The board
Flatsawn is the cathedral grain on a plain board face and moves most. Quartersawn shows straight vertical lines and moves about half as much. Rift is in between.
The moisture swing
Kiln-dried lumber comes in around 7 to 9%. Indoors, unfinished wood drifts roughly 6% in a dry winter to 12% in a humid summer. A heated, bone-dry shop or a damp basement widens the swing.
Pick a wood, a width and the moisture swing.
This is movement across the width. Along the length it is tiny and you can ignore it. The figure is for a plain board; a finish slows the swing but does not stop it, and real shops vary, so treat it as a close guide and leave yourself a little room.
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Solid wood is never still. It takes on moisture when the air is humid and gives it back when the air is dry, and as it does it swells and shrinks across its width. Glue a wide board down so it cannot move and it splits or cups instead. This works out how far a board will move between its driest and wettest, so you can leave it the room it needs.
The rule behind it
Movement is the width, times the moisture swing in percent, times a coefficient for the species and the way the board was sawn. The coefficients come from the USDA Forest Products Laboratory Wood Handbook, which measured the shrinkage of each species in a lab. A 24 inch flatsawn white oak top across a 6 percent swing works out to roughly half an inch; quartersawn, about half that.
Flatsawn, rift and quartersawn
How a log is cut decides how much the board moves on its face. Flatsawn boards, with the cathedral arches of grain, move the most. Quartersawn boards, with straight vertical lines, move about half as much, which is why wide tops, doors and drawer fronts are often quartersawn. Rift falls between the two. Pick the one that matches your board and the number changes with it.
Designing for it
Let wide parts float. Fix a tabletop with slotted screws, buttons or figure-eight fasteners so it can slide across its width. Let a solid panel float in its frame, never glued at the edges. Cut breadboard ends so only the center is pinned and the rest can move. And if you must size a wide panel tight, do it in the dry season, so it grows into its space rather than out of it.
Which way, and how much to leave
All of this is about movement across the grain. Along the length, wood barely moves, so a long rail stays put while a wide panel beside it breathes. Leave at least the movement this shows, and a touch more, because a damp basement or a bone-dry heated room swings wider than the average.
More MakerKiln tools
- board feetBoard Foot CalculatorAdds up the board feet of your whole lumber list and the cost, from thickness, width, length and how many.→
- will it bowShelf Sag CalculatorHow much a shelf will bow under the weight you put on it, and whether you need to go thicker or add a support.→
- sheet planCut List OptimizerTells you how much plywood to buy and shows you how to cut it, so you waste less and spend less.→
- drill guideDowel Jig GeneratorSet the board, the dowel and the spacing, and download a checked STL. Every hole lands dead center on the edge.→