Timber Properties

Colour

Most timbers show variation in colour between species and within species. Colour may also vary during use and can be influenced by the application of different finishes. Timber exposed to light will change colour and unprotected timber, exposed to the sun, will eventually become silvery grey in colour.

Grain

Grain refers to the general direction, size and arrangement of the wood cells, and may be used to broadly describe the timber’s appearance. Grain can be described appropriately by using other descriptive terms such as, straight, sloping, spiral, irregular, wavy, and the like, which describe some specific characteristics of the timber grain.

Figure

Figure is the term used to reference the patterns produced on the surface of timber, and is influenced by the arrangement and dimensions of the wood cells, the nature of the grain, and colour variations in the heartwood and sapwood or even may depend on how the logs were sawn. Interlocked and wavy grain create a striking figure, due to the manner in which light is absorbed or reflected at differing angles by the wood cells.

Texture

The timber’s texture may be described as being coarse, fine, even or uneven. This differentiation between coarse and fine texture is made on the basis of the size and arrangement of the wood cells. Softwoods are normally considered to be fine textured, whereas hardwoods may span the range from coarse to fine.

Density

Density is the weight of timber per unit of volume. It is an indicator of the thickness of the timber cell walls and is one of the most reliable indicators of strength, as well as several other properties, such as stiffness, joint strength, hardness, ease of machining, fire resistance and drying characteristics.

Durability

Durability of timber is the ability to perform its task for a required period of time. This means its performance when exposed to hazards such as decay (fungi) and insects (termites and borers). Timber species have different natural durability characteristics. Species are given a durability classification based upon their expected service life in the ground or outside above the ground when exposed to hazards.

Shrinkage and Swelling

The percentage of shrinkage and swelling varies between species. Shrinkage starts when cell walls of timber start to release water and swelling starts in a moist atmosphere when cell walls absorb water. Timbers that have thicker walls swell and shrink more than those with thinner walls.

As wood shrinks/swells, the loss in dimension is not the same in all directions. The longitudinal shrinkage (down the length of timber) is very minimal and it not usually regarded, however in the radial direction, (across the width of timber) depending on species, shrinkage is around 3% to 6% of the board width.

For more information see the detailed section Shrinkage and Swelling

Defects

There may be some of the natural and artificial defects in timber such as cross-grain, knots, checking and shakes, etc. All of them cause a decrease in the strength of the timber.