Wood has been a beloved building material for thousands of years. It’s no wonder. This construction medium is known for its beauty, versatility, and variety. There may be no other building material on earth with more uses than wood. It’s used to construct colossal buildings, miniature carved figurines, and virtually anything in-between. One of the challenging facts about wood is that it moves continuously. Though mankind has come up with numerous methods of trying to constrain that movement, from fastening to sealing, nothing has been able to completely stop the movement of wood. Once you realize that wood is always prone to movement, you can learn how to predict and work with it in spite of this tendency.
Why Wood Moves
In order to understand why wood moves you have to remember that wood originally comes from living trees. When a tree is living and growing, it includes straw-like components that draw moisture and nutrients up from the ground to give the tree what it needs to grow and thrive. After cutting a tree down and sawing it into pieces of lumber, the wood will begin to gradually lose that moisture. It’s a slow process that takes place over time.
When the lumber is left outside and exposed to the elements, the wood can come into contact with water or moisture in the air. When this happens, the lumber will expand as it starts to retain moisture again. Over time, the wood will eventually reach a state of equilibrium, and it will start to move less. Motion can be further inhibited by processes such as kiln drying, sealing, and finishing. Even when these methods are employed, wood will still move as the straw-like components shrink and grow repeatedly. Once you understand that wood is always going to move, you can know what to expect when you start to build with it.
How Wood Moves
Wood moves in two basic ways: tangential shrinkage and radial shrinkage. Tangential refers to wood movement along the tree’s growth rings. When you’re dealing with lumber, this would refer to movement across the grain of the wood. Radial shrinkage is the wood movement longitudinally along with the grain of the wood. When dealing with lumber, this would refer to wood’s movement along the length of the board. Wood’s radial movement is usually far less significant than its tangential movement.
Various species of wood move at different rates depending on specific conditions. This means that certain types of wood will perform better for different purposes. Each species has its own tangential/radial movement ratio. Lumber made from wood that has a lower T/R ratio would be considered more stable than lumber made from wood with a high T/R ratio. This means that low T/R ratio wood expands and contracts longitudinally and vertically at a more uniform rate. Practically speaking, this means the wood won’t be as prone to cupping and warping.
The next installment of this two-part series of articles will explore how knowing about wood movement can help with choosing the right wood for different projects. It will also explain how to troubleshoot common problems caused by wood movement.