HOW TO COPE WITH WOOD MOVEMENT

In one of his books, renowned teacher James Krenov wrote: “A true achievement is to make a fine cabinet – elegant, graceful, one with character – and fit it with drawers and doors that work properly the year round.” His advice applies to more than cabinets, of course. Joints, veneers, chests, tables or any builds that involve cross-grain structures all require careful consideration of, in Krenov’s words, the moods of the seasons.

Wood movement, if ignored, is a destructive time bomb. Here’s how we can prepare for wood movement and prevent the furniture we build from falling apart over time.

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Handling Wood Movement Challenges

Coping with wood movement should not be an afterthought. I start as early as rough lumber is dressed. Here are four common techniques I employ to cope with wood movement. You can find more coping measures from the publications listed at the end of the article in Further Reading.

Attaching a Top to its Base

This is an example of a cross-grain assembly in which members expand and contract in different directions, weakening or breaking the joint over seasonal changes. The solution is to allow one of the mating parts to move freely. For accent or side tables built over the years, I used either wooden buttons or Z-clips to attach the tops so they can move across their width. For table tops, such as the top of a contemporary coffee table I built for a “client” (my daughter), they are screwed to their bases in elongated holes.

Left: Using Z-clips to attach the tops so they can move across their width.

Right: Screwing the table top to its base using elongated holes.

Understanding Why Wood Moves

During my dry season (summer), wood shrinks because it releases its moisture content into the air. In the winter, my shop – also an overnight garage – is very humid from the melting snow brought in by the cars. Wood kept there absorbs the moisture in the air and expands. Inside my house, the humidity changes are controlled with a humidifier system. Once you know how humidity changes from season to season where you live, you can predict how wood will behave and determine what to do about it. For example, if you build a drawer during the dry season, you should fit loose as the drawer front will expand on wet days, and vice versa.

Knowing How Wood Moves

Wood movement is predictable not only in response to humidity changes, but also in terms of where the wood will expand or contract. In practical terms, wood swells or shrinks only across the grain and along the radial rays. The wider and thicker a board is, the more it moves due to moisture change. For typical projects, we can safely ignore movement along the length or along the thickness of material that is less than 1 1/2” thick.

Choosing the Wood Species and Cuts

Different wood species and cuts react differently to moisture changes in the environment. For instance, ash and birch change less in their dimensions with humidity change in contrast to red oak. How the wood is cut also affects how much it moves with moisture change: quartersawn wood moves only half as much as flatsawn in width, but flatsawn wood moves less in thickness. Knowing this allows you to select the right mix of species and cuts for the critical components of your projects.

Applying an Equal Amount of Finish

Sealing a finished piece to prevent the exchange of moisture is a myth that Krenov compared to trying to keep a person from breathing. Certain finishing can reduce but never stop wood movement. When finishing, apply the same amount of finish on all surfaces to allow for even moisture transfer in wood, keeping the risk of warping to a minimum.

Armed with these techniques, I made a nook table in sapele (this time for my wife). I milled lumber from the same cut, removed the same amount of material from each side of the top when dressing, attached the top to the base with screws in elongated holes along the grain, and applied six equal coats of varnish top and bottom, patiently waiting for each coat to dry first. That's how you should go about building fine pieces that will stand the test of time.

Text and photos by Charles Mak

Charles Mak, now in retirement, is an enthusiastic hobby woodworker, teacher, writer and tipster. He formerly worked part-time at his local Lee Valley Tools store.