Knotty problem: when cladding goes bad

Look at the new buildings in almost any city in the UK these days and you will be struck by the amount of timber cladding on show. Whether it is the flamboyant design of York St John university’s quadrangle, the hip mixed-use Bermondsey Square development or even common or garden social housing schemes, timber seems to have replaced terracotta rainscreen as the cladding of choice.

“There are always fashions for materials,” says Peter Caplehorn, technical director of Scott Brownrigg. “And timber, along with render, seems to have become ubiquitous over the past few years.” But given timber’s vulnerability to ageing, staining, shrinking, expanding and decaying, could this trend be storing up problems for the future?

The rise of timber

Christian Brash, managing director of specialist timber importer John Brash, says the demand for timber cladding is growing markedly in new-build and recladding projects. Its rise in popularity is in part simply because architects like to have another material in their palette. “It provides great diversity and a variety of colours and textures, offering lots of possibilities to the designer,” says Caplehorn.

It also suits a variety of sectors. AMA Research’s report on the UK wall cladding market found that the commercial sector accounted for most of the timber installed, but high-rise mixed-use, residential developments, Building Schools for the Future and healthcare all use it.

But perhaps the most important factor in the rise of timber is that it is seen as a statement of sustainability. External softwood cladding achieves an A-plus in BRE’s Green Guide to Specification, and international initiatives such as the Forestry Stewardship Council scheme and the Programme for the Endorsement of Forest Certification reassure specifiers and clients that the timber is sourced from responsibly managed forests. This in turn has helped it gain credits under the BREEAM energy rating and the Code for Sustainable Homes, Yet despite its popularity, there are some reservations about timber. Caplehorn says: “Aesthetics is driving the use of timber and it is not always appropriate. I’ve come across examples where bad detailing or poor quality timber has led to remedial work before the project is handed over.”

Part of the problem, says Caplehorn, is that the industry always has to go through a learning curve when it comes to using materials in unfamiliar applications. “I’d consider that in about 10 to 15 years’ time all the poor examples will be attracting expensive repair or replacement costs,” he says.

Graham Fairley, head of facades at consulting engineer Aecom, thinks expertise on using timber needs to be disseminated to the designers and subcontractors carrying out the work. “There is a lot of inherent knowledge, but it tends to be embedded in literature,” he says.

So what are the issues? To start with, there is the question of durability. Although timber rainscreen cladding may be well drained and vented, they can still become damp for long periods and at risk from fungal decay and insect attack.

Most external timber cladding is softwood, and there are “moderately” durable examples, such as red cedar, Siberian larch or douglas fir that can be used without preservatives. Brash says Western red cedar remains the most popular choice because of its resistance to decay: it has a 60-year service life, does not need preservative treatment and is straight-grained with only a few small knots.

Cheaper, less durable timber such as European redwood should only be used if treated. And, depending on the location and exposure, recoating will be necessary. This, believes Caplehorn, is a real issue. “We have timber cladding that will need inspection, maintenance and possibly repair in ridiculous locations – you just need to walk down any major city street to see that.”

Fairley points out that, from a sustainability point of view, the energy required to transport the timber needs to be taken into account. European timber has obvious advantages here, but for cladding, which needs a moderately durable species of timber, this means the choice open to the specifier is restricted.

One of the properties of timber is that its dimensions can shrink and expand as it gains and loses moisture. Green oak, for example, can shrink by up to 7%, or 10mm over a 150mm wide board. Detailing needs to take this movement into account through the use of overlapping boards, oversized holes around fixings and open-jointed designs.

Staining and weathering

One of the most obvious issues with any cladding is staining from the fixings. “If it’s properly detailed and maintained, it will last a long time,” says Fairley, “but if done poorly you can end up with unsightly staining as a result of the wrong type of fixings. Inconsistent weathering can also be an issue, which is something that the client needs to be made aware of.”

In timber with a moisture content greater than 20%, aecetic acid is produced that will corrode mild steel and galvanized steel fixings such as screws or nails, and can lead to staining on the face of the cladding. More corrosion-resistant fixings are recommended, such as stainless steel or austenitic stainless steel in coastal locations; non-ferrous fixings such as silicon bronze are often supplied with Western red cedar.

However, according to the Timber Research and Development Agency, weathering should not be regarded as a problem but rather as a natural process. Indeed, architect Feilden Clegg Bradley made use of the weathering on the cladding of the Longwell Green leisure centre near Bristol to create a random finish for a stronger architectural statement.

The problem is that, if left unfinished, many of the popular softwoods will bleach out to a grey colour, which might not be uniform. This depends on the exposure and orientation of parts of the facade, says Caplehorn. Also, where there are high levels of pollution, say from traffic, the timber surface may absorb the dirt and become discoloured.

“That’s not to say that timber cladding is bad – far from it,” says Caplehorn. “Correctly installed and used appropriately, it can last and look good for decades, but it will need to be maintained. It may seem obvious, but it’s not brickwork.”