Timber

Timber is a natural, renewable material that is strong for its weight, easy to work and pleasant to live with. It is used for doors, windows, roof trusses, formwork, scaffolding, piles in marine work and, increasingly, in engineered forms for whole buildings. Its properties come from its cellular structure: long fibres running along the trunk make it much stronger and stiffer along the grain than across it.

Timber is classed as hardwood (from broad-leaved trees, such as teak, sal and shisham) or softwood (from conifers, such as deodar and pine). The names do not describe hardness reliably. Strength depends on species, density, moisture content and defects: knots, splits, shakes, slope of grain and decay all weaken it. Freshly felled timber holds a lot of water; seasoning, by air drying or kiln drying, brings moisture down near the level it will have in service, which prevents shrinkage, warping and fungal attack. Below the fibre saturation point, around 25 to 30 percent moisture, timber shrinks as it dries and gains strength.

Preservation protects timber against fungi and termites, by pressure treatment with approved preservatives or by detailing to keep it dry. Fire resistance of large sections is better than many expect, because a char layer forms and insulates the core. Engineered timber products, such as plywood, glued laminated timber (glulam), laminated veneer lumber and cross-laminated timber, reduce the effect of defects, allow large spans and are made from fast-growing plantation wood.