When does wood expand?
The Wood Guide · Calculating with wood
Wood knowledgeWhen does wood expand?
The short answer: when the humidity rises. The useful answer is how much, in which direction, and what you do about it.
- Moisture, not heat
- Tangential ≈ 2× radial
- Length is negligible
The short answer
Wood expands when it takes up moisture, and it takes up moisture when the relative humidity rises. Temperature plays a part, but a much smaller one than most people think.
Unlike metal — which reacts mainly to heat — wood moves almost exclusively with moisture. A wooden tabletop in a warm room with dry air shrinks; the same top in a cooler room with humid air expands. The thermometer tells you nothing about it; the hygrometer does.
And wood does not expand evenly. Across the width of a board it moves noticeably, along its length hardly at all. That difference is the key to every design that works with solid wood.
Tangentially — along the growth ring, usually the width of a flat-sawn board — the movement is greatest. Radially, across the ring, it is about half that. Along the length it is so small that you can ignore it. That is why a board gets wider and narrower but never longer.
- Main cause
- change in relative humidity
- Secondary cause
- temperature — much smaller effect
- Tangential movement
- the largest; across the board width
- Radial movement
- about half of that
- Longitudinal
- negligible
- Rough rule of thumb
- about 1% change in width per 4% change in moisture
To be fair: you cannot switch wood movement off, only plan for it. Lacquer, paint and oil slow down moisture exchange but do not stop it — a lacquered top moves more slowly, not less. Any design that fixes solid wood rigidly will sooner or later tear itself apart.
Work it out
Examples
Rough rule of thumb from the page: ± 1 % change in width per 4 % change in moisture, measured tangentially. Varnish, paint and oil slow the movement down, but do not stop it.
More about how wood moves →Exactly how much does it move?
A useful rule of thumb in practice: allow for about one per cent change in width per four per cent change in moisture content, measured tangentially. For an oak top 80 centimetres wide, a change from 8 to 12 per cent moisture therefore means about 8 millimetres — enough to make a drawer stick or a joint open up.
| Situation | Moisture content | Consequence |
|---|---|---|
| Heated home, winter | 6 to 8% | Wood shrinks; gaps open, drawers run loose |
| Living space, summer | 10 to 12% | Wood expands; drawers and doors stick |
| Sheltered outdoor space | 12 to 16% | Noticeable seasonal movement |
| Exposed outdoors | 15 to 20% | Greatest movement; gaps are essential |
| Permanently damp | above 20% | Rot risk — fungal growth starts here |
In quartersawn wood the tangential direction runs through the thickness instead of across the width. As a result it moves about half as much across the board width as flat-sawn wood. For a tabletop that has to stay flat, that is worth the money.
How you notice it
Doors and drawers that stick
In summer, when humidity is high. In winter the same doors run loose and you see gaps.
Gaps in a floor
A wooden floor shrinks in winter and gaps appear between the boards. In summer they close again. That is normal, not a defect.
A top that cups
If one side takes up or releases moisture faster than the other, the board cups or bows. Hence the rule: always treat both sides the same.
Cracks in the end grain
End-grain surfaces release moisture ten to fifteen times faster than flat faces. So they shrink before the core does and split open.
How to allow for it
- Acclimatise before workingLeave wood for one to two weeks in the room where it will go. That way it reaches its equilibrium moisture content before installation rather than after.
- Let it moveTabletops with slotted holes or tabletop fasteners, floors with an expansion gap along the walls, decking with gaps of 5 to 8 millimetres.
- Treat both sidesIncluding the underside of a top and the back of a panel. Uneven finishing gives uneven moisture uptake and therefore cupping.
- Keep the humidity stableIndoors between 40 and 60 per cent. In winter that often means humidifying — it spares your floor, your furniture and your musical instruments.
The full explanation of how wood moves and which distortions exist is on how wood moves.
Frequently asked questions about wood expansion
Does wood expand with heat or with moisture?
Almost exclusively with moisture. Temperature has a small effect, but the relative humidity is decisive — unlike with metal.
How much does wood expand?
As a rule of thumb, about one per cent change in width per four per cent change in moisture content. For a top 80 centimetres wide, that easily comes to 8 millimetres between winter and summer.
Does wood also expand in length?
Hardly — that movement is so small that you can ignore it. All the problems arise across the width.
Why is my board cupping?
Because one side takes up or releases moisture faster than the other. Always treat both sides the same, including the underside you don't see.
Expansion in perspective
Wood moves across its width, not along its length. Design accordingly.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Boards with sticks between them air-drying outdoors — Johann Jaritz, CC BY-SA 4.0 · Quercus — Tomticker5, CC BY-SA 3.0