What Causes Wood to Warp?
A few months ago, you purchased a bunch of lumber to build some patio furniture. Now that you’re ready to tackle the project, you find some of the wood that you purchased and left in the garage is now warped.
Why Does Wood Warp?
Wood warps when the moisture content in the wood changes unevenly. Think of it this way: You have a 2×4 that gets wet. As it dries, one part of the board dries faster than the other, causing the drier area to shrink faster. That causes stress on the wood and can cause it to warp or bend.
While some types of wood have a relatively high moisture content, other types have a very low moisture content. The expanding then shrinking of wood is caused by both the wood’s moisture content and the humidity level in the air. As humidity increases, the moisture content of wood will also increase and the wood expands. As humidity decreases, moisture content will also decrease and the wood will shrink. This is known as the Equilibrium Moisture Content, or EMC.
The size of the wood, along with the grain can also impact EMC. Wood that is thicker takes longer to both absorb and lose moisture. Quarter sawn lumber will shrink and then expand about half as much as lumber that is flat sawn.
Some types of wood are less likely to warp than others. Cedar is a very dense species meaning it is less prone to cracking and warping. Fir is considered seasoned when it reaches EMC so it shouldn’t warp much. Redwood has a natural chemical that protects it against excess moisture.
Prevent wood from warping by storing wood in an area with proper ventilation. Lumber should also be stored in flat piles and stored in a cool, shaded and dry location.