Why does wood shrink and swell?
Timber is hygroscopic and never stops responding to the air around it. What that means for width, thickness and length, and why almost every timber problem in a house traces back to it.
- Difficulty
- beginner
- Time
- 10 min
- Read
- 4 min
Short answer
Wood absorbs and releases water vapour from the air until it reaches equilibrium with it, and it changes size as it does. It moves substantially across the grain — most across the width of a flat-sawn board, about half that through the thickness — and effectively not at all along its length. That is the entire explanation for gapping floors, split panels, sticking doors and cracked joints.
Timber is a bundle of cellulose tubes, and the walls of those tubes take on and give up water depending on the humidity around them. This never stops. Every piece of timber in your house is a fraction wider in a damp August than in a dry February with the heating on, and everything that goes wrong with timber in service is a consequence of that.
Step by step
- Understand equilibrium moisture content.A piece of timber constantly exchanges moisture with the air until its moisture content is in balance with the humidity around it. In a heated British house that equilibrium is roughly 8 to 12 per cent; in an unheated garage or outdoors it is far higher. Move the timber and the target changes.
- Learn the three directions and their very different magnitudes.Tangentially — around the growth rings, which is across the width of an ordinary flat-sawn board — movement is greatest. Radially — through the thickness, or across a quarter-sawn board — it is roughly half. Longitudinally, along the length, it is so small it can be ignored.
- See why boards cup.On a flat-sawn board, the face that was nearer the outside of the log has more tangential movement in it than the face nearer the centre. As the board dries, that face shrinks more and the board curls away from the heart side.
- See why panels split.A wide board glued or screwed into a rigid frame cannot get narrower when it wants to. Something has to give, and it is the board — it splits, usually starting at a fixing.
- See why cross-grain joints fail.Where one piece has its grain running across another's, one wants to move and the other does not. Fixing them rigidly together guarantees a split or a broken glue line eventually. Traditional joinery is largely a set of solutions to this problem.
- Understand the fibre saturation point.Below roughly 30 per cent moisture content, water is leaving the cell walls and the timber changes size. Above it, the extra water sits in the cell cavities and the timber does not shrink further. This is why very wet timber loses a lot of weight before it starts visibly shrinking.
- Accept that you cannot stop it, only allow for it.No finish stops movement — a good film finish slows the rate of exchange so the timber responds to the season rather than to the week, but it does not change the endpoint. Design so the movement has somewhere to go.
Tips
- Wide boards move a lot; narrow boards move a little. A panel glued up from several narrow boards moves the same total amount as one wide one, but the cupping is much better behaved.
- Quarter-sawn timber moves roughly half as much across its width as flat-sawn, which is why it costs more and why it is worth it for wide, flat, visible parts.
- Man-made boards — MDF, plywood, chipboard — move very much less than solid timber, and are the reason modern furniture can have wide flat panels that stay flat. They still swell if they get properly wet, and that swelling is permanent.
Common mistakes
- Fitting timber straight off a cold delivery lorry into a heated room — It is at a much higher moisture content than the room will impose on it. It shrinks in place over the following weeks, opening every joint you just cut so carefully.
- Screwing a wide solid top down through a fixed frame at both edges — It cannot get narrower, so it splits. Use slotted holes, buttons or figure-of-eight fasteners so the top can move across its width while staying held down.
- Finishing only one face of a board — Finishing slows moisture exchange, so a board finished on one face only responds faster on the bare face and cups. Finish both faces, even the one nobody sees.
Questions people ask
How much does timber actually move?
It varies by species and by how the board was cut, but a rough working figure for a flat-sawn softwood board is a change of around one to two per cent of its width across the swing between a damp summer and a dry heated winter. On a 300mm wide board that is a visible few millimetres; on a 50mm stud it is nothing.
Does old timber stop moving?
No. A three-hundred-year-old beam still responds to humidity. What old timber has done is finished its initial drying shrinkage, so it is stable in the sense of not shrinking any further overall — but the seasonal swing continues for the life of the timber.
Do plywood and MDF move?
Far less across their width, because the plies or fibres are arranged in opposing or random directions and restrain each other. They do change slightly with humidity and they swell permanently if wetted, which is a different failure mode from solid timber's reversible seasonal movement.