Wood flooring boards change dimension when environmental conditions associated with temperature and relative humidity are modified. Owing to the structure of wood used in flooring, the effects of contraction and expansion are greater on the width and thickness of the wood boards than on their length. Wood is a hygroscopic material made up of fibers that swell or shrink when humidity is absorbed or lost.

Specialists have established the contraction and expansion coefficient inherent in each species of wood. However, variations in dimension will never be perfectly uniform in every board used in a given surface of flooring.

The variations are also subject to the position of the growth rings of the trees used and the way in which each board of wood is cut. Some boards will change mainly in length, others more in thickness.

Normal Conditions	Excess Humidity	Loss of Humidity
This drawing illustrates how boards change in dimension, based on the way in which growth rings are laid out. The arrows indicate movement caused by changes. The thick arrows show the main reaction and the thin arrows show the secondary or minor reaction.