Several factors determine the amount and direction in which a board warps, but the primary cause boils down to an uneven distribution of moisture among the wood cells. Wood fibers shrink when they dry out, and depending on which part of a board dries most quickly, various warping patterns develop. You can correct the longitudinal warping, known as bowing, by restoring moisture to the cells, straightening the board and allowing it to dry evenly.
Types of Warping
Uneven drying can make the edges or ends of a board curl up or bend sideways. Cupping occurs when the edges curl toward each other, and bowing occurs when the ends curl upward. A third type of warping is called crooking. It is similar to bowing, but the board tends to curl more toward one end than the other. The tree species, the location in the trunk from which a board is sawed, the method of sawing and the method of drying all have an effect on the amount of warping a board sustains.
A board that comes from the center of the trunk is more likely to bow than one from the outside. One side of such boards is primarily heartwood while the other side is sapwood. Because heartwood is younger and contains more water than sapwood, it tends to shrink more when it dries out, and the heartwood side of the board contracts more than the other side. Wood that comes from a branch or a log that isn't straight can also bow due to irregularities in the grain of the wood from which it was sawed.
Straightening a Bowed Board
The cure for bowing is to restore enough moisture to the wood cells to allow them to expand and make the wood pliable, and then to bend the board back into shape and hold it there until the wood dries more evenly. Wood tends to absorb moisture better when it's warm, so steaming the board for a prolonged period is a more effective way to get water into the cells than simply soaking it. Once it's pliable, you can straighten and hold it with clamps while it dries in a well-ventilated area out of direct sunlight.
The best method for steaming a board depends on its size. If it's too large to place over a pot of boiling water on the stove, you can steam it by building a fire under a 55-gallon drum filled with water. Once it's pliable, you need to clamp it to something rigid that won't interfere with the drying process. Two lengths of galvanized pipe, one placed along each edge of the board, will hold it straight while allowing the side against which they are clamped to breathe. The board should remain in a low-humidity environment until it dries.
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