Guides
Understanding Wood Movement
Updated 3 min read
The short answer
Wood moves because it is hygroscopic — it absorbs and releases moisture to match the air around it, and it changes dimension as it does. In Calgary, where indoor humidity swings from 40% in summer to 15–25% in winter, wood movement is more pronounced than in most cities. Understanding how much and in which direction is the difference between millwork that lasts and millwork that fails.
The physics of wood movement
Wood is a bundle of hollow tubes running lengthwise, held together by lignin. When the air is humid, water molecules fill the spaces between these tubes, causing the wood to swell across the grain. When the air is dry, those molecules leave, and the wood shrinks. The change is negligible along the grain (lengthwise) but significant across it (widthwise and thicknesswise).
The amount of movement depends on the species (ring-porous species like oak move more than diffuse-porous species like maple), the width of the piece (wider boards move more total distance), and the humidity change (a 25% swing in relative humidity can move a 300mm-wide board by 3–5mm across the grain).
In Calgary the relevant humidity range is extreme by North American standards. A heated house in January may be 15% RH; the same house in August with windows open may be 40–45%. That 25–30% swing moves wood more than it would move in Vancouver, where indoor humidity stays in a narrow band year-round.
How much to expect
| Board width | 15% to 35% RH change | 15% to 45% RH change | Notes |
|---|---|---|---|
| 100mm | ~1mm | ~1.5mm | Barely visible |
| 200mm | ~2mm | ~3mm | Noticeable at joints |
| 300mm | ~3mm | ~4.5mm | Visible gap at mitres |
| 400mm | ~4mm | ~6mm | Significant; requires design accommodation |
| 600mm | ~6mm | ~9mm | Will cause problems if restrained |
These are approximate values for white oak. Ring-porous species move more; diffuse-porous species move less.
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Construction techniques that accommodate movement
- Floating panels in frames
- A door panel should sit in a groove within the frame without being glued, allowing it to expand and contract freely. A panel that is glued or pinned will crack itself or the frame.
- Slotted screw holes
- When attaching wide boards to a fixed structure (a breadboard end on a table, a panel to a frame), elongate the screw holes across the grain so the wood can move through them.
- Coped inside corners
- A coped joint closes when the board shrinks; a mitred joint opens. On baseboard and crown moulding, coped inside corners are the standard technique for this reason.
- Expansion gaps
- Hardwood flooring, wide-panel wall cladding, and large tabletops all need a gap at the perimeter to accommodate seasonal movement. The gap is usually concealed by baseboard, trim or a breadboard end.
- Cross-grain attachment
- Attaching a wide board perpendicular to another (a table top to its frame, a shelf to its sides) requires allowing for movement in the perpendicular direction. Buttons, Z-clips, and figure-8 fasteners all accomplish this.
Answers
Frequently asked questions
- How do I reduce wood movement in my house?
- Control indoor humidity. Holding your home at 35% relative humidity through winter — with a whole-house humidifier or portable units — dramatically reduces the seasonal swing that causes movement. This benefits millwork, hardwood floors, musical instruments and the people living there.
- Does engineered wood move less than solid wood?
- Yes. Plywood, MDF and other engineered sheet goods are dimensionally more stable than solid wood because their layers are oriented to resist movement. This is one of their primary advantages. However, they are not immune — MDF swells when wet, and plywood moves slightly with humidity changes.
- Why do my cabinet doors stick in summer?
- Because the door panel expanded in the higher summer humidity and the gap between the door and the frame has closed. This is usually a sign that the door was built with insufficient clearance or that the panel is restrained (glued or pinned) rather than floating freely.
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