Moisture in timber

One of the most important factors affecting the performance and properties of wood is its moisture content. The amount of water present in wood can affect its weight, its strength, its workability, its susceptibility to biological attack and its dimensional stability. It is estimated that over 80% of the in-service problems with wood are in some way related to its moisture content. The importance of the interaction between water and wood cannot be understated and, if not properly understood and taken into consideration can result in the need for expensive remedial measures.

Living trees and freshly felled logs contain a considerable amount of water. It is an essential chemical constituent of the wood not simply a remnant from water conduction in the tree. About 25 to 30% of the water in wet wood is chemically bound in varying degrees to the wood fibres; the rest is present in cell cavities. However the actual amount of water will vary with the species and time of year - some living trees and freshly cut or ‘green’ timber can contain more water than they do timber.

The weight of water in timber is expressed as a percentage of the oven dry weight (wt) of the wood:

Thus a piece of wet timber whose weight is half dry wood and half water will have a moisture content of 100%.

Above the 25 - 30% moisture content level, water fills or partially fills the cavities of the wood cells (fibres), see Figure 1. When wood dries, this water is lost first. This reduces the weight of the piece but does not change its dimensions. When the cell cavities are empty but the cell walls still retain all their bound water, the wood is said to be at fibre saturation point (fsp).

Further drying below fibre saturation point results in shrinkage of the wood as the walls of the fibres contract, see Figure 1. The process happens in reverse if dried wood is put into a wet or moist environment and it absorbs water.

Figure 1 What happens as wood dries