Many people are often surprised to learn that wood moves with the seasons. The reason for this movement lies in the fact that wood is a hygroscopic material. This means that it is always in a state of flux with its environment as it is constantly pulling in moisture from the air or releasing moisture back to the atmosphere. You probably have noticed this in your house without realizing what was behind it. For instance, have you ever had a cabinet door or a drawer on a dresser that was difficult to open in the summer and loose in the winter? This is caused by the wood’s tendency to expand in the summer months when the air is relatively humid and its tendency to shrink in the winter when the air is quite dry.
When building with wood one must take this seasonal change into account as well as considering the way in which the wood naturally wants to expand and contract. It is foolish to proceed with a flawed design assuming that the expansion and contraction will not affect the work. Even with adequate glue, screws, nails, and other restraining devices, a flawed design will have sufficient wood movement over time to break these bonds or split the wood itself.
Wood expands and contracts most across its grain and somewhat in a radial manner. There is little to no expansion or contraction with the length of the grain. The movement issues are greatly limited or rendered negligible in man made products such as plywood. This is due to the fact that the cross layering of the plys freezes the natural tendency for the wood to shrink across its grain as each layer is keeping its neighbors in tension. When building with plywoods and MDF, wood movement issues are not an issue of much concern and this is one of the chief qualities of these products in addition to their ability to span large areas and provide strength.
Many people desire the use of solid wood because of its natural beauty and innate warmth. Wood movement issues must be addressed when using solid wood and it is necessary that a customer be educated as to why a given design will or will not work. When movement is considered from the beginning of a project one not only accepts it as an eventual reality but actually takes precautions and steps to greatly minimize its potential destructive force upon the project.