Wood movement

Also called: moisture movement, shrinkage, dimensional movement, seasonal movement

Wood never stops moving. Even after a tree is felled and the timber dried, the wood keeps shrinking and swelling as its moisture content changes, and it does so unevenly: this is why joints, panels and whole timber buildings have to be built to allow for it.

Shrinking in three directions

Wood shrinks by a different amount depending on which way you measure it. Along the grain, the direction most boards run their length, shrinkage is very small. USDA (2021) states plainly: “Longitudinal shrinkage of wood (shrinkage parallel to the grain) is generally quite small.” Across the growth rings, the curved lines visible on the end of a log, wood shrinks more than it does the other way across the log, straight out from the centre. Timber with grain that runs off true, rather than straight along the length of the board, tends to shrink more along its length than straight grained wood does. A single board can also contain reaction wood, a kind of timber that shrinks unusually along its length, sitting right alongside ordinary wood that barely shrinks that way at all. Some treatment can cut a wood's normal across-grain shrinkage to about a fifteenth of the usual figure, at a small cost to its stability along the grain.

Building movement into a joint

A joint that is not designed to allow for wood movement can fail, or damage the wood itself. A stronger, dimensionally stable material such as metal can be used to restrain wood as it swells and shrinks. Doweling changes how a board joint fails: without dowels, shrinkage leaves a visible gap and, with nothing to hold the edges level, a board's edges can cup above the face. Doweled, the same kind of joint still opens a gap as it shrinks, but the dowels keep the faces of the boards level with each other. In framed-and-panelled work, adding muntins, the narrow bars that divide a panel into smaller widths, narrows each panel enough that its moisture movement shows less, which makes damage from that movement less likely.

Cabinets, drawers and doors

In cabinet or case construction, the direction the grain runs is chosen to head off particular movement. Orienting the long grain horizontally across a case bottom stops shrinkage from making drawers stick. Keeping the long grain running vertically in cabinet side panels stops shrinkage from making doors bind.

Timber-frame houses

In timber-frame building design, shrinkage adds up to real, predictable movement across a whole structure. Builders must plan for how much a frame's height will drop as its horizontal members season, since how much a timber-frame house settles vertically depends on the depth of its horizontal framing members, such as girders and floor joists set on their sides. This settling happens because wood shrinks mainly across its width and thickness, not along its length, so a stack of horizontal timbers loses height as each one dries. Running posts continuously through several floor levels, rather than stacking one floor's framing on the last, largely removes this height loss from the frame. Where horizontal members remain at every floor, shrinkage along a frame's length and width usually shows up as small gaps at many joints, rather than one large gap, and uneven shrinkage between joints is one reason thick sub-floor or ceiling boards have value.