Woodworking Tips
Woodworking Tips Index
Wood Movement, Part 1
The change of seasons is here. Winter is gone. It's raining outside, and I turned off my whole-house humidifier inside. At this time of the year, I'm always reminded that wood "moves."

There's nothing more frustrating than putting a lot of time into a project only to find out six months later that a panel split apart, doors won't close right, or joints have opened up. It's all because wood moves. It expands and contracts with changes in relative humidity, and there's nothing you can do to stop it. But what you can do is take wood movement into account when designing and building a project. This might mean special joinery. Or certain hardware that allows wood to move freely. It might even mean a different way of gluing parts together.

Wood moves because it acts like a sponge. When the surrounding air is damp, wood absorbs moisture from the air and expands. When the air is dry, it releases moisture and contracts. And this movement can be considerable. As a rough rule of thumb, a 12"-wide piece of hardwood can expand or contract as much as 1/8" (or about 1%) across its width. (It moves very little along its length.) So with the seasonal changes in humidity, furniture is taking in and releasing moisture all the time. If a project isn't designed to handle this, then you're asking for trouble down the road.

Fortunately, there are a number of things you can do to allow for this movement. I got some suggestions from the guys at Woodsmith about how they have handled wood movement situations on some Woodsmith furniture projects.

When trimming out a cabinet with molding, there's one thing to remember. Molding should never be glued across the grain of a solid wood panel, such as the quarter round molding under the top panel on our Pie Safe. (Note: This doesn't apply if the panel is built from plywood. Plywood is very stable and moves very little with changes in humidity.)

The problem is that a solid panel (like the top of the Pie Safe) can expand and contract quite a bit across its width. But the strip of molding will change very little along its length. If the molding is glued to the panel and the width of the panel changes, the glue joint can break causing the molding to come loose or fall off. There's even a chance the panel could split because the molding is preventing the panel from moving.

On the Pie Safe, we didn't glue the molding to the solid top at all. That would have prevented the top from expanding and contracting. Instead, we pushed the molding up tight against the top and nailed it (using wire brads) to the frame that makes up the side of the Pie Safe. The stiles of the side frame are narrow enough that they will move very little. And even if they do, there's enough "give" in the brads to allow for movement while holding the molding in place.

Next week, we'll look at table tops and blanket chest lids.

Go to Tip #58
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