In an effort to bridge the “expectation gap” that many experience when viewing wood, we’re going to take a look at wood movement. We can’t change it, but we can learn to expect it and use it to our advantage. When we do, we’ll not only be in a better place to appreciate the natural beauty of real wood, but we’ll avoid some of the common pitfalls associated with it.
Let’s start with an overview of wood movement.
Wood Movement Basics
Imagine a bundle of straws, and you’ll get the basic idea; the way a tree is designed, its fibers soak up moisture from the ground and transport it throughout the tree. Those same straw-like fibers are still present once a tree is harvested and milled. When lumber is milled, it will begin to shed water from the freshly cut ends.
Back to our straw analogy, each straw-like fiber will shrink, becoming narrower as it sheds moisture. Wood fibers will continually swell as they take on moisture and shrink as they shed it, until the wood comes into equilibrium with the moisture level of its environment.
Lessening Wood Movement
Even once a board comes into an equilibrium with the surrounding humidity level, its fibers will continue to expand and contract as the humidity levels shift. This process can be slowed in a variety of ways, but it cannot be stopped; in fact, any attempts to stop it may result in case hardening, which in turn can cause structural problems such as cracking. While proper kiln-drying can help alleviate wide swings of moisture content, slow and steady drying is key.
Wood Movement According to Species
Two basic types of shrinkage occur: tangential shrinkage and radial shrinkage. Tangential shrinkage is how much the wood moves along the growth rings, amounting to side-to-side swelling of the straws; as you might guess, this is the greatest degree of shrinkage.
Radial shrinkage is wood movement that’s perpendicular to growth rings, or along the medullary rays that were once used to transport nutrients to the interior parts of the tree. Each wood species has a general ratio of tangential to radial shrinkage, and the closer the numbers are to being equal, the more stable the species. Species with T/R ratios with numbers far different from one another will be more likely to experience warping and cupping.
You might be thinking that if you can’t eliminate wood movement, what can you do? You can anticipate it and take steps to lessen it. As you anticipate wood movement, you’ll be lessening that “expectation gap” we’ve discussed previously. You’ll also be able to plan well, realizing what you’re up against. As far as what you can do to safely reduce the amount of movement, we’ll discuss the steps you can reasonably take in Part 2.
