That timber cladding continues to gain popularity is reflected by the large numbers of cladding-related enquiries through the TRADA advisory line from architects’ practices. In some cases, particularly for large areas of cladding on public buildings, the norm is increasingly to specify cladding in the unfinished state.

Many architects choose to do so because they want the wood to ‘weather’ to grey as part of their design aims and the absence of a finish avoids the need for regular repainting, of course, minimising maintenance costs.

The timescale of weathering can be somewhat 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. The rate of colour change will also vary between species (hardwoods tend to take longer than softwoods, particularly if it is a very dark timber to begin with) and can sometimes vary within the same species.

Wood containing a high proportion of tannin (a water soluble extractive), 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 wash off surfaces exposed to rain after some time, but may remain trapped, and often visible, behind a translucent finish.

Normally the weathering process will give an even appearance, provided that the detailing is correct and the local environmental conditions are considered in the original design. Weathering generally tends 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 in close proximity. In most cases this works reasonably 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, too, may prevent even weathering. Overhangs will also prevent rain water washing the surface clear of dirt particles. Nor should unfinished wood be used in locations where there may be pollution from nearby traffic or other forms of atmospheric pollutants.

Given these trends and uncertainties, TRADA has just completed the second phase of a research project entitled “Investigation into the discolouration of external unfinished timber cladding, its prevention and remedial treatment” which aims to identify the causes of the various forms of unpredictable discolouration of unfinished exterior cladding in service. In addition, by means of site-based trials in phase three, it also seeks to establish the effectiveness of preventative treatment or design measures to prevent discolouration occurring in service and, finally, to develop appropriate strategies for the remediation and long-term prevention of discolouration on exterior cladding. The ultimate aim of the project is to improve the aesthetics of timber cladding in service.

Where cladding is exposed to the weather, its surface colour changes over time, with this change normally being predictable. The rate of colour change is related to species, level of exposure to direct sunlight and rainfall. (See TRADA Wood Information Sheet Specifying timber Exposed to Weathering).

However, our experience over the years shows that in many instances the surface colour change of cladding is much less predictable and the end colour can be undesirable to the client. The first phase of the research was to collate and re-examine more than 40 TRADA reports into disfigurement of cladding boards in service around the UK. Of the 32 reports that contained a detailed analysis of causation, mould colonisation, extractive staining and iron staining were responsible for discolouration that resulted in TRADA providing consultancy to concerned clients.

This work has identified areas of buildings where cladding is likely to be more susceptible to mould colonisation, including areas where wood remains wetted for longer, or on areas of sapwood. By understanding where the mould is likely to occur, it should be possible to design out the problem or use a treatment that inhibits mould growth.

Preferential growth of surface mould on areas of exposed sapwood on oak cladding

Many clients wrongly associate mould growth with wood decay. However, although mould is likely to indicate that wood moisture content in the affected areas is elevated periodically and may be more likely to decay, it does not mean that decay has been established.

Typically, mould growth on exterior cladding is caused by the growth of a yeast fungus from the genera Aureobasidium or Hormonema which exists in a highly melanised or black form, producing the characteristic black discolouration.

This type of fungus can use naturally occurring sugars present in the wood, as well as the breakdown products from delignification of wood (caused on cladding by weathering). It also requires wood to have suitably high moisture content in order for growth to be sustained.

Discolouration caused by this fungus is therefore more prevalent on unprotected aspects which are more liable to periodic wetting from rainfall and which are subjected to high levels of solar radiation. The weathering will provide a food source in the form of products from wood delignification and since the wood will wet up more frequently then it will support fungal growth. It will develop much less so beneath areas which are sheltered from direct sunlight and rainfall such as beneath eaves overhangs and canopies.

Often mould discolouration can be seen as a distinct zone immediately above overhanging cills or above ground level within the splash zone where cyclic and repeated splashback can contribute to the moisture regime of the cladding in these areas.

Differing porosity can also create undesirable effects and can be a particularly troublesome form of discolouration if the cladding is made from smaller finger-jointed sections where the porosity and water absorption characteristics of adjacent pieces differs, creating areas of differential mould growth in distinct visual boundaries between adjacent pieces.

The third stage of the project is to investigate cladding at sites where mould colonisation is known to be a problem and explore whether surface cleaning will remove sufficient mould foodstuffs to prevent recolonisation. We will also examine the efficacy of products that may confer remedial protection.

In addition, TRADA will investigate whether any uncoated modified timbers resist discolouration from the growth of moulds. This will include thermally and chemically modified timbers and those treated with oil.

We would therefore be keen to receive photographs of sites where cladding is performing well or is disfiguring in service. Please email them to pkaczmar@trada.co.uk.

Finger-jointed sweet chestnut cladding (open jointed) showing differential mould growth on adjacent pieces

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