Natural unfinished timber is the cladding material of choice for fashionable architects these days, but not everyone knows how to use it. Choose the wrong type of wood or an inappropriate fixing method and the rot could set in … 
At the turn of THE millennium, there are few cladding materials more in vogue than natural timber. Not just the green fringe of the architectural profession, such as ECD Architects, Penoyre & Prasad and Feilden Clegg Bradley, but mainstream practices such as Foster and Partners, Michael Hopkins and Partners and David Chipperfield Architects, have chosen to clothe some of their prize buildings in unfinished timber boarding. Even the high priest of high-tech, Nick Grimshaw, has succumbed to the most low-tech, rustic material – cedar shingle – to clad his visitor centre at the Eden project in Cornwall, and he has two more timber-clad buildings on the drawing board.

Patrick Hislop, who runs a consultancy service within the Timber Research and Development Association (www.trada.co.uk), has assisted all the above-named architects with their timber cladding. He claims that enquiries have quadrupled since the opening of Chipperfield's Rowing Museum at Henley in 1996.

Timber that is untreated, or treated only with preservative or translucent stain, not only looks natural, it is natural. It is warm to touch and to look at. And it is potentially the greenest of cladding materials – apart from straw and reed thatch, timber is the only renewable building material, and processing trees into building boards or panels consumes little energy and causes little pollution or unusable waste.

Architects are now keen to explore the potential to use timber cladding for more than the traditional British application of narrow, overlapping weatherboarding, as seen in the typical garden shed. White-painted tongue-and-groove weatherboarding, which was widely used in 1960s council housing and has since degenerated into cheap, dismal American-style PVCu "siding", is even more taboo among today's architects.

British designers are experimenting with vertical boarding, diagonal boarding and even storey-height flush panels of timber. Square-edged boards can be arranged with open joints between them, while making a visual feature of large screwheads set in even larger washers. Hislop favours "board-on-board" arrangements in the Scandinavian tradition, where the inner and outer layers of square-edged vertical boards alternate.

Three softwoods – western red cedar, European larch and Douglas fir – can be used untreated externally. Common-or-garden whitewood or redwood can also be used externally, provided that it is regularly maintained with a penetrating preservative or stain. The most popular untreated hardwoods for external cladding are European oak and tropical iroko.

However, Hislop warns that to ensure long-term durability, "care needs to be taken in the specification of the timber itself, in design detailing, preservation treatment and how it is erected and fixed". Accordingly, he has written a 46-page design guide called External Timber Cladding. It is published by TRADA, which is planning a series of best-practice seminars around the country (see Information point, page 86).

After introducing the various timbers and architectural options, Hislop's guide discusses the principles of design for durability, and presents a range of generic cladding details.

Despite being a natural material, timber cladding relies on the concept of the rainscreen, just like synthetic cladding systems in steel and glass. "Any timber cladding is to some extent a rainscreen, because there has to be a ventilated cavity behind," says Hislop. "And it's often a good idea to leave the joints open between boards, as this lets rainwater through. The timber cladding can be backed by a timber frame or insulated blockwork." On the other hand, argues Hislop, architects often don't appreciate the limits of a natural product. "They don't understand how to deal with moisture movement in timber. And they often E E try to overspecify the quality of timber without visual blemishes such as knots and/or wide variations in grain and colour. They forget that it will all fade to the same colour over time." A case in point is J Sainsbury's green supermarket in London's Greenwich. This was designed by Chetwood Associates and won last year's RIBA sustainable architecture award – but vertical buckling occurred in the tongue-and-groove boarding of American oak.

Several months after installation, every sixth board started to buckle outwards, as either the timber had been supplied pre-seasoned for internal use and expanded as it soaked up moisture, or the boards had been fitted too tightly. The problem was solved by removing the buckled boards, machining deeper grooves at their edges, and replacing them.

Hislop also sees difficulties on the contracting and supply side. "There's a real problem with subcontractors, because joinery skills are vanishing. But timber cladding needs to be fixed by somebody with better knowledge than your average contractor's chippie. Subcontractors should ideally set up specialist teams for timber cladding with the right tools and skills." He also thinks that sawmills and timber merchants could do more. "At present, it is normal for boards to be cut to order, which means that the wood is green and will shrink. But if sawmills pre-cut sections in advance of contracts, it would mean that the most suitable sizes would be available from stock and they would have been allowed to dry out naturally."

Oak at the Earth Centre

Architect Feilden Clegg Bradley used unfinished English oak slatting to clad the restaurant block at the Earth Centre near Doncaster. Horizontal boards, with chamfered edges and air gaps between them, screen an insulated aluminium curtain wall. Oak slats have also been used in an angled, wider-spaced formation as hinged louvred panels to shade the glazing.

Sainsbury’s knotty problem

Unfinished oak cladding was used to achieve a suitably natural effect at Chetwood Associates’ award-winning J Sainsbury eco-store in Greenwich. Traditional horizontal tongue-and-groove boarding was used. However, the American oak used had been seasoned for internal joinery, with the result that it absorbed moisture and expanded. A few months after installation, every sixth board had buckled outwards. The problem was solved by removing the bulging boards and machining them to fit.

Scandinavian-style larch at Greenwich Millennium School

Unfinished larch was specified by Edward Cullinan Architects for the new Millennium School in Greenwich, as it was more resistant to bashing and smoother to the touch than western red cedar. The vertical boarding was detailed in a variation of Scandinavian “board-on-board” system. Prefabricated panels were made by pairing two 130 mm wide boards with hidden ply tongues. Plywood backing panels were also used to enable the boarding to overlap the window frames.

Which timber to use?

Western red cedar
The only softwood to be classed as durable, untreated western red cedar has a 60-year life if sapwood is excluded. It bleaches to silver-grey. Being rather soft and brittle, it is unsuitable for uses where it may be exposed to impact or vandalism. It scores lower than many other softwoods in BRE’s Green Guide to Housing Specification on account of the energy consumed in transporting it from western North America. European larch
A very strong, robust softwood. Classed as moderately durable, it can be used untreated for cladding if sapwood is excluded. Being stronger, denser and smoother than western red cedar, it can better resist wear and tear but is more expensive. Douglas fir
A straight-grained, robust wood, available in long lengths, it is rated as moderately durable and can be used untreated for cladding if sapwood is excluded. Home-grown Douglas fir is less durable and should be treated with preservative. European redwood and whitewood
A non-durable softwood with large, loose knots, European redwood should be impregnated with preservative if used for cladding. European whitewood can be used in a similar way and has smaller knots. European oak
European oak is the classiest hardwood that can be used untreated for cladding – but it is expensive. It is fully durable and more robust than softwoods, with a dense texture and grain and bleaches to a silver grey. Iroko
Iroko is less expensive than oak – although less well known and cheaper tropical hardwoods can also be used. Ensure that it comes from a sustainable plantation by obtaining a certificate from the Forest Stewardship Council (www.fsc-uk.demon.co.uk).

What could go wrong?

Patrick Hislop’s seven most common design pitfalls in external timber cladding: 1 Boards too wide (more than 150 mm) or too long, amplifying natural moisture movement of the wood
2 Horizontal boarding fixed to curved surface, causing boards to crack or become loose
3 Cavity behind the boards closed off, so lack of ventilation causes timber to rot
4 Tongue-and-groove boarding used inappropriately – flimsy timber tongues could be damaged by secret nailing
5 Nailed hardwood or screwed softwood – hardwood should be screwed and softwood nailed, both in stainless steel
6 Timber kiln-dried to internal joinery standards, causing excessive moisture movement and buckling of boards
7 Timber cladding too close to the ground, allowing splashing rain to cause rot

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