What will winter do to your wood?

Do…

– Allow wide panels, such as tabletops and door panels, to float freely as relative humidity and moisture content change.
– Pay attention to the boards you choose so that only boards with similar growth-ring orientation are edge-glued together to make panels.
– Choose quartersawn lumber, or at least riftsawn, for higher-risk cross-grain construction situations that come up in your designs.
– Use veneered plywood when necessary to eliminate wood movement.
– Make calculations to estimate wood movement so that you are able to design your projects around it intelligently.

Don’t…

– Design with wide areas of cross-grain construction in solid wood without engineering some method of free movement into the joinery.
– Glue a solid-panel tabletop to its base. Use cleats or some other method to allow the top to expand and contract freely.
– Glue panels into frame-and-panel door frames unless the panels are plywood.
– Glue plywood bookshelves into dados cut into solid-wood gables, or vice versa. Solid wood and plywood can only be combined in certain ways because solid wood moves and plywood restricts movement.
– Ignore wood movement or you’ll watch your project slowly self-destruct over time.

Calculating wood movement

Wood technologists have come up with a wood-movement formula that helps you calculate such things as how much a drawer front is likely to expand and contract annually or how much your 36″-wide tabletop will move. The following is the formula I use:

D = ΔD x R x ΔMC ÷ fsp

It means that the change in dimension (ΔD), which is what we’re trying to calculate, equals the current dimension (D), multiplied by the rate of movement (R, which varies by species and the particular cut of lumber), multiplied by the change in moisture content the piece of wood is going to be subjected to (ΔMC, which depends entirely on relative humidity conditions), all divided by the fibre saturation point of that species (fsp).

Before you panic, let me simplify the formula by inserting one fixed number: 0.28 for fsp, a number that wood scientists consider a fair number to use for this variable. While it does vary slightly from one species to another, this is an acceptable number to use.

The ΔMC in the formula is dependent on the area in which you live, and you can calculate this number with the help of weather statistics or a book such as R. Bruce Hoadley’s Understanding Wood: A craftsman’s guide to wood technology. For the Toronto area, which isn’t far from where my workshop is located, I use a figure of 0.05 for ΔMC, or 0.06 if I’m being extra-cautious. That means that I’m assuming that the wood in a finished project, stored indoors, will be subjected to a moisture-content change of no more than six per cent over the year. So, the wood might take on an moisture content of something like 10 per cent at the height of summer if you don’t use air conditioning or a dehumidifier; four per cent would be quite possible in January or February in a forced-air furnace environment with little or no humidification.

The best way to illustrate the use of the formula is to give an example. Let’s suppose you intend to build a frame-and-panel door with a panel that is 16″ wide. It is made of flatsawn hard maple, which has a rate of movement of 9.9 per cent. (See the statistics in “Moving Around” on the next page.) The calculation would go as follows:

ΔD = 16″ x 0.099 x 0.06 ÷ 0.28

The rate-of-movement value of 9.9 per cent was converted to a decimal to become 0.099.