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Specifying Timber: ' The green agenda; timber technology; design innovation and recent legislation'

Martin Spring examines recent developments in timber and joinery, and related products, as well as their design and specification. He also considers the impact of environmental and economic influences and recent changes in legislation

The green agenda

No building material is riding higher on the green agenda than timber. Eminently renewable and carbon-neutral, timber is a splendid thermal insulator, structurally strong, and when left exposed feels and looks like the natural product it is (AJ 29.04.10). Architects love it because it is easy to machine, allowing wide scope for purpose-designed one-offs. And even supermarket giants - first Sainsbury’s and now Tesco - have jumped on the bandwagon, with two timber-built stores in Devon that openly flaunt their green credentials.

But all that does not make timber immune from the economic downturn. According to the Office for National Statistics, prices of all construction timber rose by 26 per cent between March 2005 and October 2008, and they have fully surmounted the recessionary dip of five per cent that they plunged into after that. On the other hand, the current fuel price-hike might come to timber’s rescue because, unlike steel or concrete, it demands negligible energy during production.

Timber technology

If one product encapsulates the current vogue for wood in buildings it is cross-laminated timber (CLT) (AJ 05.02.09 and AJ 03.12.09). In basic terms, this is a structural plywood that can be produced up to 27 layers and 334mm deep, and in panel sizes up to 3m wide and 18m long. The thrill for architects and engineers is that CLT is as close to an all-in-one material as you can get: the same component doubles up as structure, fabric and finish. As an inner cladding material, the face can be left exposed, even where half-hour fire resistance is required. To contractors and clients, this panelised system offers speed of construction.

CLT is invariably imported from Austria, Germany and Switzerland by market leader Finnforest and an expanding gaggle of competitors. Panels are cut to precise size and shape at the laminating plants by computer numerically controlled machines, while all fabrication, whether on or off site, is undertaken in the UK by firms such as B&K Structures and James Jones & Sons.

Design innovation

Despite its continental production, UK architects and engineers have been pushing out the boundaries of CLT applications ever since Waugh Thistleton Architects and structural engineer Techniker conjured up an eight-storey apartment block in north London entirely out of CLT wall and floor panels in 2008 (AJ 14.06.07). Currently, these panels are used to cover large halls in new schools, while its wide-span potential has been exploited in several small show-homes at Scotland’s Housing Expo, running throughout August in Inverness.

Even so, Peter Wilson of Napier University’s Centre for Timber Engineering points out difficulties that CLT poses for UK project teams. ‘There’s no standard code of practice among manufacturers, and it’s not easy to find detailed information. What codes there are were set up to meet European standards, so some British building control officers don’t accept their fire tests. And British quantity surveyors have no idea how to cost it.’

Added to that, one of the latest buildings to use CLT for its roof exposes the Achilles heel of all timber products in matters green. The £4.9 million Woodland Trust Headquarters in Grantham, Lincolnshire, by Feilden Clegg Bradley Studios, due for completion in July, achieves a BREEAM ‘excellent’ rating by passively absorbing the excess heat generated by the offices during the day. However, since timber is a relatively lightweight material with minimal thermal mass, precast concrete floor slabs had to be used to absorb the heat.

Among advanced timber products, heatmodified wood, such as Platowood and ThermoWood, is likely to be used more often. At the Gillespie Park nature reserve in Highbury, north London, the heat-modified softwood entrance canopy, by AREA Landscape Architects, has the same durability as hardwood but at a considerably lower cost.

Research and legislation

Turning to the most traditional of timber building products, research just released by Imperial College London demonstrates that, even with low maintenance, timber windows have a service life of at least 60 years. Over that period, they can save seven per cent in wholelife costs compared with PVC-U windows. The research, conducted for the Wood Window Alliance, covers windows manufactured to basic BSI British Standards. On the same theme, Dulux Trade has just brought out a new formula for its Weathershield paints and stains that doubles the protection period offered for exterior wood from five to 10 years.

In matters of legislation, the British Standard for the structural design of timber frames was last month replaced by Eurocode 5 (EC5) (AJ 18.03.10). To help designers cope with the change, the Timber Research and Development Agency (TRADA) has converted its Timbersizer online design tools to meet EC5 requirements (www.trada.co.uk/ ec5software). In August, TRADA will publish a portfolio of 40 thermal details for insulated timber frames to meet the upgraded thermal requirements of Part L, due to come into effect in October (AJ 17.12.09).

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