Solid wood moves because it takes in moisture from the air when humidity rises and gives it back when the air dries. As it does, it swells and shrinks, almost entirely across its width rather than along its length. A wide tabletop can change by a quarter to half an inch over a year. Good furniture is built to let that happen, with tops that can slide on their fasteners and panels that float in their frames. Furniture that fights the movement is the kind that cracks.
What is happening inside the wood
A tree is a bundle of long, hollow fibres that carry water. When the wood is dried, the water leaves the cell walls and the wood shrinks. Once dry, it never stops exchanging moisture with the air around it. In any steady climate it settles at a balance point, called its equilibrium moisture content.
In a typical heated and air-conditioned US home, that balance sits around 6 to 9 percent. In a humid summer it rises; in a dry, heated winter it falls. A swing of three or four percentage points across the year is common, more in regions with very humid summers and cold winters.
Direction matters
Wood does not move equally in every direction.
- Along the grain (the length of a board), movement is so small it can be ignored in furniture.
- Across the grain, around the growth rings (tangential), movement is greatest.
- Across the grain, through the rings (radial), movement is roughly half the tangential amount.
This is why a flat-sawn board, whose face runs around the rings, moves more across its width than a quartersawn board, whose face runs through them. It is also why a board can cup: one face moves more than the other as the rings straighten.
How much a tabletop moves
Woodworkers estimate movement from the published shrinkage of each species. Using a common method, with indoor moisture moving between about 8.5 percent in winter and 12 percent in summer, as it might in a humid southern climate:
- A 36-inch (91 cm) wide flat-sawn white oak top: about 1/2 inch (12 mm) across the year.
- The same width in flat-sawn black walnut: about 3/8 inch (9 mm).
- A 12-inch (30 cm) shelf in flat-sawn oak: about 5/32 inch (4 mm).
In drier climates, or in homes where humidity is kept steady, the figures are smaller. They are never zero.
How good furniture allows for it
None of the following details is new. They are the ordinary vocabulary of solid wood construction, and they are worth asking about on any piece you consider.
Tops that can slide
A tabletop is fixed firmly near its centre and held toward the edges by fasteners that allow sideways movement: wooden buttons in a groove, metal clips, figure-eight fasteners, or screws through slotted holes. The top stays tight to the base but can grow and shrink across its width. A top glued or screwed rigidly all round has nowhere to go, and something gives.
Breadboard ends
A breadboard end is a board fitted across the end of a top, its grain running the other way. It keeps the top flat and covers the end grain. Because the top moves and the breadboard does not, the joint is glued only at the centre and pinned through elongated holes elsewhere, so the top can slide within it. Done well, you may see the top sit a fraction proud of the breadboard in summer and a fraction short in winter. That is the joint working.
Floating panels
Cabinet doors and side panels are often made as a frame with a panel set into a groove. The panel is not glued; it floats, with a little room in the groove to expand. In winter you may see a thin unfinished line at its edge as it shrinks. Finishing panels before assembly avoids that line.
Drawers and doors
Drawers are fitted with a small gap that allows for summer swelling. A drawer that slides freely in winter and a little tighter in summer is fitted correctly. One that is perfectly snug in a dry month will stick when the weather turns.
Slabs and wide boards
Live edge slabs move most of all, because they are wide and often flat-sawn. They need the most generous allowance at the base, and any end checks should be stabilised with inlaid keys.
What movement looks like, and what it does not
- Normal: drawers that tighten slightly in summer; a hairline gap at a panel edge in winter; a top that sits a touch proud of a breadboard end; very slight cupping that eases as the season changes.
- Worth a closer look: a crack that keeps opening, a joint that has pulled apart, or a top that has lifted from its base. These usually point to a fixing that cannot move, or to a piece kept beside a heat source.
What you can do at home
- Keep relative humidity roughly between 35 and 55 percent, and steady. A humidifier in winter and air conditioning or a dehumidifier in summer do most of the work.
- Keep pieces away from radiators, fireplaces and heating or cooling vents, which dry one side faster than the other.
- Give a new piece a few weeks to settle in your home before judging small seasonal changes.
- Avoid plastic covers or tablecloths left on permanently, which trap moisture on one face.
Movement is not a fault in solid wood; it is the reason solid wood lasts when it is built with care. Browse our dining tables and console tables, and if you would like to know how a particular piece is put together, ask us; it is a fair question, and we are glad to answer it.