Wood movement: how much a board changes, and how to design for it

Estimate seasonal movement with the Wood Handbook's dimensional change coefficients, then choose joints and fasteners that let solid wood move.

Solid wood takes on and gives up moisture with the air around it, and it swells and shrinks as it does. Nearly all of that change happens across the grain. Along the grain, a board’s length changes so little that furniture design can ignore it. Across the grain, a wide panel can change by a quarter inch or more between a dry winter and a humid summer.

Movement cannot be stopped with glue, finish or clamping force. Pieces that last are built so the wood is free to move where it wants to.

Why wood moves

Wood is always heading toward an equilibrium moisture content (EMC) set by the relative humidity and temperature around it. As its moisture content falls below the fibre saturation point (roughly 25 to 30 percent), the cell walls themselves shrink. Kiln-dried furniture lumber is usually dried to around 6 to 8 percent, and from then on it follows the seasons in your house.

The amount of movement depends on direction:

Longitudinal (along the grain): negligible for furniture.
Radial (across the growth rings, from the pith outward): moderate.
Tangential (along the growth rings): usually about one and a half to three times the radial movement, depending on species.

That is why the way a board was sawn matters. The width of a flatsawn board is mostly tangential, so it moves the most and tends to cup. The width of a quartersawn board is radial, so it moves roughly half as much and stays flatter.

Estimating the movement

The USDA Forest Products Laboratory’s Wood Handbook gives a simple estimate, valid between about 6 and 14 percent moisture content:

change in width = width × coefficient × change in moisture content (percentage points)

Use the tangential coefficient for flatsawn boards and the radial coefficient for quartersawn boards. For glued-up panels with mixed grain, the tangential figure gives a conservative answer.

The table lists the Wood Handbook coefficients for common furniture species, and what they mean for a 12-inch-wide board whose moisture content changes by 4 percentage points.

Species	Tangential	Radial	12” flatsawn, 4 points	12” quartersawn, 4 points
Red oak	0.00369	0.00158	0.177”	0.076”
White oak	0.00365	0.00180	0.175”	0.086”
Hard maple	0.00353	0.00165	0.169”	0.079”
Soft maple	0.00289	0.00137	0.139”	0.066”
Black cherry	0.00248	0.00126	0.119”	0.060”
Black walnut	0.00274	0.00190	0.132”	0.091”
White ash	0.00274	0.00169	0.132”	0.081”
Hickory	0.00411	0.00259	0.197”	0.124”
Yellow-poplar	0.00289	0.00158	0.139”	0.076”
Eastern white pine	0.00212	0.00071	0.102”	0.034”
Douglas-fir	0.00267	0.00165	0.128”	0.079”

These coefficients are averages. Individual boards, and different species sold under one name such as “red oak”, vary. Treat the result as an estimate to design around, not a measurement.

How large is the seasonal swing?

The Wood Handbook recommends an average moisture content of about 8 percent for interior woodwork in most of the United States (individual pieces 6 to 10 percent), about 6 percent in the dry Southwest, and about 11 percent in damp, warm coastal areas. Heated homes in cold climates get dry in winter and humid in summer, so furniture there cycles every year.

A swing of 3 to 4 percentage points is a common planning figure for furniture in a heated house. A room with tight humidity control sees less; an unheated cottage or a humid basement sees more. If you own a moisture meter, measure your stock when you build and think about which way it will go from there.

A worked example

An 18-inch-deep top glued from flatsawn cherry, built in a dry winter at 6 percent and heading for 10 percent in summer:

18 × 0.00248 × 4 = 0.179”, a little under 3/16”.

The same top in red oak: 18 × 0.00369 × 4 = 0.266”, just over 1/4”.

Built in winter, that top will get wider. Built in the humid season, it will shrink. Leave room in the direction it will travel.

Designing for movement

Trouble starts when wood is restrained across its width by something that does not move the same way: a rail glued across the grain, a cleat screwed tight at both ends, a panel glued into its frame. Something eventually gives, usually as a crack or a failed joint.

Frame-and-panel. The panel floats in grooves without glue. Size it so it can expand without pushing the frame apart and shrink without pulling out of the groove. A panel made in winter needs more room at its edges than one made in summer.
Tabletops. Fasten tops with buttons, clips or slotted holes that let the top slide over the aprons. See attaching a solid-wood top.
Breadboard ends. Glue only a short section at the centre. Pin the outer tenons through elongated holes so the top can move under the breadboard.
Wide rails. A rail joined to a leg is glued across the leg’s grain. On wide rails, limit the glued width, for example with two smaller tenons instead of one wide one. See sizing a mortise and tenon.
Solid drawer bottoms. Run the grain side to side, let the bottom slide in its grooves, and fasten it only at the back with a screw in a slot.

Plywood and MDF are made to resist this movement and can be glued or screwed rigidly. That is one reason case backs, drawer bottoms and panels are so often plywood.

Checking a design

Before you cut, look at every wide solid part and ask two questions: which way does its grain run, and what is it fastened to across that grain? In GrainDraft, each part carries a species and a grain direction you can inspect, so a quick pass through the wide parts of a model is a good time to ask those questions. Then check the joint or fastener that connects each one against the list above.

Numbers here are starting points from common shop practice and published references. Check your own stock, tools and hardware instructions, and cut a test piece before committing project wood. Spotted an error? The contact address is on the About page.