Q: We are getting a lot of cupping in some of our lumber. What is causing this and what can we do to fix it?
A: Cupping, which is warp across the width of lumber, is a natural event for wood, with the bark side of lumber shrinking more than the heart side (or side closest to the center of the tree). This fact means that perfectly flatsawn lumber cups the most; perfectly quartersawn does not cup at all. Also, the closer the lumber is to the center of the tree, the greater the difference in shrinkage between the two sides and therefore the greater the cupping tendency. This fact means that lower grade lumber, which is usually closer to the center of the tree, will cup more than clearer lumber from further away.
When drying lumber, if the wood is dried fairly quickly, meaning at a low RH, then the outer fibers will be dry, which means they are fairly strong. These dry strong fibers resist cupping. However, if partly dried lumber is rewetted, which means the fibers become weaker, or if the drying conditions have a high RH, meaning that the fibers are not as strong, then cupping will be more severe.
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Gene Wengert, aka The Wood Doctor, troubleshoots wood related problems, and explores lumber and veneer qualities and performance, species by species, in Wood Explorer, inside FDMC's Knowledge Center.
In my experience, cupping is often the result of mixing MCs in a kiln. The high RH used to protect the wetter lumber will weaken the fibers on the drier lumber and the drier lumber will cup.
Cupping is also accentuated if the lumber is over-dried. Even if the MC is increased after over-drying, the cupping does not moderate much. Over-drying can occur when MCs are mixed in the kiln and the drier lumber's MC is not monitored.
So, as a short answer, check for anything that you are doing to rewet the partly dried lumber and check to see if some of the lumber is being over-dried.