Which way does wood move, and how much?

Timber is dead but it is not inert. The cell walls take up and give off water vapour until they are in balance with the air around them, and as they do, the cells get fatter or thinner across their length. Because the cells run along the tree, that dimensional change happens across the grain and almost never along it. Once you hold that one fact, table tops, panelled doors, breadboard ends, drawers that stick in winter and cracked carvings all become the same story.

Step by step

1. Learn the three directions and their relative sizes.Along the grain: negligible, small enough to ignore in furniture. Radially, across the rings: moderate. Tangentially, around the rings: roughly twice the radial figure. Those ratios hold across almost all species; only the absolute numbers change.
2. Know how the board was cut, because it decides the behaviour.A flat-sawn board has the rings running roughly parallel to the face, so it moves mostly in width and tends to cup away from the heart. A quarter-sawn board has the rings running through the thickness, so it moves less in width, stays much flatter, and mostly changes in thickness.
3. Understand that indoor humidity is the driver, not rain.The moisture content wood settles at follows the relative humidity of the air around it. A centrally heated house in January is very dry; the same house in a humid August is not. Timber in an unheated garage lives at a different equilibrium again.
4. Expect the biggest movement in the first year after a change.Timber bought from a heated merchant and taken to an unheated garage, or vice versa, moves substantially as it re-equilibrates. Let boards sit stickered in the room they will live in for a week or two before working them.
5. Design so movement is unrestrained across the grain.Anything that fixes a wide board rigidly across its width — a screwed batten, a glued cross-piece, a rigid frame — is fighting a force that will win. The wood splits, or the fixing tears out.
6. Do not worry about movement along the grain.Rails, stiles and legs do not change length in any way you need to allow for. This is why frame-and-panel construction works: a stable frame of long-grain members carries a floating panel that is free to move.
7. Use the movement to your advantage where you can.A drawer fitted snugly in a damp month will run freely in a dry one. Fit drawers with a whisker of clearance in summer, and a little more if the piece will live somewhere humid.

Tips

• Species differ substantially. Oak and beech move a great deal, mahogany and teak comparatively little, and stable species are what traditional makers reached for on wide unsupported surfaces.
• Plywood and MDF are dimensionally stable across their width because the movement is cancelled out by cross-laid layers or ground fibre. That is exactly why they are used for carcass backs and drawer bottoms.
• You can estimate movement for a species from published shrinkage figures, but a simpler practical rule is to allow a few millimetres of movement per 300 mm of width across the grain on solid timber indoors, and design so it does not matter if you are wrong.

Common mistakes

• Treating kiln-dried timber as finished moving — Kiln drying brings the timber to a moisture content, not to a permanent state. It carries on responding to the air around it for the rest of its life.
• Gluing a solid wood cross-piece across the width of a panel — The panel changes width and the cross-piece does not. One of them has to give, and it is usually the panel, which splits.

Questions people ask