For many architects, designers and self-builders, timber is a material they often have less familiarity with when used as an exterior feature. But as timber cladding installations mature it’s time to reflect on what has and hasn’t worked. Tom Barnes of Vastern Timber explains how decisions about timber species and design can help ensure timber cladding installed today looks good and performs for many years to come.
The use of timber cladding has boomed in recent decades in both the commercial and domestic markets. In many cases timber cladding still looks beautiful 5 – 10 years later, but in other cases it has not fared as well. The reason is often that specifiers don’t always have a strong enough understanding of the different types of weathering – both natural and human influenced; how particular species react in modern environments; and the effect of elevation and aspect on the speed and consistency of weathering.
Timber cladding is often used in a natural and unprotected state because the common cladding species used are naturally durable and do not need protection if they are not in ground contact. This is a key benefit as the application of surface treatments is costly and time consuming. But 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.
Common unexpected changes in the appearance of timber cladding 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; black and green mould caused by a lack of sunlight and perpetually damp conditions; blackening due to particulates in 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 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.
For example, 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 many cases these species have been used to clad the top storey of buildings while the lower storey has featured bright white render. However, rain will cause the tannins and oils to leach out from the wood, often resulting in staining on the render. Similarly, 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. So the species itself is not problematic. However, its inappropriate use in the wrong environment and a failure to consider weathering factors certainly is, and can result in an unattractive patchy appearance.
It is common to see timber cladding used on large buildings with numerous elevations and overhangs. However, 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, so there will be a huge amount of differential weathering to consider. 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. Additionally 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 architects and designers thinking of specifying timber cladding is to take a broader view of the project and the specific environmental influencers that will affect the timber used. These include aspect of the elevations, the environment (is it rural or urban?), 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 more reactive. Larch and thermally modified timbers are less reactive and therefore potentially better suited to harsher urban environments and more complex designs.
Ultimately, timber cladding provides a beautiful and highly desirable facade for both urban and rural buildings. However, as a natural material it needs a little extra understanding and consideration to ensure long term successful implementation, but it’s certainly worth the effort.
As seen in RCI Magazine