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.

The top on this table shrunk more than 1″ in width as it dried.

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About wunderwoods

Hi! My name is Scott Wunder and I am the owner of WunderWoods Custom Woodworking. We build wine cellars, built-ins and furniture from local woods, here in St. Louis, MO. Recently, I finished a three-year term as the President of the St. Louis Woodworkers Guild, which had me writing a monthly article for our newsletter. I love to write, especially about wood, and found that I still had more to say. Every day I run into something wood related that I realize some of my customers don't know and this seems like a great forum for sharing what I have learned (instead of telling the same story to each person). The main thing to remember is that I try to keep it light and as my wife always reminds people that have just met me, "He is joking."

4 responses to “Considering Wood Movement”

  1. Bob Moske says :

    Scott,
    Does MDF move more than poplar?

    • wunderwoods says :

      Bob,

      Do you have an application mind? I want to give you a complete answer but there are several ways that I could compare the two. Give me as much info as you can, so I can focus my response. Thanks

      • Bob Moske says :

        Scott,
        We are using 5 1/4″ MDF baseboard from Koetter in the corridors of a nursing home. So, alot of full length, 16′ or so pieces. With the season change the building has a humidity swing that opens up our scarf joints pretty bad. Shrinkage over 16′ can add up pretty quick. If we were to use the same base in a finger jointed poplar or pine, do you think we would have the same issue? Problem is the test period is six months or so. Let me know your thoughts and questions. Thank you

      • wunderwoods says :

        Bob,

        MDF is often recommended for its stability based on the fact that it shrinks and swells uniformly in all directions. Solid wood is considered less stable because the wood moves different amounts in each of the three directions. In this case, however, that is not such a bad thing because one of those numbers, which is the movement in length, is basically zero in solid wood. The movement in the length of MDF is greater than zero because MDF is made from wood fibers that are going in all directions. I have no hard numbers on this to officially compare the two, but I am confident in recommending solid lumber over MDF in this situation.

        As a footnote, I have made panels for wainscotting out of MDF and run into a similar situation. The MDF shrinks the same amount in all directions and produces noticeable seams in the winter. In this wainscotting, the panels are MDF and the frames just happen to be made of poplar, which puts our two contenders side by side and indicates the MDF is shrinking more.

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