If timber is to be the new face of urban design, then the industry must address its misuse. TiC reports on two initiatives designed to carve its credentials into the mindsets of 21st century designers.

IT’S AN unfortunate but common sight.

A black deposit leeching its way out of a sunset-grained western red cedar-clad residence as it sits warped and cracked in the open air.

For Wood for Good (WfG) project manager, Tony Traynor, each time he sees such degredation is like a kick in the teeth for him. “There should be no fear in using timber for cladding if you follow the correct procedures,” he says.

Traynor is leading WfG into a joint initiative with the Trada to educate the industr y on the issues of building with timber.

Initially, for the proverbial white van man, Traynor is keen to stress that the need to have quality and factual information about specifying and installing timber is as important for the smaller builder as it is for the Persimmon Homes Group.

The industr y is tarred with the same brush, he says: when somebody sees a black deposit leeching out of a timber-clad building, it damages its profile in the industry.

Specification
Traynor says the most common errors when using exterior wood cladding can be grouped into three main areas: choice of species, detail in the fixing and the maintenance. “If these concepts are understood, you shouldn’t go far wrong,” he says.

Softwood continues to be the popular choice for external cladding, he says, but there is sometimes ignorance in the market place over softwood and hardwood specification.

“A lot of people think generally that hardwoods will be more durable than softwoods, but that’s not always the case. Balsa wood is a hardwood but it is not durable at all. On the other side, Parana pine is a softwood but it’s extremely hard and durable. The definitions have a biological grounding rather than to be taken practically.”

Traynor says specifiers must have a firm understanding of what they want the product to do and achieve in terms of its practicability, maintenance and durability.

“When you’re talking about using the right species for the job you’ve got to look at what its durability characteristics are, hence you wouldn’t use balsa wood for exterior cladding because it wouldn’t last five minutes.”

Despite the popularity of using softwoods for cladding due to their natural durability and ease of availability, Traynor says specifiers shouldn’t dismiss temperate hardwoods, such as European oak and indigenous UK species sweet chestnut. “They can be used without treatment,” he says, “but with the exception of the sapwood, which is not ver y durable. They’re readily available and you can get ver y good specifications.”

Traynor says that a failure to choose and use the right species can result in variations over the board lengths increasing the threat of moisture and decay.

Most softwoods are available up to a maximum of 4.8m. While most temperate hardwoods are available up to 3.6m, and tropical hardwoods between 2.1m and 4.2m.

“If you’re an architect designing a façade that has got an average length of 4.5m, ideally you want to try and get an unbroken board in that length,” says Traynor. “It’s no good choosing a species that is only 3.9m because you’re going to have joints all over the place. Not only does it not look good from an aesthetic point of view but there is a bigger threat to water ingress.”

Availability in the desired lengths and inherent durability is why softwoods generally have the lion’s share of the cladding market, although architects often see it as a series of tradeoffs over costs.

Jason Martin, associate at architect Hawkins\Brown explains the need for finding a species that was both affordable and durable when specifying cladding on the £1.7m project to build the Wysing Arts Centre in Cambridgeshire.

“The budget was very tight,” he explains. “Siberian larch glulam was chosen over other options for a balance of cost, as it was cheaper than solid hardwood sections and durability as larch is a moderately durable timber.” The availability of continuous 7m lengths and the structural stability of larch was also taken into account.

Despite being moderately durable, Martin says the larch has been preservative treated and stained and will require re-application every five to seven years.

Modified to last
The need to find a species that is affordable, with good specification and needs minimum maintenance has created the market for modified wood products. BSW’s Accoya has introduced a new range of softwoods that achieve greater durability without the use of chemicals.

Head of business development at BSW, John Alexander, says modified wood will be the growth sector in the cladding market over the next five years. “It is common knowledge that people have been looking for a long-life sustainable product,” he says. “On the one hand you’ve got long life, which could be aluminium, or on the other end of the spectrum, is sustainable timber. But there’s been a gap between those requirements and Accoya bridges the gap. It fits that market very well.”

Alexander says the initial growth in Accoya has been in the social housing sector.

Twenty-one homes in Tweedbank on the Scottish borders have just been clad in Accoya for Eildon Housing Association. The product has been independently verified as having a minimum 60-year service life in cladding applications.

Behind the scenes
Peter Mayer, research and development manager of construction insurance specialist BLP, says that he’s often seen UK specifiers skimping on the thickness of cladding.

“In the UK they’re very mean,” he says. “You can sort of get 16mm or 19mm thick timbers and they will work but they are more prone to cutting and splitting more than say 25mm or 30mm thick. Thinner is more liable to twist, split or crack.”

In order for the timber to stand the test of time, fixing detail is crucial, says Traynor. “It doesn’t matter what you’re doing whether it’s vertical, horizontal or diagonal fixing, it’s about the behind the scenes fixing process and making sure you’re creating the correct cavity and vapour barrier to allow for water to drain off.”

Key, says Traynor, is to leave relevant gaps for air circulation. “A cavity should be formed behind the cladding to allow moisture and water to escape,” he says.

WfG recommends that preservative treated and structurally graded battens should be fixed behind the cladding and a weatherproof membrane fitted behind that. “By doing this you are creating air flow behind the cladding and at the same time keeping it watertight in as much as if any water did ingress it would drain down through the gaps and out of the bottom of the building.”

Splash-back
With the right specification and species it is then in the hands of the installer where standards have often been falling down. Black staining around the fixings of the wood is likely to have been caused by the wrong use of nails and screws.

A common misuse is using iron fixings on western red cedar, which in a short order will give black staining and disfiguring. WfG recommends that both annular ring shank nails and round head nails be used to fix softwoods, while stainless steel screws are preferred for hardwoods.

When using hardwoods, Traynor says slight over-drilling for the screw holes will allow for movement in the wood. “You have to be careful about how you fix it so you can allow it to expand and contract.”

When protecting the cladding from rainwater, a key factor is to leave as much of a gap from the floor as possible, says Peter Mayer. “In the UK you’ll usually have a 150mm clearance off the ground,” he says. “But the continental practice is to have the timber higher off the ground. Although 150mm stops quite a lot of splash-back you can get regular high splash-back from rainfall splashing off the ground.”

The Government’s drive to deliver 240,000 new homes a year will provide opportunities for timber manufacturers and their supply chains over the coming years.

Timber’s renewable and thermal properties make it not only good enough in the bid to build zero carbon homes, but make it a viable option to clad our existing energy guzzling housing stock. “It is proving to the housebuilders, architects and designers that if you tell them how to use it properly there is no fear in using timber for cladding,” says Traynor. “The message is already starting to get through, but there is still more work that needs to be done.”