Wood on the Move
Even the driest wood can change shape due to moisture, no matter what you do to it. But you can learn what to expect.
Where you'll find the water
Wood remains dimensionally stable if its moisture content is above the fiber saturation point (FSP). The FSP is the condition where the woo's cell walls are completely wet, but the cavities within the cell walls are dry. If the wood loses moiture from the cell walls, it shrinks. If the cell walls gain moisture, the wood swells.
Wood Shrinkage I
What shrinkage does to wood
Woodworkers call the change in shape of a piece of wood warp. And it takes several common forms, all of which distort the wood.A board has cup when it is no longer flat from edge to edge. Cup always occurs in the opposite direction of a flatsawn board's annual growth rings.
Bow, as its name implies, describes the lengthwise curvature of a board -- end to end along its face.
When a board has crook, all the curvature runs from end to end along its edge.
Twist means that all of a board's corners won't lie equally flat.
Although not a distortion like any form of warp, checking refers to small splits along the grain. You'll most often see checks in the ends of boards, but they can occur on surfaces, too. That's because as wood dries, it loses moisture along its length about 10 times faster than across its width. So end grain dries more rapidly and shrinks faster, causing these small ruptures.
Wood Shrinkage II
How wood shrinks
Unlike a dissolving sugar cube, a block of wood doesn't behave the same in all directions as it shrinks. As shown in the illustration, wood shrinks most in the direction of the annual growth rings (tangentially). It shrinks about half that much across the growth rings (radially). And shrinkage with the grain (longitudinal) is minimal. The result: Combined radial and tangential shrinkage distorts the shape of any piece of wood because of the difference in the two shrinkage rates and the way the annual rings curve. A number of variables affect how and to what degree wood shrinks, But in general, the denser the wood, the more it shrinks.
Finishes slow moisture absorption
To limit the defects caused by natural shrinkage of green wood, lumber producers preshrink it by carefully supervised seasoning and kiln-drying. They, and you, would rather have the wood shrink before it is made into a piece of furniture.
Woodworkers apply finishes to wood because -- despite the drying -- wood will both take on and lose moisture. There isn't a finish, though, that completely blocks moisture from re-entering things made of wood. As you can see in the chart, below, finishes only slow the process.
You can't change wood's tendency to shrink and swell; only plan for it. Design with dimensional change in mind. Use wood dried to the average moisture content it will see in use -- 8 percent indoors and 12-15 percent outdoors. Finally, apply the most moisture-resistant finish you can that's consistent with the piece's intended use, and coat all surfaces.
|Laboratory tests show finish effectivenenss
in keeping moisture out *
|FINISH TYPE||NO. OF COATS||% OF MOISTURE-EXCLUDING EFFECTIVENESS|
|1 day||7 days||14 days|
|Gloss Enamel Paint||2||91||64||43|
|* Testing by the U.S. Forest Products Laboratory in Madison, Wisconsin, compared the moisture-excluding effectiveness of different types of finishes. Tests were conducted on dry Ponderosa pine boards that were coated, then exposed to the moisture vapor of 90 percent humidity at 80° F for from 1-14 days. the results listed here show how only the most common woodworking finishes of the many tested performed.|
Take wood movement into account
Experience taught woodworkers of old how to deal with wood's dimensional changes due to moisture. The answer was joinery that allowed for seasonal wood movement. And despite today's super-strong glues and moisture-fighting finishes, that's still the answer.
Frame-and-panel construction for cabinet and doors, wall panels, and sections of furniture, for instance, didn't come about by accident. Joiners, as woodworkers were called centuries ago, figured out that a rectangular panel could be maintained in poisition with a solidly secured frame of wood. However, the panel must not be glued or nailed in place in the frame. Instead, it has to "flot" in grooves, free to shrink and swell with changes in atmospheric moisture.
Today, some professional woodworkers talk about "nickel and dime reveals" on flush-fitting cabinet doors and drawers. These refer to the space you should leave between the wood that you expect will shrink or swell -- the doors or drawers -- and the carcase or frame of the piece. "If you build in winter, make the reveal the thickness of a nickel," they say. That leaves room for the wood to swell when the humidity goes up. On the other hand, "Build in summer, use a dime," means that you're allowing for the shrinkage that will come in winter.