Selection of the correct fixing nails is important to the performance and appearance of a finished wall.
Hot dipped galvanised or other non-corrosive nails should always be used to fix timber cladding. Plain steel nails are likely to rust, causing unsightly stains and gradual deterioration of the timber around the nail, and loosening of the joint.
However, these nails may be suitable where they are punched and puttied, and the cladding is finished with an oil based paint system. Nails should be driven with care. Heavy and excessive nailing distorts the wood and may cause splitting during weather changes.
If a nailing gun is used, care should be taken to ensure that the nails are not driven too deeply and that excess pressure does not distort the boards. Cladding should be fastened so that the boards are free to shrink and swell individually and so reduce the chance of cupping, cracking and splitting. Nails should not fix two adjacent boards together.
Refer Figure 5 for typical nailing patterns. Flathead nails may be used to provide additional restraint of treated pine and western red cedar cladding.
To avoid splitting, some cladding boards may require pre-drilling of nail holes at the ends of boards.
CCA treated pine and western red cedar cladding should be fixed using hot dipped galvanised, silicon bronze, monel or stainless steel nails. Secret nailing or the use of shorter nails is not recommended because they do not provide adequate long term fastening. Deformed shank or longer nails should be used where cladding is fixed to softwood frames. Copper nails must NOT be used as the extractives from the timber will react with the copper causing them to deteriorate.
None of the building adhesives currently available are suitable for fixing solid timber cladding.
Joints between boards
Long edges: When grooved or rebated profiles are fixed horizontally or diagonally the groove must always be on the lower edge of the board (facing down) to avoid trapping water.
When tongue and grooved or lapped cladding is being installed vertically, the joint should face away from the prevailing weather to reduce the ingress of windblown water. Sealers or mastic should not be used to seal edge joints.
Ends: Wherever possible, single length boards should be used on exposed walls. Short lengths of cladding can usually be used between windows etc. or on sheltered parts of the wall under verandahs, eaves or awnings.
Tight fitting joints are achieved by cutting a board slightly overlength, bowing to get it into position, and snapping into place. Refer to Figure 6.
If a natural or stained finish is to be used all cut ends of boards should receive a supplementary coating of water repellent preservative before fixing. If a paint finish is to be used, end-joints should be sealed with a compatible mastic or a timber paint primer applied to the ends of boards before fixing.
When boards are fixed horizontally it is important to protect the ends of each board. Where cladding abuts masonry, clearance should be left to prevent moisture in the masonry from being taken up by the boards. Alternatively, ends should be sealed and the joint protected by a coverstrip set in mastic. The ends of diagonal boards must be well drained and protected from water. Figure 7 illustrates some typical construction details.
Finishing and Maintenance
Timber cladding that is exposed but left uncoated will absorb moisture during wet weather and give off moisture in the dry. This results in swelling and shrinkage that causes small cracks, or surface "checks," or cupping of the boards. The regular use of water repellent finishes will reduce these
Weathering will also change the surface colour of uncoated timber to silver grey. The graying of timber cladding will vary depending on the exposure to sun and rain of various parts of the wall. Uncoated timber remaining damp for extended periods may develop dark discolouration due to surface mould.
A natural weathered appearance is attractive to some people, and certain types of timber cladding will, with minimum finishing and maintenance, perform well. Timber specified for such applications should be a naturally durable species or CCA preservative treated softwood. All timber, however, requires some finishing and regular maintenance to give extended service life.
Pressure impregnation of timber with CCA preservative, whilst giving long term protection against insect attack and decay, does not prevent surface checking of the timber, colour change, or other effects of weathering.
Generally, it is recommended that all timber cladding exposed to the weather be protected with a suitable coating.
WARNING: Linseed oil or mixtures containing a large proportion of linseed oil should never be used on timber cladding as a natural finish. Mould and fungi feed on the oil and will discolour the timber.
Removal is difficult and it is often necessary to scrub the cladding with a bleach and stiff brush before re-coating with an acceptable finish.
If a natural appearance is required, liberal application of a clear water repellent preservative will help to maintain the timber in good condition. Timber finished only with a clear water repellent preservative needs more frequent maintenance than stained or painted timber during the first few years of exposure, although successive applications over a period of time build-up water repellent preservative compounds in the timber surface and extend the maintenance interval.
Where reliable long term performance and a high standard of appearance are required, timber cladding must be protected by the use of quality acrylic or oil based paints or stains, formulated for exterior timber finishing, and applied in accordance with the manufacturers specifications.
The frequency of maintenance and re-finishing of painted or stained cladding will depend on the colour and type of finish being used and the amount of exposure to weather. In areas of high temperature, pale coloured finishes are recommended as they absorb less heat and thus provide greater protection to the timber.