Wood never stops reacting to the air. It takes in moisture when the air is damp and gives it up when the air is dry, and as it does so it changes size. A tabletop that fits its frame in a humid summer may be a quarter inch narrower in a dry winter. Pieces that are built as if wood were a fixed-size material crack, cup and pull joints apart. Pieces built to allow for the movement last for generations.
Which direction wood moves
Wood is made of long fibres that run along the trunk. It moves very little in length: about 0.1 percent across its whole range of moisture. It moves much more across the grain. Movement in width depends on how the board was cut from the log.
| Direction | Approximate change, dry to wet |
|---|---|
| Along the grain (length) | Under 0.2 percent |
| Radial, across quarter-sawn faces | 3 to 5 percent |
| Tangential, across flat-sawn faces | 5 to 10 percent |
A flat-sawn oak board 12 inches wide can change by 3/8 inch or more between a dry and a humid season. A quarter-sawn board of the same width changes about half as much. The woods database gives a movement rating for each species, from low to moderate to high, and the quarter-sawn cut further reduces it.
Seeing the grain
Look at the end of a board. If the growth rings run roughly parallel to the wide face, it is flat-sawn, and the board will cup, with the edges curling away from the heart side. If the rings run at right angles to the face, it is quarter-sawn, and it stays flat and moves less. Rift-sawn boards are in between. Quarter-sawn lumber costs more, since it produces less material per log, but it is the choice for wide panels, door frames and tabletops where flatness matters.
How a board cups
A flat-sawn board tends to cup toward its bark side, because the side nearer the bark shrinks more than the side nearer the heart. When you glue up a wide panel from flat-sawn boards, alternate the ring direction, heart side up and down, and the cups offset one another. Or cut the boards narrower, since narrow boards cup less in absolute terms. Four 4-inch boards behave better than one 16-inch board.
Breadboard ends and cleats
A solid top must be held flat without being locked in place. Breadboard ends are cross-grain strips on the ends of a top, joined with a tongue and groove. The centre of the tongue is glued and the rest is fixed with screws in slotted holes or pinned through an elongated hole so that the top can expand and contract along its width while the breadboard stays the same length. The result keeps the top flat and covers the end grain.
Fastening a top
Do not glue or screw a solid top tightly to its base. Use one of the methods that lets the top move.
- Figure-8 fasteners pivot on a screw in the rail.
- Z-clips or tabletop clips hook into a groove cut along the inside of the rail.
- Slotted screw holes in a cleat or rail allow the screw to slide. Drill the hole oval in the direction of the grain movement, with a washer under the head.
- Wooden buttons fit in a groove in the rail and overlap the top, with a gap to slide.
Fix the top at the centre of the front rail and let it grow outward on both sides, for the best balance.
Frame and panel
A door panel that is glued into the frame cracks the frame when it swells. In frame and panel construction, the panel floats in grooves in the frame. It is cut slightly smaller than the groove, leaving a gap of 1/16 to 1/8 inch, so it can move without pushing on the rails. A small rubber bead or a dab of flexible material in the groove keeps it centred. The panel does not need glue. It is one of the oldest ways of making something wide from wood that moves.
Drawers and cabinet sides
Cabinet sides and drawer fronts are cut so that the grain runs along the long dimension, which means they move across their height. Give the drawer a small clearance top and bottom, about 1/16 inch, so it does not stick in summer. Cabinet backs made of solid wood should be in narrow boards with shiplap or tongue and groove, allowing each to move a little, instead of one wide board.
Plywood moves less
Plywood has layers with alternating grain, so it barely moves. It is the preferred material for the backs of cabinets, the bottoms of drawers and shelves that need to stay flat in a wide span. Solid wood edging covers the layers on the front.
Acclimating wood
Bring boards into the room where the piece will be used a week or two before cutting. Stack them flat with thin sticks between layers for air flow. The wood will settle near the moisture level that it will have in use. Cutting and gluing boards that have just come from a damp yard or a dry store gives a piece that moves soon after it is finished.
Planning the cut list
When you plan a cut list, allow for movement in the widths: a top that is glued up at 30 inches wide can vary by 1/4 inch or more. Fit parts that sit in grooves with a margin. The cut list packer places each part on a board, and adding a small allowance at the planning stage avoids surprises later. The joinery database identifies which joints allow movement, such as tongue and groove, and which do not, such as a glued end-grain joint.