Wood movement
How far a panel will expand and shrink between a dry winter and a damp summer, and what that means for the joint holding it.
Across the grain
Cathedral grain on the face, rings running roughly parallel to it. The cheapest way to cut a log and the one that moves most — this uses the tangential coefficient.
The northern US and inland Canada. Forced-air heat dries a house out hard in January.
Seasonal movement across a 24" board
11/16"across the grain
Driest
23.663"
Dampest
24.337"
Moisture swing
7.7%
At 25% humidity the wood settles at 5.4% moisture content; at 70% it reaches 13.1%. Over that swing, white oak plain sawn moves 0.00365 of its width per percentage point — which comes to 2.81% of the board.
Length along the grain is not shown because it barely changes — a few thousandths over several feet. Wood moves across the grain, and only across the grain, for any purpose a furniture maker cares about.
What to do about it
Fastening a top to a base. Every fastener outside the centreline needs to slide. Elongate the screw holes across the grain, or use figure-8 fasteners or buttons in a groove, and give each one at least 3/8" of travel in each direction. Fix the top at the centre so the movement splits evenly to both edges instead of walking the overhang to one side.
Panels in a frame. A panel this wide has to float. Leave at least 3/8" of space at each edge in the groove, use space balls or a dab of silicone to keep it centred, and never glue a panel in — a glued panel is a panel that will split its own stile out.
Breadboard ends. With 11/16" of swing the breadboard end has to be pinned, not glued. Glue the centre two inches only, drill the outer pin holes oversized across the grain, and accept that the ends will sit proud in summer and shy in winter. That step is the joint working, not the joint failing.
When to build. Build to the middle of the range rather than to today's reading. A top glued up in a damp August shop and delivered into a dry January house is the classic way to find out how much a panel can shrink, and the answer is: enough.
Working white oak
Closed pores make it the one to use for anything that holds liquid or lives outdoors. Quartersawn white oak gives you the ray fleck that Arts and Crafts furniture is built on, at roughly double the price of flatsawn. It is hard on blades and it will react with steel — wipe up swarf before it stains.
Tangential 0.00365 · radial 0.00180 per 1% moisture content. Reference figures from the Forest Products Laboratory; individual boards vary, so treat the answer as the size of the problem rather than a guarantee.
About wood movement
- How much does wood actually move?
- A 24" wide plain sawn white oak panel moves about 11/16" between a dry winter and a damp summer in a house with real seasons — a 25% to 70% humidity swing. In a milder climate, or a house held near 50% all year, it is nearer 3/8" or 3/16". That is the whole reason a solid top cannot be screwed down at both edges: it is not a rounding error, it is a visible, structural amount of movement that will split the wood if nothing gives.
- Does quartersawn wood really move less?
- Yes, by roughly half. Wood moves about twice as much tangentially — around the growth rings — as it does radially, across them. Quartersawn boards have their rings running through the thickness, so the width of the board is the radial direction and gets the smaller coefficient. That stability, not just the ray fleck, is the argument for paying more for it.
- Can I stop wood moving with a finish?
- No. A film finish slows the exchange of moisture, which helps against a sudden change — a radiator switched on, a week of rain — but nothing available to a furniture maker stops the wood reaching equilibrium with the air over a season. Finish both faces equally so the two sides move together, then design the joint to allow the movement rather than resist it.
- How do I fasten a tabletop so it doesn't split?
- Fix it at one point — usually the centre — and let everything else slide. Elongate the screw holes across the grain, or use figure-8 fasteners, wooden buttons in a groove, or Z-clips in a slot. Each fastener needs at least half the total seasonal movement of travel in each direction. Fixing at the centre rather than one end splits the movement evenly to both edges, so the overhang stays symmetrical.
- What humidity should I build to?
- The middle of the range the piece will live in, not the humidity of your shop today. A top glued up in a damp August shop and delivered into a January house heated to 25% humidity will shrink the entire seasonal amount at once, and a breadboard end or a tight frame will find out about it. If the piece is going somewhere very different from where it is built, let the wood sit there first.