Wood as a construction material has many outstanding properties.

It is highly versatile, has good thermal performance, and is aesthetically pleasing, legal and sustainable and a carbon store. At a time when sustainable development is at the forefront of construction and key issues such as climate change dominate our future thinking, wood is an outstanding renewable material that offers a range of environmental benefits. The continued future success of wood products in construction use will depend on fostering and adopting innovations to improve durability, consistency and stability in damp, exposed conditions of what is a natural and therefore variable material. This will improve cost effectiveness and quality, representing the sustainability advantages of wood products as well as indicating good design and its durability in construction.

Traditionally, the selection or specification of exterior coating systems for timber has, in the main, relied upon brand awareness, availability and the initial low cost of the coating material alone. These short-term criteria may have been taken in isolation, with little consideration of its interactions with the selected timber product, an integrated coating system approach or the longterm benefits of planned maintenance programmes.

A combination of correctly selected timber substrates and compatible integrated coating systems, with planned maintenance programmes can present a strong challenge to competing materials such as plastic and concrete that may be less sustainable due to an inability to store carbon or high energy processing.

Species of timbers most commonly used for cladding in the UK are softwoods such as European redwood, European whitewood (usually preferred as it is very resistant to moisture ingress), or Western red cedar. European redwood and whitewood are similar in colour and appearance except that redwood generally has fewer, but larger, knots than whitewood. The difference is not usually noticeable unless the two species are mixed and subsequently treated with a translucent finish.

Western red cedar is a durable timber which can be left to weather to a natural silver grey colour or it can be finished with a decorative stain.

Douglas fir has some similar characteristics to Western red cedar and, although rated moderately durable, is accepted by the BSI as suitable for external trim without treatment.

Hemlock has a straight grain and even texture but does require treatment if it is to be used externally.

Timber cladding can be used to achieve a variety of patterns, textures and colours in buildings but, in all cases, the species chosen should be straight grained and free from open defects such as excessive splits and open knot holes which could allow water ingress.

Regardless of the selected timber species, cladding generally falls into two definitions: semi-stable i.e. some movement of the timber boards permitted, for example tongue and groove cladding; and non-stable, where free movement of the timber is permitted, for example overlapped cladding.

As timber cladding experiences natural movement it is important to provide a protective coating that absorbs into the wood or is sufficiently flexible to cope with the movement.

Generally, low build woodstains absorb better into timber rather than high build woodstains that sit on the surface and may lead to cracking, resulting in flaking or peeling as the timber moves.

Low build woodstains will provide very good protection from rain and sunlight by absorbing into the wood.

In addition to low build woodstains, the new generation of high solids woodstains such as Sikkens Cetol Novatech provide a significant film build up but are flexible enough to cope with timber movement.

Apart from their flexibility these products have the additional benefit of a long wet-edge time. This means that when decorating cladding from a ladder, a dozen boards can be coated before moving the ladder and coating the next section and the wet edge will be kept going. With other woodstains it is possible only to coat three or four boards before the wet edge begins to dry. Also, if the decorator fancies a tea break, the job can be left without breaking the wet edge and subsequently showing messy brush marks. If drips, flies, brush hairs, scratches or other marks accidentally get into the coating, even after half an hour the particle can be removed and it will be possible to brush the coating back and achieve a smooth finish.

A traditional woodstain would dry out and form a skin with removal resulting in an uneven finish.

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