Wood is always taking on and losing water
Living trees hold a lot of water, both inside the cell cavities and in the cell walls. As lumber dries, the free water in the cavities leaves first and causes little dimensional change. Once the cell walls themselves start to lose water, around a moisture level often called the fiber saturation point, the wood begins to shrink. Even dry wood keeps trading moisture with the air, absorbing it when humidity is high and releasing it when humidity drops. This is why a board that was the right size in summer may be a little narrower in a heated house in winter.
Which direction does it move?
Wood does not change evenly in every direction.
- Along the grain (length): very little, usually so small it is ignored in ordinary work.
- Across the growth rings (tangential): the most movement, often roughly twice the radial amount.
- Across the radius (radial): less than tangential but still noticeable.
In practice, a wide board changes in width far more than in length. This is why a long tabletop can get slightly wider or narrower with the seasons while staying the same length. The difference between the two directions is also why flat-sawn boards tend to cup, and why quarter-sawn boards, whose growth rings run more nearly perpendicular to the face, tend to be more stable. The way wood grain runs through a board matters for movement as well as for cutting.
How much movement to expect
It depends on the species, the width of the board and the change in humidity. As a hypothetical example, a wide board could change by a few hundredths of an inch to well over an eighth of an inch across its width over a year in a climate with large seasonal swings. A narrow board changes proportionally less. Rather than relying on a general figure, many woodworkers look up shrinkage data for their species and measure humidity in the space where the piece will be used. A simple hygrometer shows the humidity of your room, and a moisture meter shows the wood’s level.
What goes wrong when movement is ignored
- Cracked panels: a solid panel glued rigidly into a frame cannot shrink without splitting.
- Open joints: cross-grain glue joints, where one part moves and the other does not, may fail over time.
- Sticking doors and drawers: swelling in humid months can bind parts that fit well in dry months.
- Buckled floors and decks: boards with no gap to expand into can push against each other.
- Warping: one-sided moisture exposure makes boards cup, as seen in warped and twisted boards.
Designing for movement
The goal is not to stop wood from moving, which is unrealistic, but to let it move without causing damage.
- Float panels. In frame-and-panel construction, the panel sits in a groove, unglued, with a little space to grow and shrink. A small dab of glue at the center can keep it centered.
- Use slotted holes or buttons. Tabletops attached with screws through oversized or slotted holes, or with sliding fasteners, can shift with the seasons.
- Avoid cross-grain gluing of wide parts. Gluing a wide board across the grain of another is risky. Wood joinery methods often account for this. Plywood is more stable and is sometimes used where cross-grain parts must be fixed.
- Leave gaps in flooring and decking. Small gaps between boards and expansion room at the edges are common practice for flooring.
- Finish all surfaces. A finish slows the exchange of moisture but does not stop it. Coating every face evenly, including the underside, keeps movement more even. Choices in wood finishing influence this.
- Choose stable cuts. Quarter-sawn or rift-sawn boards move less in width, though they are often more expensive.
Start with settled wood
Building with wood that has already adapted to your environment gives movement less to do afterward. The article on lumber acclimation describes how to let boards settle. Even so, seasons keep changing, so allow for it in the design.
A realistic mindset
Small amounts of movement are normal, and tiny gaps that open and close are not necessarily a defect. Aim to build pieces where movement shows up as a harmless change, not a crack.