The four kinds of distortion
When people say a board is warped, they may mean any of four different things. Each is fixed differently, so identify which you have first.
- Cup: the board curves across its width, like a shallow trough.
- Bow: the board curves along its length when viewed on the flat face, like a rocking-chair rail.
- Crook: the board curves along its length when viewed on its edge, so one edge is concave.
- Twist: the corners are not in the same plane, so the board rocks on a flat surface like a propeller.
Sight down each board from one end and lay it on a known-flat surface to see which ones apply. Some boards have more than one.
Why boards distort
Wood shrinks and swells unevenly. Moisture loss from one face, uneven drying, and the way the board was sawn from the log all play a part. Wood tends to shrink more across the growth rings than along the radius, which is one reason flat-sawn boards cup toward the bark side as they dry. Internal stresses released when a board is cut can also cause it to bend. The idea behind wood movement is the same: wood responds to humidity changes, and one-sided exposure makes it move. Reaction wood and grain running off-axis also add tension. Good drying and storage reduce the problem but rarely remove it completely.
Check before you buy
Pick through the stack. Sight down the edge and face of each board and set aside anything clearly twisted. Mild cup or bow is often workable, while heavy twist is the hardest to fix and wastes the most wood. Buying a little extra length lets you cut away the worst sections. Boards that have sat for some time in a stack held flat typically behave better than those left leaning against a wall.
Flattening with machines
Flattening is done in a particular order.
- Joint one face flat. A jointer removes high spots and, over several passes, makes one face flat. A board with cup or bow is usually run with the concave side down so the cutter takes the high edges first. Twisted boards tend to rock on the table, so they need shallow passes and care, and some builders shim the low corners first.
- Plane the second face parallel. Once one face is flat, a thickness planer makes the opposite face parallel to it. A planer alone usually does not flatten a warped board, because its feed rollers press the board flat as it passes and it springs back afterward. A benchtop planer can still do the job if you first flatten one face, or use a carrier sled. The same idea applies in thickness planing generally.
- Joint an edge square. With a flat face as the reference, joint one edge, then rip the board to width on a table saw and clean up.
Take the cuts in light passes, and stop if the stock begins to chatter or burn. Wear eye and hearing protection and a dust mask, and keep hands clear of the cutterhead. Follow the manual, and use push blocks where the machine calls for them. Cutting deep enough to flatten a heavily cupped board also means you end up with a thinner board, so expect to lose thickness.
Flattening with hand tools and sleds
Without a jointer, a hand plane can flatten a board by working across the high spots and checking with a straightedge or winding sticks, a pair of straight sticks that make twist easier to see. A router sled can also flatten a board: the board is shimmed to avoid rocking and a router passes over it on rails. Both methods are slower but work for wide boards that will not fit a jointer.
Work around the problem
Not every distorted board needs flattening.
- Cut it shorter. Short pieces from a bowed board are often much straighter.
- Rip it narrower. Narrow strips cup less than wide boards, so a cupped board can become several usable narrow ones.
- Use it for hidden parts. Warped boards can serve as shop stock for jigs or blocking.
- Glue up narrower pieces. Panels made from several narrow boards tend to stay flatter than one wide plank.
Prevention
Store lumber flat, supported at regular intervals, with spacers called stickers between layers so air circulates. Let boards adjust to your shop before cutting, as described in lumber acclimation, and seal or finish all faces of a panel evenly, so moisture changes do not act on one side only. Some warping is a normal part of working with wood, and an extra margin of stock gives you room to correct it.