Timber has an appealing aesthetic for specifiers, not to mention the environmental credentials which are a key driver in its selection. Therefore it is a popular choice as a building finish, being frequently specified for use as a rainscreen cladding on residential and commercial buildings. Phil Nash, Product Development Manager for Finnforest explains that, because of the weathering characteristics of wood and the effects of atmospheric moisture, there are important considerations for the architect to ensure that the resulting installation delivers a long service life.
Weathering is the term applied to the chemical and physical changes that occur when timber is exposed to rain, snow and ice, sunlight, UV rays and wind. Key to the architect’s consideration is the effect of moisture on the performance of timber as suitable building envelope material selection that will not only look appealing but will also protect the structure.
As a hygroscopic material, timber attracts, takes on and loses water according to the local climatic conditions. In fact BS942: 2007 ‘Timber in Joinery – General Requirements’, states that the moisture content of external timber is expected to range between 12% for summertime and 19% for wintertime in the UK.
It is therefore important to allow for movement in the timber components and supporting framing to accommodate seasonal variations in moisture content. Specifiers should look to select timber that is installed at around 16% in moisture content in order to minimise movement as a result.
Moisture does not only affect the structural performance and longevity of timber cladding boards, it can also impair the smart appearance. Extractive staining is a visual result of the movement of moisture along cladding boards. It is exacerbated by the application of signage and lamp fittings to the walls for example, but it can be minimised through good design, helping to prevent degradation of the building’s appearance and improve the performance of the cladding.
Because of the considerations of moisture when dealing with timber it is important to ensure timber is specified as a rainscreen: a component to intercept rain and direct it away from the inner leaf of the building, rather than a cladding to directly protect the structure.
As such there should always be a drained and ventilated cavity between the rainscreen and external walls, for both timber frame and masonry constructions. For timber frame constructions there should be further protection in the form of a breather membrane. The detailing should also incorporate a drained and ventilated cavity that extends from the lower to the upper edge of the wall, being open from both edges to allow for the vertical circulation of air.
In order to ensure adequate ventilation is achieved there is important detailing needed for the cavity behind the rainscreen too. It should be no more than 19mm wide and the width of this gap is determined by the size of the battens used to fix the boards.
The cladding needs to be fixed securely to the building. Therefore when installed on the elevations of tall buildings, or when fixed to masonry constructions for example, thicker battens are recommended. Either way, horizontal support battens nailed directly to the inner leaf should be chamfered to direct water away from the building structure.
Timber should also be specified with appropriate flashings and guttering to prevent excess water discharging onto the timbers and to remove rainwater from the building efficiently. Wetting from indirect sources is also a consideration, to avoid this it is recommended to specify the installation of timber to 150mm above ground level. Finally, end sealing once the cladding is installed will help to prevent moisture ingress through the end grains of the timbers.
When detailing around openings it is advisable to specify the sizes of windows and doors in multiples of the chosen timber boards. This will prevent the need for the installer to split or notch boards, which not only looks less slick but also requires more treatment of the exposed timber.
The specifier should also consider the fixing method. Stainless steel nails are most desirable. Mild steel and even plated nails can deteriorate in moist conditions, not only leading to structural compromise but also unsightly staining around the nail head.
It is of course vital to select timber that is durable. This is governed by the requirements of BS EN 350-2 1994, ‘Guide to natural durability of solid wood species of importance in Europe’. Preservative treatment is usually regarded as necessary in order to render any timber durable enough to meet this standard. However, thermally treated wood such as Finnforest’s Thermowood is timber that has been dried at temperatures in excess of 200°C. This causes the hemicellulose content of wood to degrade, resulting in a material that is far less susceptible to the decaying effects of wood-rotting fungi. The main advantage here is that further chemical preservative is not needed, so there is no impact on the environmental credence of the product.
There are natural effects of weathering that will be visible on even the best-designed timber rainscreens. However, fading, cracking and algae growth can be minimised through the specification of a surface treatment. Pigmented translucent stain provides UV protection and helps to retain the original colour of the wood. The alternative is an opaque wood stain. Importantly any wood stain must be specifically for external use.
There are many reasons to select timber as a building façade solution. It has a natural appearance that is highly prized for its ability to blend with both modern and more traditional architectural vernaculars. When procured under strict conditions and specified with environmental caveats, it is one of the most sustainable construction materials available. By following these steps to specification, the architect or building designer can avoid the risks associated with weathering to deliver a rainscreen that enhances the building’s appearance and lasts for a long time.