WORKBENCH GUIDE
Wood movement across the grain
Boards change width with every change in humidity. Estimate how much from published shrinkage, and design so the movement does no harm.
Why wood moves
Wood shrinks and swells mostly across the grain. The Wood Handbook gives a longitudinal shrinkage from green to oven-dry of only 0.1% to 0.2% for most species, while tangential shrinkage, along the growth rings, exceeds radial shrinkage across them: for the species on this site, by 1.3 to 2.2 times. That is why a flat-sawn board changes width and cups more than a quartersawn one.
Shrinkage starts only below the fibre saturation point, around 30% moisture content. In service, wood usually moves between about 6% and 14% moisture content, a range in which the Wood Handbook treats the relation between shrinkage and moisture as linear.
Estimating the change
Wood Handbook Eq. 4-9 assumes dimensions change linearly between the fibre saturation point and oven-dry. For a change of a few percentage points, the width change is approximately width × total shrinkage ÷ 100 × moisture change ÷ fibre saturation point. Use tangential shrinkage for flat-sawn boards and radial shrinkage for quartersawn ones; for mixed or unknown orientation the Handbook suggests the tangential value.
Individual boards vary: the Handbook puts the coefficient of variation of shrinkage at about 15%, and lumber is rarely perfectly flat-sawn or quartersawn. Treat the result as an estimate, not a tolerance. The table shows a board 300 mm wide changing by 4 percentage points, for example from 12% in a humid season to 8% in a heated room.
| Species | Flat-sawn | Quartersawn |
|---|---|---|
| Teak | 2.4 mm | 1.3 mm |
| Mahogany (Swietenia) | 1.9 mm | 1.4 mm |
| Iroko | 2.8 mm | 1.8 mm |
| Black walnut | 3.1 mm | 2.2 mm |
| European oak | 3.8 mm | 1.7 mm |
| Red oak | 3.4 mm | 1.6 mm |
| Hard maple (sugar maple) | 4 mm | 1.9 mm |
| European beech | 4.4 mm | 2.1 mm |
| Eucalyptus grandis | 3.9 mm | 2.2 mm |
| Norway spruce | 3 mm | 1.4 mm |
Designing for movement
Quartersawn stock moves less in width, which is why vertical-grained flooring shrinks and swells less and its edge joints open less (Wood Handbook, chapter 6). Species with low shrinkage, such as teak or mahogany, move less than beech or oak of the same width.
A glued panel moves like a single board of the same width, so a 600 mm tabletop moves twice as much as a 300 mm one. Give that movement somewhere to go: slotted or floating fixings, room for panels inside frames, and clearances in drawers and doors fitted during a dry season.
Moisture before machining
Movement after assembly comes from the difference between the moisture content at cutting and the moisture content in service. Measuring before machining, and letting stock acclimatise to its future surroundings, removes most of that difference before it reaches the joints.
A flat-sawn European oak tabletop 600 mm wide going from 12% to 8%: 600 × 9.7% × 4 / 31 ≈ 7.5 mm narrower (Tropix shrinkage and fibre saturation point).