Wood

Wood is an extremely useful building material, however the nature of timber demands that the species, the protective coating systems and the conditions of service are chosen carefully to ensure success.

Timbers are commonly classified as softwoods or hardwoods.

Softwoods and hardwoods

Softwoods are from coniferous trees: these have needle-like leaves and are mostly evergreen - for example, Pine, Fir and Spruce. They predominate in cool temperate climates and are the main building timbers in Europe, North America, New Zealand etc. Softwoods are not always mechanically soft.

Hardwoods are from broad leaved trees like Oak, Ash, Elm, Beech and Chestnut. Tropical timbers like Teak, Mahogany, Sapele, Meranti etc., are also hardwoods. Hardwoods are not always mechanically hard.

Sapwood and heartwood

The older wood in the centre of the tree is known as the heartwood. In this area none of the living processors - conductinv sap, etc. - occur, these being carried out in the younger, outer sapwood. In many timbers the heartwood is darker in colour and is readily observable, although some trees the two are the same colour. During formation of the heartwood, chemicals not found in the sapwood are formed, whereas the sapwood contains more soluble foodstuff. Both of these factors make the sapwood more prone to attack by both fungi and wood destroying insects. Many species of fungi and insects confine their attack to the sapwood.

In summary: wood, being a natural material, is susceptible to attact from:

• Moisture uptake - leading to dimensional movement and risk of splitting.
• Fungal attack - from destructive wet and dry rots to disfiguring surface moulds normally when moisture content exceeds 20%.
• Sunlight (ultra violet and visible light) - causing degradation of the wood.
• Wood destroying insects (main risk in the UK is untreated interior wood in older properties e.g. roof timbers, joists, floorboards etc.

When choosing the type of wood for any particular application, it is essential that its cost, condition of service, durability and appearance are considered at the design stage.