If you work with wood, understanding that it moves and the different ways that it can move are imperative. While the wood movement, expanding and contracting, isn’t always easy to predict, it helps to understand how the seasons play a role. If you don’t account for this and you aren’t prepared for it, your projects can end up being completely destroyed. What is seasonal wood movement? Why does it happen? How can you plan for it? Keep reading for the answers to these questions.
What to Know About Seasonal Wood Movement
What is seasonal wood movement?
Wood movement describes the way in which wood changes its shape and structure as a response to changes in its surrounding environment (temperature and humidity). Movement can happen while the wood is being processed (like when it is being kiln-dried, for instance), as well as after it has been crafted into a finished project (a chair, a table, or a dresser, for example).
Why does seasonal wood movement happen?
As most people know, wood comes from trees, and trees are living things. Like all living things, trees are derived from cells, and therefore, pieces of wood contain cells. A substance known as lignin holds the walls in the cells together. It is lignin that is responsible for giving wood its strength, durability, and rigidity. In other words, it’s why wood has been used to create a wide range of products for millennia.
Though lignin is vital, it is important to understand that it can be impacted by changes in air temperature and humidity levels. When the surrounding air is hot and humid (high moisture levels), lignin expands, and as such, the wood swells. When the weather is cool and dry, lignin contracts, which makes wood shrink. In other words, as the weather changes, the lignin that holds the cell walls in wood together expands and contracts. In turn, the wood itself expands and contracts as well.
How does wood move?
There are several ways in which the weather can affect wood movement. Some of the most common examples include:
Width-wise
The width of the wood is most affected by the weather, and its thickness is least affected.
Outward swelling
Wood warping is most commonly caused by outward swelling. When the wood swells, big gaps between the boards can occur. Or, boards that have been mounted together can start pulling away from one another. Furthermore, the wood can split along the grain.
Buckling and warping
Temperature and humidity can completely change the shape of the wood. When straight boards pop up and twist, it is known as warping. This can cause a variety of issues. Buckling occurs when wood bends and curls as a result of high humidity levels.
Planning for Seasonal Wood Movement
While you can’t stop wood movement from happening, there are things that you can do to avoid major issues. Here are some tips to help you plan for some popular wood projects.
Plan for movement in deck boards
Accommodate movement in wood deck boards by spacing out moisture-treated boards with 1/8 inch nails and dry boards with a carpenter’s pencil.
Plan for expansion space in floor planks
Generally speaking, floating wood floors that aren’t glued or nailed down need about ½-inch of space along the perimeter. Floating wood floors also need enough clearance at obstructions, such as doorjambs and thresholds. Solid wood floors should also have a minim of a ½-inch wide expansion space around the perimeter.
Allow for acclimation
Because humidity levels can vary significantly from the space wood was stored to its final resting place, you should allow the wood to acclimate. In other words, bring the wood to the space where it will be installed and allow it to sit before working with it. Wider and thicker boards will need more time to acclimate, while narrower, thinner boards will need less time.
Skip wide boards
While there may be times when you will need or want to use wider wood boards, it’s important to understand that the wider the board is, the more it will move. That movement will be visible. As an alternative, instead of using wide boards, consider gluing together a collection of narrow boards to create one wide board, which will end up being more stable.
Pre-finish tongue-and-groove
If you’re working with tongue-and-groove, understand that it’s going to expand and contract. As a result, the gaps will change. If the boards aren’t pre-treated, exposed raw wood on the tongue will be visible when the wood shrinks. By finishing the boards before you work with them, you can eliminate this issue.
Additional reading: The Effects of Heat on Architectural Woodwork (AWI)
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