While timber cladding is a technique that has been used for millennia, its use has boomed in recent decades, driven largely by a push for more sustainability in building, as well as a renewed appreciation of its aesthetic benefits. In most cases timber cladding still looks as beautiful today as when it was installed 10 or 15 years ago, but in some cases it has not fared as well. The main reason for the occasional failures is that specifiers don’t always have a strong enough understanding of the different types of weathering – both natural and human influenced – that can affect timber. Wood species react differently in modern environments, while elevation and aspect can have a considerable effect on the speed and consistency of weathering. With better understanding of these specifics, those thinking of specifying timber cladding can look forward to longer lasting and more effective installations.

Timber cladding is often used in a natural and unprotected state because the common cladding species are naturally durable and do not need protection if they are not in ground contact. This is an advantage because surface treatment can be costly and time consuming. However, when left in its natural state timber will obviously weather more quickly than when protected, and will be more prone to environmental influencers, such as UV, moisture and pollution.

Oak and sweet chestnut have become very popular woods for cladding projects. These species are heavy in tannic acid, a soluble organic compound which is very reactive and brown in colour. In many cases these species have been used to clad the top storey of buildings while the lower storey has featured bright white render. Rain will cause the tannins and oils to leach out from the wood, often resulting in staining on the render.

Other unexpected changes in the appearance of timber cladding include; black and green mould caused by a lack of sunlight and perpetually damp conditions; unattractive blackening due to particulates, particularly ferrous particulates common in urban road pollution, settling on the cladding surface; and surface greying (bleaching) which, while an attractive feature, can also be undesirable if it is uneven due to different levels of exposure to rain and sunlight.

Most of the problems associated with untreated timber cladding can be addressed prior to specification or installation through a better understanding of the behaviour of various wood species and their suitability for different environments, as well as more consideration given to the effects of elevation and shading. Additionally there is always the option to apply a surface treatment to the timber cladding which will further mitigate weathering problems.

Canadian cedar is the most popular species for cladding. In unpolluted rural areas and on designs that have adequately considered weathering factors, uncoated Canadian cedar will develop a beautiful and highly desirable silver/grey colour. However, its inappropriate use in the wrong environment and a failure to consider weathering factors can result in an unattractive patchy appearance instead of an overall uniform silvering.

The use of timber cladding on large buildings with numerous elevations and overhangs is commonplace. Yet usually with such large and complex projects, each panel will invariably face a different set of weathering factors, such as exposure to rain, snow, ice, and differing amounts of UV light and wind. As a result there will be a huge amount of differential weathering to consider, and these differing weathering effects can be highly noticeable if not enough consideration is given to aspect and elevation, or where detailing serves to compound the issue.

Overhangs, recesses and other detailing must be considered and their effect will be accentuated by a northern elevation. For species which leach extractive compounds during the early stages of weathering, the semi shelter afforded by overhangs can prevent the brown liquid from being washed away, leading to unsightly and prolonged staining.

With so much to consider for architects and designers thinking of specifying timber cladding, the correct approach is to take a broader view of the project and the specific environmental influencers that will affect the timber used. These influencers will include aspect of the elevations, the environment (is it a rural or urban installation?), the proximity of other buildings or trees that might provide shade to some areas, the design and management of water run-off, and of course a consideration of the reactivity of different species, particularly in polluted urban environments. Oak, sweet chestnut and western red cedar are the more reactive species. Larch is less reactive and therefore potentially better suited to harsher urban environments and more complex designs.

New thermally modified woods are providing a solution which negates the majority of issues with weathering and the particular natures of different species. These highly robust timbers are extremely stable and of a high quality and present a far less problematic cladding solution for those who are either unsure or are presented with extremely complex designs and demanding environments. Not only are they highly stable, but their aesthetic is consistent and remains so after installation.

Timber cladding provides a beautiful and highly desirable facade for buildings in both urban and rural settings. However, architects and designers should always appreciate that, as a natural material, wood needs a little extra understanding and consideration to ensure long term successful implementation. That extra understanding is certainly worth the effort, and the knowledge and experience gained will serve to ensure subsequent specifications and installations deliver long-lasting, attractive, and highly desirable results.

As seen in FC&A Magazine