HOW TO COPE WITH WOOD MOVEMENT



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.

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.

Using a moisture meter

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Using a Moisture Meter


I use a moisture meter for two key purposes: 1) to check that the dressed lumber has stabilized, and 2) to ensure that all of the project parts have similar moisture content. For indoor furniture subject to average heat and humidity, use lumber that has a moisture content between 8% and 12% or so. The hotter and drier the conditions will be, the lower the moisture content should be. The correct way to use a moisture meter is to poke the pins into side grain.

Using a moisture meter.

Top: Wood Movement Guide. Bottom: Japanese-style box.

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Calculating Wood Movement


We use a wood movement guide to determine the amount of dimensional change. Using a Japanese-style box I recently finished as an example, in the sidebar below I lay out the steps I followed to calculate the wood movement for the box’s lid. If you don’t have a reference guide, you can find an online shrinkage calculator for the movement value.

Expansion Allowance for a Box’s Lid

Wood:

Douglas fir

Lid:

Panel width: 8 3/8”

Moisture content fluctuation for my place (about 16 °C at relative humidity of 35%): 5%

Movement value (tangential) from the guide: 0.0026

Projected movement (with a 25% safety margin): 8 3/8” x 0.0026 x 5 x 1.25 = 0.136”, or 9/64”

The box opening is built 8 1/2” wide, so with the safety margin there is room for the lid to move during different seasons.


Top: Wood movement guide. Bottom: Japanese-style box.

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.

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.

Orienting the Boards with Care


In general, a board cups in the direction opposite to the rings' curves. When I built a coopered box for a set of tools, I oriented the sides with their heart side facing out so if they ever move, they can only tighten the joints.


Coopered box the author built

Coopered box the author built.

Handling the Panel Movement


In case work, I rely on one of two trusted ways to keep panel movement at bay. The simplest method is to use plywood stock for the panel. For instance, I simply nailed the back panel to the carcass for a tool cabinet, with no worries of any self-destruction.


Tool cabinet with back panel attached by simply nailing it in place


Tool cabinet with back panel attached by simply nailing it in place.

When solid wood panels are the right call, however, I use tongue-and-groove construction, as in the case of the Japanese-style storage box I made. This construction method prevents the common misery of cracks and splits seen in so many older case constructions.


Tongue-and-groove construction


Tongue-and-groove construction.

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.



Further Reading

Cummins, Jim. “Cross-grain Constructions”. Fine Woodworking. Sept./Oct. 1988.

Frid, Tage. “Textbook Mistakes”. Fine Woodworking. Spring 1976.

Krenov, James. The Fine Art of Cabinetmaking. Sterling Publishing Co. Inc. 1992.

Lee Valley Tools. Wood Movement Booklet. Lee Valley Tools. 2003.

Coping with Wood Movement

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