Why wood moves and why you need to know

When humidity is high, wood absorbs moisture and swells. When humidity drops, wood shrinks. This “movement” is gradual, so you probably won’t notice weekly changes. But seasonal changes cause problems you can’t miss, like sticking doors, ugly gaps in woodwork or a crack in a tabletop. This movement occurs whether wood is fresh from the mill or centuries old, whether it was kiln dried or air dried, and it can exert a tremendous force that is almost unstoppable. But with a little knowledge of wood movement, you can minimise its potential consequences. Here we will explain the basics of wood movement and show you real-world solutions to the problems that it can cause.

Wood moves as its moisture content changes. Wood doesn’t move much lengthwise, so you don’t have to worry a lot about boards getting shorter. But a board can move quite a bit across its width. A board that’s 150mm wide during a humid summer might shrink by nearly 0.8mm in winter. That’s not much, but it’s enough to cause a crack in a tabletop or create gaps between floorboards.

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When wet wood dries, it shrinks. The amount of movement is determined by the type of wood and the degree of change in its moisture content. Applying a sealer or paint can moderate wood movement. But it’s nearly impossible to seal wood so completely that its moisture content stays exactly the same.

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Movement within a board isn’t uniform; one section might move more than another. That leads to warping, twisting and cupping. Most of these changes happen in the initial drying phase, but wood can change shape later, too.

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How a board is cut from the log also affects how much it moves. Quarter-sawing yields “vertical-grain” boards, while plain-sawn boards have “flat grain.” Inspecting the grain pattern on the end of a board will reveal whether the board has vertical grain or flat grain. A flat-grain board will move about twice as much as a vertical-grain board with the same change in moisture content. But because cutting quarter-sawn lumber is much less efficient, vertical-grain boards are expensive and can be hard to find.

Mitre joints hide end grain and look more refined. But the effect is ruined when the mitres don’t meet tightly. Huge changes in humidity, and wetting and drying from rain and sun, cause wood to move more outdoors than it would indoors. So whereas a mitre joint will look good for decades indoors, it may start to look really bad after only one season outdoors. That’s why it’s usually better to avoid mitres outdoors whenever possible. Use a butt joint instead.

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Deck boards can shrink or expand after they’re installed, depending on how much moisture they contained when you fastened them down. To allow for this, space wet treated boards with a 16d nail (3mm) and dry boards such as cedar decking with a carpenter’s pencil (7mm).

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If you’re installing an engineered wood floor, follow the instructions carefully – they include all the information you need about spacing. In general, floating wood floors that aren’t nailed or glued down require about a 12mm space around the perimeter and enough clearance at thresholds, doorjambs and other obstructions to allow movement. Solid wood floors also require at least a 12mm wide expansion space around the perimeter.

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One of the most common errors that beginning woodworkers make is to securely fasten wood tabletops to the underlying frame. Tabletops tend to be wide, and wood moves a lot across its width. Restricting the movement with screws or nails can cause the top to crack as it shrinks. To avoid this, use special tabletop fasteners or some other method that holds the top down but still allows the top to expand and contract.

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Since the relative humidity of your indoor space may be quite different from the humidity where your trim or flooring was stored, you should always allow time for the material to acclimate. The wider and thicker the trim or flooring boards are, the longer you should leave them in the space before installation. Thin, narrow trim may only take a day or two to reach equilibrium with the room’s relative humidity. Wide or thick boards should be left in the room for at least four days before you install them. Of course the trim or flooring will still move a little after it’s installed, but at least most of the change will have occurred beforehand.

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