Why Solid Wood Moves (and Why That's Normal)
Solid wood never stops responding to the air around it. It takes on moisture and swells slightly when the air is humid, then releases it and shrinks when the air is dry. This is normal, and furniture built by people who understand it will look after itself for generations. Here's what happens and what to expect.
Wood follows the humidity
Wood gradually settles at a moisture content that matches the relative humidity (RH) of the room. This is its equilibrium moisture content. At room temperature (70°F, 21°C), the USDA Wood Handbook gives these figures:
| Indoor relative humidity | Wood moisture content | Typical conditions |
|---|---|---|
| 30% | 6.2% | Heated homes in winter |
| 50% | 9.2% | Spring and fall |
| 70% | 13.1% | Humid summer days without air conditioning |
Furniture lumber is kiln-dried to around 6–8% moisture content before it's built, close to the average for a heated home. The finish slows moisture exchange but doesn't stop it, so a piece follows the seasons slowly.
Wood moves across the grain, not along it
- Along the length: almost nothing. Wood shrinks only about 0.1–0.2% along the grain from green to dry, so a table doesn't get noticeably longer or shorter.
- Across the growth rings (radial): moderate movement.
- Along the growth rings (tangential): the most movement, about twice the radial figure (usually 1.5 to 2.5 times).
That's why the way a board is cut matters. Plain-sawn boards have the growth rings running across their width, so they move tangentially: the most. Quartersawn boards have the rings running through their thickness, so their width changes radially: about half to two-thirds as much.
How much a tabletop moves
The table shows how much a 36" (91 cm) wide solid top changes in width when its moisture content changes by 3%. That's typical when indoor humidity stays between about 30% and 50% through the year; homes that reach 70% in summer can see roughly twice as much. Figures use the Wood Handbook's dimensional-change coefficients.
| Species | Plain-sawn top | Quartersawn top |
|---|---|---|
| Black walnut | 0.30" (7.5 mm) | 0.21" (5.2 mm) |
| White oak | 0.41" (10.3 mm) | 0.21" (5.3 mm) |
| Northern red oak | 0.33" (8.3 mm) | 0.15" (3.8 mm) |
| Hard maple | 0.38" (9.7 mm) | 0.18" (4.5 mm) |
| Black cherry | 0.27" (6.8 mm) | 0.14" (3.5 mm) |
| White ash | 0.30" (7.5 mm) | 0.18" (4.6 mm) |
Real tops usually move a little less than this, because they're glued from boards with mixed grain angles and a good finish slows the seasonal swing.
How good furniture allows for movement
- Tabletops attached with buttons, clips or slotted holes. These hold the top down but let it slide as it expands and contracts. A wide top screwed tight is likely to crack.
- Breadboard ends pinned through elongated holes. They keep a top flat while letting it move underneath.
- Frame-and-panel doors and sides. The panel floats in grooves, unglued, so it can change width inside the frame.
- Drawer bottoms in grooves, fixed only at the back with a screw in a slot.
- Matching grain direction. Parts glued together have their grain running the same way, so they move together.
What's normal, and what's not
- Normal: drawers and doors that fit a little tighter in humid weather; a thin, unfinished line appearing at the edge of a floating panel in winter; breadboard ends that stand slightly past the edges of the top in winter and slightly short of them in summer; small checks in the end grain of a thick slab.
- Worth a call to the maker: a crack running along a tabletop, a top that cups noticeably, joints opening, or doors that stop closing. These usually mean movement is being restrained, or the piece is somewhere much drier or damper than the rest of the room.
Keeping solid wood happy
- Keep the humidity steady. Steady humidity matters more than an exact number. Health Canada recommends 30–50% indoor humidity for a healthy home, and that range suits solid wood too. The woodworking industry's standard for installed woodwork allows 25–55%.
- Use a humidifier in dry winters if your indoor humidity regularly falls below about 30%, and a dehumidifier or air conditioning in damp summers.
- Keep furniture away from heat and airflow: heating vents, radiators, fireplaces and baseboard heaters dry one side of a piece faster than the other.
- Limit direct sun. Sun heats and dries surfaces unevenly, and it changes wood colour.
- Give new furniture time. A piece that has travelled from a workshop to a home with a different climate will settle over its first few seasons.
