好色先生TV

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 鈥� 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 鈥� western red cedar, European larch and Douglas fir 鈥� 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 鈥� 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."