The ability of wood to adapt to climate change is one of its more admirable qualities. Understanding this quality is essential to a sturdy lasting structure. Every species of wood will move differently and every cut of wood will move on its own. These movements are predictable and controllable if not altogether preventable. Wood is going to move no matter how it is treated. Preparation to deal with that movement needs to happen before use in a project. If the correct precautions are not taken, the moving wood may cause health hazards.
The Wood Moves!
Wood moves in three directions (thank goodness only three). A piece of wood is best viewed as a bundle of straws. The fibers absorb and leak water according to the humidity of the air around them. This expansion and shrinkage is what causes the movement in wood. Knowing the exact amount a type of wood will move is important while using it. The lumber industry has data on every species to help.
Tangential Shrinkage: This is the movement of wood along its growth rings. Expansion or shrinkage in this direction is what causes swelling. A sticky door jam or difficult window is the result of tangential shrinkage. This is the most common movement type.
Radial Shrinkage: This is the movement perpendicular to the growth rings. The fibers are moving water from the outside in and reverse for the wood. The water moves along the radial or medullary rays of the wood to get nutrients to the interior of the tree.
Longitudinal Shrinkage: While wood will move in this way, stretching longer and shorter, it is the least amount of movement and not a concern.
The organic nature of wood is that it will move regardless of man’s intervention. Air and Kiln drying will stabilize wood, while some stains and seals help decrease movement. Completely stopping this movement will not happen. Thus, taking the right precautions when working with lumber is crucial. Any lumber should acclimate on the job site before being added to the job. This time sitting in the climate allows the wood to stabilize and settle into a normal state. Once that state is reached, the wood will not move as much and the project’s stability is insured.