Wood Expands and Contracts: Designing for Seasonal Movement

Wood moves. Accept this truth or fight your projects forever. Every board in your shop is responding to the humidity right now, swelling when moisture increases, shrinking when it decreases. Ignore this movement, and your carefully crafted joints will self-destruct by spring.

The Science of Movement

Wood cells are essentially bundles of tiny tubes that absorbed water when the tree was alive. After drying, these tubes still absorb and release moisture from the air. As they absorb water, they swell. As they release it, they shrink.

The movement isn’t uniform. Tangential movement, across the growth rings, is typically twice the radial movement, through the rings. This differential causes flat-sawn boards to cup and quartersawn boards to stay flatter. Neither stays still.

Movement along the grain is negligible, usually less than 0.1%. This is why we can use long rails and stiles without worrying about length change, but must account for width change in panels.

Measuring Your Risk

A 12-inch-wide red oak panel might vary by 1/4 inch or more between summer humidity and winter heating season. Cherry moves less, around 3/16 inch. Teak, one of the most stable woods, might only move 1/8 inch. These numbers assume typical indoor humidity swings from 30% to 70%.

Your specific environment matters more than averages. A house with forced air heating and no humidification can drop to 20% relative humidity in winter. That same house in a humid summer climate might hit 80%. The wider the swing, the greater the movement.

Joints That Accommodate Movement

The floating panel is the classic solution. The panel sits in grooves in the frame, free to expand and contract without stressing the structure. Never glue a panel into its frame unless you want cracks or buckles.

Breadboard ends use a similar principle. The tongue along the panel edge slides within the breadboard groove. Only the center should be glued; the outer portions must float on elongated slots that allow the panel to move beneath the fixed breadboard.

Tabletop fasteners, the metal Z-clips or wooden buttons, connect tops to aprons while allowing cross-grain movement. Screws through slotted holes accomplish the same purpose less elegantly.

Grain Orientation Strategy

When joining solid wood, align grain directions so movement is parallel. A frame-and-panel door works because all frame members move in the same direction while the panel floats independently.

Cross-grain construction, where one board’s grain runs perpendicular to another’s, invites disaster. The classic example is attaching solid wood edging across a plywood panel. The plywood stays stable while the solid wood tries to move, resulting in gaps, buckling, or splits.

Seasonal Building Considerations

Building in summer humidity and delivering to winter heating creates the worst scenario. The wood you’re cutting has extra moisture that will leave during winter, causing everything to shrink. Drawers stick now but will have gaps later.

Ideally, acclimate your lumber to its final environment before building. Let boards sit in the destination space for several weeks. When that’s impractical, build during the season closest to average conditions and accept some variation at extremes.

Design Solutions

Wide panels are more problematic than narrow ones. Two 8-inch boards joined together move twice as much as a single 8-inch board. Consider whether your design can use narrower panels or additional frame members to manage movement.

Strategic placement of fixed points controls where movement appears. Glue the center of a panel and let the edges float, and the expansion splits evenly to both sides. Glue one edge, and all movement goes to the opposite edge. Choose based on where gaps would be least visible.

Wood movement isn’t a defect to overcome but a material property to respect. Design with movement in mind, and your furniture will look as good in ten years as it does today.