|Prevent Wood Movement Disasters in 4 Easy Steps|
David: Wood movement can ruin a seemingly well-constructed project. Luckily getting ahead of the material and calculating wood shrinkage and expansion is fairly easy - all you need to do is multiply three numbers together. Here's how to find them.
How to Calculate Wood Shrinkage and Expansion
Wood movement can ruin a seemingly well-constructed project. Luckily getting ahead of the material and calculating wood shrinkage and expansion is fairly easy-all you need to do is multiply three numbers together. Here's how to find them.
Step 1: Measure the board's width.
.Know that wider boards expand and contract more than narrower ones.
.Wood expands and contracts mostly across its width. Measure Width in inches.
Step 2: Find the average yearly change in moisture content.
.Wet air expands wood, dry air shrinks it.
.Use a moisture meter to note the highest moisture content (MC) in your wet season and the lowest in your dry season.
.Subtract the smaller number from the larger one to find the yearly change. Most climate-controlled houses change 3 percent to 4 percent MC during a year.
Step 3: Determine the dimensional change coefficient of the species.
.Dimensional Change Coefficient::A number that reflects how much a certain species of wood will change in width.
.Formula works only when wood is between 6 percent and 14 percent moisture, but this is a fair range for furniture.
.First, determine if your board is mostly flatsawn (F/S) or quartersawn (Q/S). Few boards are entirely one or the other, so make a best guess. The best place to look is in the end grain (see drawing at right).
.Now look up the right number in the chart of common species.
Step 4: Do the math.
.Multiply the width of the board (in inches) by the annual change in MC (highest annual MC minus lowest). Then multiply the result by the number from the chart.
.Because few boards are entirely F/S or Q/S, calculate both numbers and shoot for something between.
Example: The workbench in this issue is 24" wide. The annual change in moisture in our shop is 3 percentage points (12 percent minus 9 percent). The top is mostly Q/S yellow pine (the numbers from the chart are .00176 and .00263).
So our equations are:
Q/S: 24 x 3 x .00176 = .127" (about 1/8")
F/S: 24 x 3 x .00263 = .189" (about 3/16"