We are all familiar with this type of design for a door, also known as panel door. This one has four panels, but you'll often see them with six, and occasionally with more. And as you've probably guessed by reading through this series, those panels were traditionally not placed there for aesthetics, but for functionality. This is a time-tested, very clear-cut example of how you use design to cope with wood movement.
First off, let's get some basic terminology down by looking at a smaller version of the same concept: A cabinet door.
As you can see, the horizontal crossmembers are called rails and the verticals are called stiles. If we look at it in section, we see the panels have tongues that ride inside grooves in the rails and stiles:
This tongue-and-groove arrangement allows the panel to "float" within the frame, expanding and contracting with humidity changes. The depth of the grooves of course have to be calculated using the techniques we've been discussing; you don't want the panel's expansion running out of room and cracking, nor do you want it shrinking so much that you can see daylight between the pieces.
Let's look at a more complicated door. You can ignore the top half of it with the glass panes, we're only interested in the frame-and-panel portion of the lower half:
If we zoom in on that door's midsection, we can see just how crafty wood joiners have become over the years to get all of these pieces to fit together while still allowing for movement:
Obviously this requires careful forethought. Panel doors like the one below look simple, but the red arrows indicate all the places where you can get joint failure through improper design and construction:
This is why it's important to understand how to estimate shrinkage or swelling of panels and frames. In the last entry we included an image of a graph you can use to determine the equilibrium moisture content (EMC) of wood; here's a link to a more comprehensive explanation of how to use it.
In any case, the design concept behind the joinery of panel doors can be applied to all kinds of furniture, objects and flooring. The basic idea is this somewhat Buddhist-sounding principle: UYou have a piece that is attached, yet not attached, to another piece. This can be accomplished through tongue-and-groove, as with drawer bottoms, which often ride in slots and are only tacked on one end, so they can expand in the other direction (usually front-to-back, as opposed to side-to-side.) There are also tricks like using screws driven through a slot in Part A while its threads bore down into Part B, yielding a connection that can move. Or you may cut a mortise a carefully calculated distance wider than the tenon it's taking.
There are also new products on the market (with varying degrees of uptake) that builders from 100 years ago could have never imagined. Space Balls are little rubbery spheres that you can shove into the grooves before you stick a panel in; since the spheres can expand and contract with the panel, they prevent it from rattling in the door when it shrinks, and they keep the panels visually centered. (They're also, from what we hear, a bit tricky to install.) Other builders have worked up their own hacks, using silicone to draw a circumferential bead that does the same thing.
And yes, there are commercial attempts to deal with wood movement through the use of chemicals. One stabilization treatment used by woodworkers is polyethylene glycol, a.k.a. PEG. At room temperature it resembles a wax, and it is very water soluble. When you soak a fresh-cut piece of lumber in PEG, the PEG replaces water molecules in the wood and so the lumber remains in its freshly swollen state. In theory, PEG should halt the natural shrinkage of the lumber. However, there are limitations to this process. The lumber has to be very fresh and have a high moisture content, and it ought to be a relatively thin cut, around one inch thick depending on the wood density. For a thorough discussion of PEG limitations and benefits see this pdf from the U.S. Department of Agriculture.
Still, the point of this series is to get you thinking about what wood, itself, is. To get you to know your material. When designing for movement it helps to choose stable cuts, as I've discussed in previous posts. Also it's good to know species of wood that are fairly stable to begin with. For instance, mahogany is more stable than beech. Teak is more stable than maple. Check out this list of wood species and the relative stability. Overall, as a rule, the closer the grain lines (the alternating lighter and darker regions within the trunk) the more stable the wood. So hardwood tends to be more stable than softwood.
The bottom line is, we need to remember that wood is what it is—and working with a natural fiber is always going to be tricky and at times very frustrating. But similar to our relationships and interactions in all areas of life we need patience and understanding in order to thrive and be happy in those relationships.
The best book we've found on the subject of wood is Bruce Hoadley's Understanding Wood, a fantastic resource and one that goes much further into depth than we can in this series. And his philosophy is compelling:
I began trying to use wood "in the raw," but I discovered I didn't know how. Then I became eager to learn ways to "overcome" all the "problems" that wood has, and I experimented with wood stabilizers and chemicals, impatient to improve upon nature's product. In time, however, I realized a certain distaste for trying to make wood into something it isn't and for trying to make it do things other materials do better. I'm back to using wood "as is" now but with a different point of view. I concentrate on learning what wood is, rather than worry about what it isn't; I try to work with it, not against it. Whether this makes me a better woodworker I am not sure, but I am more satisfied, for certainly part of the reward of working with wood is accepting the challenge of understanding it. The dimensional behavior of wood should be looked upon as simply a property of wood to be taken in stride, not as a problem to be corrected.
We hope that this series will encourage you to do more research on your own. And we hope that you can develop designs that work with the properties of wood, not against it.