Considering Wood Movement
Running a sawmill and drying my own lumber has taught me a lot about wood movement. I know that some lumber, like cedar, dries quite flat and straight, and other lumber, like sweetgum, will dry like a potato chip. Quartersawn white oak lumber almost always dries with the ends pulling away from the center of the log, making a board with a nice sweeping bow. I also know that boards that are sapwood on one side and heartwood on the other will dry with a distinct cup. These are things that happen when wood dries from green to air-dry or about 20% moisture content, which is where the dramatic movement occurs.
Even after air drying and kiln drying wood still moves with humidity changes, and as woodworkers we have to consider this seasonal movement. This is the kind of thing I think about a lot, and recently I gave myself the chance to really think about it. I was building a table and the wood I wanted to use was not dry. It was white oak which sat on sticks air drying for a little while, but was still wet enough to be considered “green”. I decided that I new enough about wood movement to make it work, and since the table was a little rustic, that I should go for it.
The bases for the table were simple. I made sure all of the grain was going the same direction, so the shrinkage would happen evenly. It also worked out nicely that all of the pieces would have air movement on all sides so they would dry out at the same rate. So far all was good. I proceeded with the top. It was a glued-up top with breadboard ends (one board on each end perpendicular to the field). No big deal. If you do your breadboard ends correctly, everything can slide and allow the wood to move. I took all of that into consideration.
There was one thing I did not take into consideration. You see my brother tends to use my shop at night. I usually leave stuff in his way–after all, it is my shop. He has to move what I am working on to do his work, and he often does. He paid no attention to the fact that my wood was not dry and had no previous reason to, since I don’t usually use wood that is so wet. The parts that I carefully stacked with sticks so that they would have air movement on all sides were now just laying down flat on a table. In one night all of the parts developed an amazing cup because the sides exposed to the air were drying and the sides on the table were not. This all happened in one night. And since my brother usually works late, it was a short night at that. I was able to get the lumber back to flat by soaking the side that was up and drying the side that was down. All ended fine, but this created a lot of extra work and a lot of extra consternation on my part. And, I thought I worried about wood movement before.
This is an extreme example of wood movement, but the lessons learned apply even to dry wood. Make sure that pieces that need to stay flat have air movement on all sides, especially before the project is assembled. And try to hustle. If you take the time to make flat pieces, get them assembled before they can move. Remember, things can literally go bad overnight.
For fun here is the table the was moving on me. I had to go back twice to trim the breadboard ends flush as the boards shrunk, even after I left them 1/4″ short on each end to allow for some of the shrinkage.