While technically, a tree is no longer living once it’s cut down, it continues to move and give to its environment, as long as it remains in solid form. We can restrain its movement in various ways, yet the very fibers of its being continue to move. Instead of trying to keep it from moving, we do best when we study how and why it moves and respond to its movement by careful installation that allows for necessary movement. The more we understand about this important topic, the more equipped we’ll be to harness the natural strength and beauty which lumber offers us.
Basic Wood Movement
Acting like a bundle of straws, wood fibers constantly absorb and shed moisture from the ground, transporting water and minerals throughout the tree. When a tree is cut down and lumber is milled, the straws continue to act as conduits for moisture. At first, they shed water from their ends, resulting in shrinkage and narrowing of the boards. If they’re in an area where they’re subjected to rain or the climate becomes more humid, they suck in more water, causing them to grow.
Essentially, the wood will always be trying to come into equilibrium with its environment. This constant shedding-and-absorbing-moisture process can be slowed through many methods, including Kiln Drying, End Sealing, and various Finishing techniques. However, nothing can completely stop wood from expanding and contracting. The best we can do is to anticipate this movement as we build.
Two Kinds of Wood Movement
Two key terms related to wood movement refer to the direction of movement: tangential and radial shrinkage. In the above discussion of wood as a bundle of straws, Tangential shrinkage refers to wood movement along growth rings, or the swelling of the straws. Radial shrinkage is the movement of wood along the medullary rays, or perpendicular to growth rings.
As you have probably already guessed, tangential shrinkage is more significant than radial shrinkage; however, both are still significant. The ratio of tangential movement to radial movement, or T/R ratio, will reflect the stability of a given species. The more closely the numbers match one another, the less the chance of warping and cupping. So you’ll want to find out the T/R ratio of the species you’re using and make the appropriate allowances.
Allowing for Wood Movement
While we know that wood moves predictably, it does not do so uniformly. The cut ends, in particular, will dry out more quickly than the middle — which is precisely why wood tends to twist, cup, bow, and warp. Enough time must be allotted for the middle to dry out as much as the ends—not just after the lumber has been milled, but each time it’s moved.
Whether the change in locale is from one climate to another or from one end of the lumber yard or job site to the other, any adjustment can become an opportunity for warping. While the moisture-exchange process will never completely stop, it will slow significantly once the board reaches a point of equilibrium with the environment and itself.
To learn more about wood movement, continue to Part 2.
Read the Entire Series
- Imperfect, Perfect Lumber: Lumber Continues To Move, Part 1
- Imperfect, Perfect Lumber: Lumber Continues To Move, Part 2
- Imperfect, Perfect Lumber: Large Timbers Will Have Cracks, Part 1
- Imperfect, Perfect Lumber: Large Timbers Will Have Cracks, Part 2
- Imperfect, Perfect Lumber: Lumber Continually Gives, Part 1
- Imperfect, Perfect Lumber: Lumber Continually Gives, Part 2
J. Gibson McIlvain Company
Since 1798, when Hugh McIlvain established a lumber business near Philadelphia, the McIlvain family has been immersed in the premium import and domestic lumber industry. With its headquarters located just outside of Baltimore, the J. Gibson McIlvain Company (www.mcilvain.com) is one of the largest U.S. importers of exotic woods.
As an active supporter of sustainable lumber practices, the J. Gibson McIlvain Company has provided fine lumber for notable projects throughout the world, including the White House, Capitol building, Supreme Court, and the Smithsonian museums.
Contact a representative at J. Gibson McIlvain today by calling (800) 638-9100.