04 Jun 2015

Out of the woods


While timber cladding has boomed in recent decades, it has become particularly popular in the self-build market, and for good reason. Here, cladding expert Vastern Timber explains why.


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Applying wood cladding is one of the easiest ways to give a building design that extra ‘something special’. The range of textures and finishes is literally infinite, depending on the species and grades used, as well as the natural and unique weathering effects that will occur dependent on exposure and location – not forgeting the added bonus of wood’s inherent sustainability, especially when it comes from well managed UK woodlands.

For many self-builders, the sustainability of wood is of course one of the main attractions. Many self-build projects incorporate a wide range of energy efficient and sustainability technologies and techniques and it is often the case that self-build projects push the envelope with regards to sustainability.

However, self-builders planning on incorporating timber cladding into their designs need to understand that there are very specific influencers on how the cladding will behave over time. Many examples of cladding still look beautiful 10 years after installation, but in other cases it has not fared as well. So to achieve a satisfactory result, people embarking on self-build projects should gain a better understanding of the different types of weathering – both natural and human influenced; how particular species will react in the modern environment; and the effect that elevation and aspect will have on the speed and consistency of weathering over time.

Changes in appearance

Timber cladding is often used in a natural and unprotected state. Common cladding species typically used are naturally durable and do not need protection if they are not in ground contact. This is an important benefit, especially for self-builders, as the application of surface treatments can prove costly and time consuming. However, when left in its natural state, timber will naturally weather more quickly than when protected, and will also be more prone to environmental influencers, such as sunlight, moisture and even pollution.

Common unexpected changes in the appearance of timber cladding can include extractive staining in woods like oak, sweet chestnut and western red cedar, where soluble organic compounds in the wood migrate in the presence of rain to leave staining. Also, black and green mould which is caused by a lack of sunlight and perpetually damp conditions. Particulates in pollution can settle on the cladding surface to create a blackening effect. Finally, natural surface bleaching – which can be an attractive feature – will be undesirable if it is uneven due to different levels of exposure to rain and sunlight.

Understanding species

However, most of the problems associated with untreated timber cladding can be addressed through having a better understanding of the behaviour of various species and their suitability for different environments, as well as giving more consideration 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.

Oak and sweet chestnut have become very popular woods for cladding projects. These species are heavy in tannic acid, which is very reactive and brown in colour. In some cases these species have been used to clad the top storey of buildings while the lower storey has featured bright white render. In such cases rainwater can cause the tannins to leach out from the wood, often resulting in staining on the render. Also, ferrous particulates common in urban road pollution will react with these extracts to cause an unattractive blackening of the surface.

Another example is Canadian cedar, which is the most popular species for cladding. In unpolluted rural areas, and on designs that have considered weathering factors, uncoated Canadian cedar will develop a beautiful silver/grey colour. The species itself is not problematic. However, its inappropriate use in the wrong environment and a failure to consider specific weathering factors can result in an unattractive patchy appearance.

Location is key

Timber cladding is often used on large buildings with numerous elevations and overhangs. With such 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 weathering effects can be highly noticeable if not enough consideration is given to aspect and elevation, or where detailing serves to compound the issue.

The effects of detailing, such as overhangs and recesses, 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, the correct approach for self-build architects and designers thinking of specifying timber cladding for their projects is to take a broader view of the project and the specific environmental and design influencers that will have a direct effect on the timber used. These key influencers include the aspect of the elevations, environment, the proximity of other buildings or trees that might provide shade to some areas, 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 more reactive. Larch and thermally modified timbers are less reactive and therefore potentially better suited to harsher urban environments and more complex designs.

Long lasting finish

Timber cladding provides a beautiful and highly desirable facade for self-build designs and will undoubtedly continue to grow in popularity as the self-build market increasingly embraces sustainability and aesthetics into projects. As a natural material, timber needs a little extra understanding and consideration to ensure long term successful implementation, but it’s certainly worth the effort, and correct application of species will always result in a long lasting finish that turns a self-build design into something special.

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