Wood movement: how much will your board shrink or swell?
Wood moves all its life: a 90 cm oak top can lose over a centimetre of width between the workshop and a heated living room. This tool quantifies that movement, species by species, so you design joinery that accepts it instead of cracking.
Expected change (negative = shrinkage)
-0.350in
- Yearly seasonal movement (±4% moisture)
- 0.350in
Large solid panel: the design must let the wood move (frame and floating panel, buttons or slotted cleats), or it will split.
The method
Below roughly 28% moisture content, water leaves the very walls of the wood’s cells, which tighten: wood shrinks as it dries and swells as it takes moisture back from the air. In the working range (6–20%), the change is nearly proportional to the moisture difference.
Each species has its coefficients, measured in the lab: so many percent of width per percent of moisture. Direction matters as much as species: tangential movement (flatsawn boards) is about double the radial (quartersawn) — hence cupping.
The second result gives an ordinary year’s movement: between humid summer and heated winter, indoor wood moisture typically swings ±4%. That is THE figure for sizing gaps.
In the formula: w the width (mm), c the species and cut coefficient (per % moisture), M₁ and M₂ the starting and destination moisture contents (%).
change (mm) = w × c × (M₂ − M₁) · yearly movement ≈ w × c × 4
Good to know
- This tool is for furniture making — tops, panels, drawers. Anything LOAD-BEARING (joists, beams, mezzanines) belongs to a professional’s structural design: that is not what this is.
- Wood barely moves along the grain (30 to 50 times less than across it): only width and thickness are in play.
- A finish (varnish, oil) SLOWS moisture exchange but does not stop it: it smooths the movement over the season, it does not remove it.
- Without a moisture meter, use the landmarks: dry merchant stock ≈ 10–12%, heated interior ≈ 8%, sheltered exterior ≈ 15%. An entry-level meter pays for itself with your first glued panel.
- Acclimatise the wood: two weeks in the destination room before machining beats calculating the movement… after the fact.
FAQ
- Why do tabletops crack in winter?
-
Heating dries the air, the wood loses 3–4% moisture and wants to shrink by several millimetres. If it is rigidly screwed to something that does not move (a metal base, glued battens), the stress exceeds its cross-grain strength — and it splits right where it is held. The cure: fixings that slide (slotted holes, buttons, dovetailed battens).
- Flatsawn or quartersawn: how do I tell?
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Look at the end grain. If the rings run roughly parallel to the wide face (lying arcs), it is flatsawn: cathedral figure, maximum movement, prone to cupping. If they run perpendicular to the face (straight, tight), it is quartersawn: ray fleck on oak, movement halved, a stable board.
- Do plywood and glued panels move too?
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Plywood barely: its crossed plies cancel each other out (that is its whole point) — count ten times less than solid wood. A solid-wood glued-up panel, however, moves like solid wood: gluing staves does not stop physics, it only distributes the distortion.
- How much gap around a floating panel?
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The calculated seasonal movement, split between both sides, plus margin: for a 400 mm flatsawn oak panel (≈ 6 mm yearly movement), leave 3–4 mm in each groove. And NEVER glue the panel in the groove — one pin at the centre keeps it centred.
- My species is not in the list: what now?
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Pick the closest species by density and behaviour: fruitwoods behave like walnut, common softwoods like Scots pine, dense exotics vary wildly (ipe moves little, eucalyptus a lot). For a demanding project, look up your species’ exact coefficient in the technology sheets (Wood Handbook, Tropix).