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Update on timber Structural applications of timber are increasing and ready-made products more diverse and widely available

technical & practice

In the debate on sustainability, timber can be regarded as exemplary. It has the capability to regenerate, needs minimal energy to convert it from raw material to usable product and is biodegradable. Forestry can itself be a contributing factor to the ecological balance since trees absorb carbon dioxide as they grow and emit oxygen, as well as providing wildlife habitats, recreational areas and job opportunities. About 40,000 people are employed in uk forestry and associated industries.

On the other hand, the uk imports the majority of the timber used. There are also concerns about tropical deforestation. But it is simplistic to lay the blame for the latter solely at the door of the timber industry. Forest clearance for agriculture, the use of timber as fuelwood by the local population and destruction of forest areas for other forms of development are all major contributors. The growth in population of many tropical countries places great demands on land and resources. For example, Brazil exports only about 5 per cent of its timber output, the remainder being consumed within the country.

Certification of sustainable supply is the long-term solution to this dilemma. Schemes operated by the fsc (Forest Stewardship Council) and others need to be supported by specifiers. The specifier can require timber to come from certified sources, and must be prepared to pay for this. Currently the fsc annual budget of about $1 million, which comes from governments and donors, is insufficient to allow further expansion. Certification of sustainability will only become commonplace when sufficient specifiers require it to justify the trouble and cost to the suppliers.

Although there is now twice as much forest area in Britain as in 1918 when the Forestry Commission was set up, Britain's geographical location and size preclude it ever being able to produce all the quality and quantity of timber it requires. Today about 80 per cent is imported. Of this, about 85 per cent is softwood, primarily from North America, Scandinavia and the Baltic area.

Around two-thirds of all British-grown timber is softwood (predominantly spruce, pine and larch plus a smaller quantity of Douglas fir) and one- third is hardwood (such as ash, beech, chestnut, birch and oak). British forests currently produce around six million cubic metres of timber per year. This figure is expected to double in the next 20 to 30 years.

Because of the small size of the uk forestry industry and our climate, Britain will never match the quality of softwoods from colder, more northerly regions in commercial quantities. Home-grown timber can however provide good-quality softwoods and hardwoods and there have recently been interesting developments to make fuller use of these, including using green (ie not kiln-dried) hardwoods for cladding and decking, laminating of smaller more readily available sections for both structural and joinery use, and the development of products such as oriented strand board and laminated veneer lumber (lvl). These products, together with mdf and wood chipboard, are efficient ways of optimising wood-fibre use since they have very low wastage rates. Together with glulam, they can enhance the mechanical properties of timber through reconstitution and allow the production of large sections from the smaller trees which come from forests managed as crops.

Glulam sections are currently available off-the-shelf from larger timber merchants. Lamination of joinery sections can provide stronger and more stable profiles than sawn wood and allow exposed faces to be selected from higher visual-quality grades and species. Glulam and lvl, including composite I-beams, seem likely to become a major growth area in construction, especially if the predicted growth in prefabricated components becomes reality.

Current work by bre and trada at Cardington on six-storey timber- frame structures will allow use of timber frame for larger buildings. Recent tests of disproportionate collapse involved removing ground- floor loadbearing internal and external walls. This resulted in no collapse, no visible deflection and no cracking of the brick cladding.

It is also possible to develop factory-finished large-panel and volumetric components which can be assembled on a production line without needing a large degree of repetition and which reduce the site process to one of assembly rather than construction. These techniques are common in Scandinavia and have been used in the uk. They need not reduce the architect's design freedom.

Timber's critical parameters are durability, moisture content and, where relevant, appropriate detailing. Durability can be achieved by selecting the correct species or by the use of the appropriate preservation method to upgrade the durability of less-durable species. Timber expands across the grain when its moisture content increases, and shrinks when that reduces. Most timber problems are related to moisture content, resulting from incorrect or inadequate specification or lack of weather protection on site.

In recent years there have been significant improvements in external finishes for joinery and timber cladding with greater understanding of the need for flexible paint films, so allowing seasonal movement without cracking. Opaque and translucent stains have longer life cycles. Water- based external finishes, which are more environmentally friendly and safer to use than solvent-based finishes, are more readily available.

Design and specification advice is available to specifiers from organisations such as the bre, bwf (British Woodworking Federation) and trada - see later - and from independent specialists and the industry.

Brian Keyworth is an architect working in private practice

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