What will winter do to your wood?

When you do the math, the answer is given in inches: 0.339″. That’s slightly less than 1 1/32″, or between 5/16″ and 3/8″. Here is the tricky part, though. My own experiments have shown that the formula consistently overstates wood movement when the lumber is flatsawn. I have two explanations for this. First, wood scientists assume a completely flat growth ring when stating the rate of movement of flatsawn lumber. Because the growth ring is usually U-shaped, it means that each flatsawn board is actually riftsawn for more than half of the board. (And we know that riftsawn moves less in width than flatsawn.)

Second, wood scientists calculate wood movement with unfinished wood. All finishes let moisture through slowly, but many will slow moisture transfer enough that some of the extreme high or low humidity conditions that only last a day or two won’t have full effect on the wood. In other words, the 0.06 number used for MC may very well be exaggerated.

There are many variables here and many assumptions. But, my own testing and experimentation leads me to cut in half the result from the formula when calculating wood movement for flatsawn lumber. When using quartersawn lumber, I trust the formula’s result because growth rings any less than perfectly vertical result in increased movement in width, while the effect of the finish on the wood tends to cancel it out. So, for the example above, I would assume that the panel will move year-round by about 0.165″ in width, or just under 3/16″. We assume zero movement in length because it’s longitudinal movement.

The question now is what you do with the information. If your workshop is not humidity-controlled and relative humidity conditions are currently at an annual high in your area (such as August in the Toronto area), you can assume your lumber is at its largest, swollen, year-round size. So, you can assume that all 3/16″ of annual wood movement will be contraction after the door is completed. In that case, make sure the dados holding the panel are well over 3/32″ deep per side so that the panel can’t contract out of the frame. Pinning the panel at its centre point using a small dowel or nail on the back keeps the panel centred in the fame. Also, there is no need to have a lot of extra dado depth when inserting the panel because all of the yearly movement will be contraction. You still want to leave something small, perhaps 1/16″ per side, in case your calculations are wrong.

Moving around

The chart bellow shows the rate of movement in width, based on various species and growth-ring orientation. The rates of movement are expressed as a percentage. To use the number in the wood-movement formula, move the decimal two places to the left. (So 7.1 per cent becomes .071 in the formula.)

Only rates of movement in width are given, since movement in width is always the biggest concern. To get the rate of movement in a thickness of flatsawn oak, use the number in the column showing you the rate of movement in width of quartersawn oak, as they are the same. Riftsawn lumber has the same rate of movement in both thickness and width. And remember: the rate of movement in length (longitudinal movement) is considered zero for all species for indoor furniture purposes.

Species Flatsawn Quartersawn Riftsawn

Ash, black 7.8% 5.0% 6.4%
Beech 11.9% 5.5% 8.7%
Cedar, western red 5.0% 2.4% 3.7%
Cherry, black 7.1% 3.7% 5.4%
Mahogany, South American 5.1% 3.7% 4.4%
Maple, hard 9.9% 4.8% 7.35%
Maple, soft 8.2% 4.0% 6.1%
Oak, red 8.6% 4.0% 6.3%
Oak, white 10.5% 5.6% 8.05%
Pine, Eastern white 6.1% 2.1% 4.1%
Teak 4.0% 2.2% 3.1%
Walnut, black 7.8% 5.5% 6.65%