Q. Is There a Way to Predict Which Pieces of PT Lumber Will Twist?
I find that a lot of the pressure-treated 4x4s and 6x6s I use twist badly after installation as they dry. It’s not a structural problem, but it looks terrible. Is there any way to recognize pieces that are likely to twist so they can be culled before the problem comes up?
A.Alexander Schreyer, a structural engineer and lecturer in wood technology at the University of Massachusetts Amherst, responds: The twist you’re seeing has two main causes. First, PT lumber contains a lot of water from the treatment process that needs to dry out. Second, the larger-sectioned square members are typically cut from the center of a log — you’ll often see the pith at both ends. The wood cells in the first few growth rings are prone to distortion as they dry, especially when the growth rings are wide.
To minimize twisting, select pieces that have the pith as close to the center as possible at both ends, which indicates that the grain is straight. If the post doesn’t come from the center of the log, look for pieces where the growth-ring orientation at the ends closely parallels one of the faces (see illustration, below). Again, a matching pattern at the ends indicates straight grain.
If you have time, stack the posts on stickers to allow them to air-dry before use. Coating the end grain with paint will promote even drying, and weighting the stack will help restrain movement as the wood dries.