Moisture and Movement

Wood, as a natural product of the plant kingdom, contains water within its cellular structure, as a necessary and important element of its physical integrity. An important part of the timber supply process is to dry the wood to an equilibrium moisture content appropriate to trade storage, pending final sale. Taking wood into an enclosed and warm environment such as a house, office or shop will usually provide conditions under which there will be a further migration of moisture from the timber. There are also likely to be changes between the seasons, with moisture being taken up from the air through the summer months and lost again in the winter. The effect of these adjustments to match atmospheric humidity may be observed in dimensional changes to width and to a lesser extent, thickness. This expansion and contraction may be even or uneven and can lead to unpredictable distortion such as cupping, bow, cast and twist.

We obviously hope that distortion will not occur and on most occasions there is none. If present it is usually slight and of no practicable consequence. However, one should not be surprised if it does arise since nature will always exert its inexorable and unpredictable influence. In being aware of timbers need to move it is wise to plan for this to be accommodated within your design.

This is especially important with wide timber jointed surfaces such as kitchen worktops, tables and door panels when any movement is accentuated by the great overall width of the article.

The main points to be considered are the possibility of the back edge of a worktop contracting away from an abutting wall leaving a gap and the need to fix the surface with a flexible system; if free movement is denied, it is likely that the top will pull against the fixings resulting in serious and spectacular splitting. One should also be wary of a heat source such as a cooker hob or built-under oven, refrigerator or radiator causing local and uneven moisture movement, shrinkage and splitting. When finishing timber in general and jointed surfaces in particular, it is generally considered a good idea to apply the finish at an early stage, to all sides and not just to the visible areas. This allows a more even and controlled migration of moisture. Much of the above discussion also applies to flooring.