Wood movement

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

Wood movement is the way solid wood changes size as it gains or loses moisture: it swells as it takes up water and shrinks as it dries out, but not by the same amount in every direction through the material. Left unaccounted for in a design, this movement can eventually break a joint or split the workpiece.

How wood moves, and by how much

Movement is smallest along the length of the grain: longitudinal shrinkage, running parallel to the grain, is usually minor, and structural framing lumber shrinks mostly across its width and thickness, hardly at all along its length. Across the width, the amount depends on which way the growth rings run through the board: shrinkage measured tangentially, parallel to the growth rings, always exceeds shrinkage measured radially, across them. Grain running off the true axis of a board behaves worse than either: boards with cross grain, grain running diagonally rather than straight along the board, shrink more than usual in length as they dry, and a board can contain both ordinary wood and reaction wood, wood formed under stress that shrinks excessively along its length, sitting right alongside normal wood in the same piece.

Designing joints and parts to move safely

Because movement across the grain is so much larger than movement along it, orienting a part's grain correctly does most of the work. Keeping the long grain vertical in cabinet door sides stops shrinkage from binding the door, while orienting the long grain horizontally across a drawer bottom stops shrinkage from making the drawer stick. In panelled joinery such as doors, adding muntins, the narrow members that divide a door into smaller panels, narrows each panel and so lessens the visible effect of moisture movement and lowers the risk of damage. Where wood must be restrained rather than simply allowed to move, a stiffer, more stable material such as metal can be built in to hold back the swelling and shrinkage. One particular construction method is said to cut tangential shrinkage to roughly a fifteenth of normal, at the cost of only a slight loss of stability along the grain, though what the method itself involves is not specified.

A doweled joint as an example

A doweled joint between two boards shows what happens when movement is not fully restrained: shrinkage can open a visible gap at the joint, yet the dowels keep the two faces flush with each other. Without further restraint, that same gap can let a board's edge twist up above the surrounding surface.

Movement in a timber frame

In a timber-framed building, the amount a finished structure settles vertically scales with the depth of its horizontal framing timbers, such as girders and joists, because drying mainly shortens those members across their width and thickness rather than their length. In other words, drying horizontal members mainly shortens the overall frame height, not its length or width. Running posts through several floor levels, rather than resting one floor's frame on the last, largely avoids the problem of cross-grain shrinkage building up storey by storey. Along a frame's length and width, the resulting shrinkage shows up as small gaps at each joint rather than one large gap at a single edge.