Many architects choose to leave wood cladding unfinished because they want the wood to bleach to grey and avoid the need for regular re-painting. In most cases this works well, but it is important to detail carefully to avoid localised heavy water run-off that can darken the wood.
The presence of large overhangs, for instance, may prevent even bleaching and stop rainwater washing the surface clear of dirt particles. Nor should unfinished wood be used in locations where there may be traffic pollution or other forms of atmospheric pollutants.
If wood cladding is erected at a high moisture content, there may be initial growth of surface mould which can cause blackening. This can usually be brushed or washed off and, because the ‘foods’ for this growth will diminish as the surface weathers, recurrence is unlikely. If it does reappear, an anti-fungicidal wash may be necessary.
It is impossible to define in advance whether any timber species is “prone to discolouration” as all timbers will discolour given certain conditions. Wood containing a high proportion of tannin, such as western red cedar, European oak or sweet chestnut, will exude this for some time, even where a finish has been applied. With unfinished timber, the tannin will eventually wash off surfaces exposed to rain, but may remain trapped, and often visible, behind a translucent finish.
The timescale of bleaching is unpredictable; it depends on the moisture content of the wood when erected, the aspect, the degree of exposure to sun and rain, and the time of year when erected.
Normally ‘bleaching’ will give an even appearance, provided detailing is correct and environmental conditions are considered in the original design. Bleaching and weathering generally tend to be more even on vertical rather than horizontal boards. Unfinished wood, particularly those with a high tannin content, can also suffer iron staining from pollutants such as brake dust and exhausts.
It is important to point out, however, that bleaching and weathering of wood, and even surface growth of moulds, will not affect wood’s long-term durability.
In the absence of a surface finish there may be a slightly quicker reaction in terms of moisture absorption, but with modern vapour permeable paints, moisture gain or loss
will only be slowed down, not prevented. A similar allowance for movement must be made, whether a finish
is applied or not.
If hardwood joinery rather than cladding is left unfinished it is advisable to provide temporary surface protection during building. One method is to spray a clear wax emulsion to reduce the porosity of the wood surface, reducing staining from construction dirt. This will eventually disappear from surfaces exposed to UV light, but will not flake or powder.
If a finish is applied, film-forming coatings should be avoided as they cannot take up moisture movement of the wood and will crack. Any moisture that penetrates the film will be trapped behind, resulting in elevated moisture content of the underlying wood and blackening of the surface. To maintain the original colours of the wood, clear finishes
are sometimes specified, but these will not prevent
bleaching and will themselves break down because of the UV in sunlight. A dark finish will raise the surface temperature, which can lead to excessive surface cracking and,
in some species, drawing out of resins in the wood, or sticky patches.
With any surface finish, all boards should have one coat applied to all faces before erection, preferably in the controlled conditions of a factory. Pre-coating also avoids unfinished areas being exposed if boards shrink, a common problem in site coating. This does not preclude a final coat after fixing which will provide extra protection to exposed faces and conceal surface-mounted fixings.
Non-ferrous fixings
Metal fixings should preferably be non-ferrous, such as stainless steel, as metal coatings, including hot-dip
galvanising, can be damaged by the fixing process. With high-tannin species, acid in the wood can cause mild
corrosion of steel and lead to iron-staining. But stainless steel fixings may hardly be visible in the long term, even against unfinished wood, as they weather to a matt grey.
On timber cladding, ‘low-build’ stains or paints are preferable as they penetrate further into the surface and conceal surface checking. If sawn rather than planed wood is used, ‘low-build’ finishes are essential, but they will also last longer than on planed surfaces. Fine-sawing rather than rough sawing should be specified, whether coatings are applied or not, as circular saw marking may otherwise be visible. If a coating is applied, sharp edges on the boards should be removed as surface tension will cause liquid films to pull away from the arris, reducing the coating thickness. The ideal 3mm radius may be impractical on cladding boards but, simple ‘pencil rounding’ by coarse sanding will improve paint retention substantially and reduce the risk of splintering sharp edges.
There are plenty of suitable finishes available in a wide range of colours for timber cladding, but their role is largely aesthetic. They will not preserve otherwise non-durable woods, prevent moisture movement of the wood, or extend the inherent life of the cladding, although they may limit the raised grain and coarser texture UV action can cause on unfinished softwoods.
It is difficult to forecast the frequency of maintenance required for coated surfaces. For instance, a translucent stain on planed boards on an exposed site might need more frequent maintenance than an opaque finish on a sawn surface. So it is important to consult the finish manufacturer at specification stage to establish the correct maintenance schedule.
Cladding assurance
The Timber Decking Association, with the backing of the wood for good marketing campaign, is setting up an advisory and quality assurance service for the cladding market.
The initiative is expected to be launched in the summer and is
intended to include a code of practice for cladding suppliers and installers.
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Stainless steel fixings are recommended |
