Wood is an elastic building material. In it's native form (as a tree), it can contain more than 50% water in the form of sap that flows from the roots to the limbs sending nutrients throughout the plant. With over half of the wood being water, this explains why wood shrinks when it dries.
After wood is harvested, milled and kiln-dried, it has a tendency to equalize to the humidity levels of the local environment. When in a dry area, such as the desert southwest of the United States, wood stock will eventually be much drier (and subsequently, a bit thinner in size) than in a tropical, humid environment, such as the islands of the Caribbean or Florida.
Why should this matter to you as a woodworker? If you buy stock from an online supplier (or one that is not in your local area), it would be wise to store the wood for a few weeks (at the very least) before beginning your project. This will give the wood time to reach a state of equilibrium with your local environment, which will subsequently lessen the possibility of the wood expanding or shrinking after you complete your project. This may be less of a problem if you buy the wood locally, but only if the supplier has had the stock on hand for a while. If they just received the shipment, you may still wish to store it for a period of time before beginning your project.
To learn more about shrinking and expansion of wood, along with some tips on how to plan for wood movement, read Allowing for Expansion and Shrinking.
(c) 2012 Chris Baylor, licensed to About.com Inc.
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