Hello again, my topic today is wood movement.
I can practically see the puzzled looks through the screen. “What the heck are you talking about? The wooden things in my house don’t move unless I move them.” From a woodworker’s perspective, however, all the wood in your house is moving nearly all the time. When a woodworker like me refers to wood movement, what we’re actually talking about is the fact that wood is not dimensionally stable; that is, its dimensions change with humidity. This is all related to the cell structure of wood, and it can be a genuine pain to deal with in some instances.
A board will shrink and expand across its width, but its length will remain unchanged. This simple fact is the reason behind decisions about what sort of joint to use to attach things together, as well as for design elements like raised panels and breadboard ends on tables. As I mentioned above this change is a response to the humidity of the surrounding air; I’ve even seen a homebrew hygrometer made by using a stack of cross grained pieces of wood attached to a pointer with a scale. Since this effect is rarely perfectly uniform throughout a board, it is also responsible for most warping and twisting. The effect is strong as well. Attempting to keep the wood locked in place by attaching it to something that is rock solid will end up with the board splitting or destroying the glue joint that’s trying to hold it.
One of the first ways woodworkers deal with this sometimes annoying trait of wood is by the initial design. Let’s consider a door. The simplest way to make a solid wood door would be to either find a slab of wood wide enough to be cut into a door, or to glue up a few narrower boards to get the desired width. If you did this and hung the door to fit well during the dry winter months, chances are that by the middle of August the door would be so swollen it might not even close. Depending on the type of wood (because as with all natural products the species used makes a difference), a typical solid door could expand or contract by a ¼ inch, or even more, as the humidity changes during the year.
One time honored way of solving this problem, often used on large pieces of furniture as well, is the panel and frame, which uses a structure made up of relatively narrow frame pieces surrounding large panels. The trick is that instead of being glued into grooves in the frame members, the panels are actually just set in place. The only glue is used where the frame members attach to each other. When the humidity goes up and the panel tries to expand, then the edges of the panel simply drive a little deeper into the grooves in the frames and when it gets drier, the edges slide back out a bit. This prevents the door from twisting and warping, and also greatly reduces the overall change of the door’s size to something that can be lived with.
The most common way of dealing with this movement problem in doors these days is to make the door out of plywood. Plywood is composed of thin layers of wood, or veneers, which are oriented with their grain running in perpendicular directions to each other and then glued together under great pressure. The two outside layers or “face” veneers will have their grain running along the length of the plywood sheet, then ones just under them will have their grain running across the width. In a typical 5 layer ply, the center layer would match the two face veneers. Each veneer layer is too thin to generate the force needed to destroy the glue joints between the layers so the opposite grain construction results in a large flat panel which is dimensionally stable. Apart from the ones I have made, every wooden door in our house, and I suspect for most of my readers as well, is a plywood panel door.
Another way of taming the beast is with the final finish. Since the problem is caused by the wood absorbing and releasing moisture depending on the humidity, then it follows that by restricting the ability of the wood to do this, the expansion and contraction will be decreased. Water-impermeable finishes such as polyurethane and lacquers, in addition to protecting wood from spills, are also used in part to at least slow down the transfer of moisture between the wood and air. Though few finishes seem to completely eliminate the problem, as some moisture always seems to get through eventually, such finishes can at least mitigate the effect, but generally not enough to compensate for a poor design.
The way a board is cut from the log can also affect its potential for movement. Quarter-sawn wood is inherently more resistant to motion than flat-sawn wood for example, and as I pointed out earlier, different species of wood have different amounts of motion, but it is an aspect of wood that has to be taken into account on any sizeable project.
So, when you hear someone talk about wood “breathing”, they are not entirely off base, and as it breathes, so also does it move.