Timber Species

Any specific timber can be defined through the correct use of its classification into family, genus and species. Thus oak, and beech are members of the

Softwood - Stots Pine [P'ms sylvestris)
Diffuse-porous hardwood Birch (Beiula péndula)

Ring-porous hardwood - Oak (Guerras robur)

Fig. 4.6 Cell structures of hardwoods and softwoods

Ring-porous hardwood - Oak (Guerras robur)

Fig. 4.6 Cell structures of hardwoods and softwoods

Fig. 4.7 Limed Oak - Jerwood Library, Trinity Hall, Cambridge. Architects: Freeland Rees Roberts. Photograph: Arthur Lyons

Fagaceae family; beech is one genus (Fagus) and oak (Quercus) another. The oak genus is subdivided into several species, including the most common, the pedunculate oak (Quercus robur) and the similar but less common sessile oak (Quercus petraea). Such exact timber nomenclature is, however, considerably confused by the use of lax terminology within the building industry; for example, both Malaysian meranti and Philippine lauan are frequently referred to as Philippine mahogany, and yet they are from a quite different family and genus to the true mahogany (Swietenia) from the West Indies, or Central America. This imprecision can cause the erroneous specification or supply of timber, with serious consequences. Where there is the risk of confusion, users should specify the correct family, genus and species.

Softwood accounts for approximately 80% of the timber used in the UK construction industry. Pine (European redwood) and spruce (European white-wood) are imported from Northern and Central Europe, whilst western hemlock, spruce, pine, and fir are imported in quantity from North America. Forest management in these areas ensures that supplies will continue to be available. Smaller quantities of western red cedar, as a durable lightweight cladding material, are imported from North America, together with American redwood from California, pitch pine from Central America and parana pine from Brazil. Increasingly, New Zealand, South Africa and Chile are becoming significant exporters of renewable timber. The UK production of pine and spruce provides only about 10% of the national requirements while Ireland plans to be self-sufficient early in the next century.

Over 100 different hardwoods are used in the UK, although together beech, oak, meranti, lauan, elm, American mahogany and ramin account for over half of the requirements. Approximately half of the hardwoods used in the UK come from temperate forests in North America and Europe including Britain, but the remainder, including the durable timbers such as iroko, mahogany, sapele and teak, are imported from the tropical rain forests. The

Great Oak Hall at Westonbirt Arboretum, Gloucestershire (Fig. 4.8) illustrates the use of'medieval' construction systems within a modern building by using 'green' oak fixed with dowels and wedges.

Since 1965, 6.5% of the Amazon forest has been lost, but much of this deforestation has been for agricultural purposes, with more than three quarters of the timber felled used as a local fuel rather than exported as timber. With the growing understanding of the environmental effects of widespread deforestation, some producer governments are now applying stricter controls to prevent clear felling, and to encourage sustainable harvesting through controlled logging. Other imported naturally durable hardwoods, available in long lengths, include ekki, greenheart and opepe, whilst UK-produced sweet chestnut is durable and an appropriate structural timber.

Fig. 4.8 Traditional oak construction - Great Oak Hall, Westonbirt Arboretum, Gloucestershire. Architects: Roderick James Architects. Photograph: Arthur Lyons

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