also be necessary.

Figure 3: Sarking and vapour barrier

SarkingCladding

Frame

Internal walllining

‘The snug house’ ‘The breeze house’

Figure 1: Ground clearance

Vapour permeablesarking

Grade surface away from house

150mmmin.

Figure 2: Drip initiator

Undercut bottomedge of claddingto provide dripinitiator

15º

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① AS2796.2 – Timber hardwood – sawn and milled products, Standards Australia.② AS1810 – Timber – seasoned Cypress – milled products, Standards Australia.

SpeciesCypress

Hardwood

GradesGrade 1

Grade 2

Select Grade

Medium Feature Grade

High Feature Grade

Table 1: Grade descriptions

CLADDING BOARD SELECTIONSolid timber cladding boards are available ineither seasoned or unseasoned materials.Seasoned boards are characterised by beingdressed, having a machined moulded face, andinterlocking tongue and groove (or rebated)joint. Unseasoned boards are more likely to berough sawn, have an unmachined face, andthe absence of any tongue and groove joint.Cypress can often be supplied unseasoned,with a machined face and a tongue and groovejoint designed to accommodate the expectedshrinkage. This allows for shrinkage movementin-situ. The most common options are shownin Figure 4.

Figure 4: Typical solid timber cladding profiles

30mm

15mm

16mm

12 mm

Weatherboard RebateWeatherboard

Shiplap Chamferboard

Species used for cladding include: Blackbutt,Cypress, Tallowwood, Spotted Gum, and amixture of species generally termed mixedhardwoods. Each can be purchased in specificgrades defining natural features of thecladding. (Refer to Table 1). For example,‘Select’ grade contains few knots and gumveins, while ‘Medium’ and ‘High Feature’grades contain higher proportions of thesenatural features. Ranking for Cypress follows asimilar theme. Specific details for each can befound in AS2796.2① and AS1810② respectively.

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Moisture ContentMoisture content in cladding boards influencesoverlap requirements. Seasoned boards shouldhave a moisture content between 10-18% forhardwood and 10-15% for Cypress whichensures relatively little movement once theboards are fitted – thus allowing a small rebateor overlap.

In contrast, unseasoned boards have in excessof 18% content and are prone to considerableshrinkage movement – thus creating the needfor larger rebates or overlaps.

Given this, the following applies③:

• For seasoned boards less than 150mm inwidth, a 13mm (min.) rebate or overlap isrequired. For boards greater than 150mm, a20mm (min.) rebate or overlap is required.

• For unseasoned boards a 30mm (min.)overlap is required.

Nailing Nailing requirements also vary according toseasoned or unseasoned states.

For instance, unseasoned boards must be ableto shrink independently of each other.Therefore nailing through overlaps must beavoided but must still provide restraint to theinner board – as shown in Figure 5.

Seasoned boards also should be nailed wellaway from edges to avoid splitting of thinnersections, or where tongue and groove orrebated overlaps occur. In addition, all boardends should pre-drilled to prevent splitting andflat head nails should be used to prevent pull-out.

Nails also need to be chosen to suit durabilityrequirements and hot-dip galvanised nails areoften adequate. Table 2 provides further detailson specific nail sizes for different boardthicknesses.

35mm approx.

30mm overlap

Vapour permeablesarking

Cladding

Full length packing atbottom boards and asnecessary over openings

25mm

25mm

Vapour permeablesarking

Cladding

Nail as specified

Nail as specified

Seasoned Cladding

Figure 5: Nailing distances

Cladding Thickness

15 to 22mm profiled

Sawn weatherboards

Cypress & Hardwood Framing

50 x 2.8mm plain

60 x 2.8mm plain

Softwood other than Cypress Framing

60 x 2.8mm twisted or annularthreaded

65 x 2.8mm twisted or annularthreaded

Nail Size (mm)

Table 2: Recommended minimum nail sizes

③ Details are taken from AS2796.1 which applies to hardwoods. No overlap or rebate requirements are given forCypress.

Unseasoned Cladding

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Installation Practices Installation practices must be undertaken withweather resistance in mind.

• Boards with grooves must be fixed with thegroove facing downwards.

• Sealants such as mastics should be avoided.

• Butt joints between boards should beminimised to limit moisture ingress e.g.single or long lengths should be used onwalls exposed to prevailing weatherconditions, short lengths could be usedbetween windows or on sheltered parts ofthe wall, such as under eaves and verandas.

• Butt joints should be achieved by slightlyover cutting board lengths then snapping thebowed board into position. For durability,boards should be end sealed prior toinstallation (e.g. with paint or oil basedstain).

HARDBOARD PRODUCTSHardboard products represent a special part ofthe cladding market. Different design andinstallation practices are required compared tosolid timber cladding.

These products are made of a composition offine hardwood fibres held together with naturallignin in the wood - applied under heat andpressure. Additives are included duringmanufacturing to boost moisture resistantproperties. The end product typically finishes9.5mm thick and has a factory applied primercoating to further reduce moisture penetrationand minimise site painting.

Hardboard products must be fixed usingspecial fasteners that allow flexibility andboard movement due to ambient moisturevariations.

Options in hardboard claddings include:boards, shingles or architectural panels(replicating board patterns). In each instancethe face finish can be either smooth ortextured. Due to the many variations of profile,size and texture, enquiries about detailedrequirements should be made direct to themanufacturer.

Hardboard exterior products

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RELATED DOCUMENTS (From this Series of Timber DevelopmentAssociation Publications)• Fire Requirements for Non-Domestic Fit-Out

(including information on BCA requirementsfor fire and other issues).

• Technical & Detailing Guide for Hardwoodsand Cypress (including information onmoisture management, durability,appearance and structural issues).

ACKNOWLEDGMENTS• Timber Manual, National Association of

Forest Industries Ltd., Canberra.

• Cypress and Hardwood Claddings, TimberResearch and Development AdvisoryCouncil, Brisbane.

BCA REQUIREMENTSThe Building Code of Australia (BCA)influences the ability to use timber cladding.For instance, there are ten classes of buildingeach requiring ‘A’, ‘B’ or ‘C’ type fireconstruction (depending on the rise of thebuilding in storeys). ‘A’ and ‘B’ require non-combustible materials thus preventing the useof timber cladding. A large number of otherapplications are still possible where ‘C’construction or less is required. Further detailson this issue should be sought by referring tothe list of publications detailed at the end ofthis guide.

Sponsored by the NSW Native Timber Industry Marketing and Development Fund

T D AFor additional assistance please contact theTimber Advisory Service

1800 044 529or visit the following websites:

www.timber.net.auwww.australianhardwood.net

Timber DevelopmentAssociation (NSW) Ltd