Wood & workshop
Wood movement: plan for moisture changes
Solid wood exchanges moisture with the surrounding air. Across the grain it generally changes size more than along the grain, and radial and tangential movement differ.
Understand the requirement
Solid wood exchanges moisture with the surrounding air. Across the grain it generally changes size more than along the grain, and radial and tangential movement differ. Species, grain orientation and moisture range all matter. A universal movement percentage is therefore a poor basis for designing a tightly trapped tabletop or panel.
Choose the right calculation
A simple estimate multiplies width by a species-appropriate coefficient and moisture change. It is a screening tool, not a complete model for every moisture condition. Measure representative boards, acclimatise them for the intended environment and use joints or fixings that permit movement. Do not confuse relative air humidity with wood moisture content.
Check the product and the order
Inspect timber before assigning it to a part. A board may have enough length but an unsuitable knot, split or bowed section where the finished component should be. Record the usable segment and the face you intend to show. Keep moisture and machining allowances separate from nominal dimensions; final fit is decided by the finished piece.
Worked example
A transparent planning example
Use these inputs to follow the calculation. The result below explains what can and cannot be concluded from it.
| Width | 300 mm |
|---|---|
| Illustrative coefficient | 0.0025 |
| Moisture change | 4 percentage points |
The example predicts 3 mm of width change. The coefficient is hypothetical, not a recommended value for all oak or all softwood. Use species and grain-specific published data before sizing a real joint.
Compare the scenarios
The following comparison isolates the quantities or assumptions discussed in the example. It is a planning illustration, not measured performance data.
Values are calculated from the illustrative inputs above. Different products or dimensions change the result.
Planning checklist
- Measure the actual project and record the units next to every dimension. Identify the regions or parts counted so nothing is included twice.
- Confirm the exact material format and its product instructions. Replace the example inputs with declared dimensions, coverage or yield from your chosen product.
- Calculate the net requirement first. Show cutting or application allowance separately and check whether the example method fits your geometry.
- Convert to whole supplier units once, then price the complete order including compatible accessories, delivery and any separately specified repair stock.
Mistakes to avoid
Do not mix millimetres and metres in the same multiplication. Do not apply a reserve twice, and do not treat a rounded purchase unit as proof that every part fits. Recheck the specific limitation described in the worked example before paying for the order.
Questions and answers
Can I copy the example quantities directly?
No. The examples demonstrate the calculation, not your exact room, layout or product. Replace the inputs and check the specific limitation explained with the result.
What should I verify before ordering?
The example predicts 3 mm of width change. The coefficient is hypothetical, not a recommended value for all oak or all softwood. Use species and grain-specific published data before sizing a real joint.