WHY BOARDS CUP AWAY FROM THE CENTER OF A TREE
Discover a clever paper model from a 1936 issue of The Woodworker that visually explains how and why wood moves as it dries. Learn how flat-sawn, square, and quarter-sawn boards react to moisture changes so you can plan stronger, more reliable woodworking projects.
As a woodworker, I sometimes have a hard time wrapping my head around how wood moves. I know a flat-sawn board will cup away from the center of the tree, for example, but it's difficult to explain why.
Then the Lost Art Press vlog pointed to an ingenious way of illustrating wood movement found in a 1936 issue of The Woodworker magazine. Lucky for me, I own The Woodworker, which they beautifully republished in four volumes.
A Paper Model of Wood Movement
Here’s how the demonstration works:
I folded a piece of paper like a fan so I could wrap it around into a circle. The fan folds also create the same type of tension a log is under as it dries. Logs don’t just shrink into perfect miniature versions of themselves. Wood shrinks along the annual rings—called tangential movement—at about twice the rate that it shrinks across the rings, which is called radial movement.
This means the circumference of the log is getting smaller at twice the rate of the log’s thickness.
These tangential stresses are very much like the tension created by the fan folds in a paper circle, and in a log those stresses usually lead to a split.
Adding Boards to the Model
Now that I’ve explained how the model works, let’s add some boards as they would exist inside a log.
On the top, we have your typical flat-sawn boards.
On the bottom left, we have a square timber.
On the bottom right, we have the more desirable—but also more wasteful—quarter-sawn boards.
Now let’s release the tension and see how drying affects these different boards:
The flat-sawn boards cup away from the center of the log.
The square timber distorts in shape.
The quarter-sawn boards show exactly why that orientation is so prized by woodworkers—the shrinkage is much more even and predictable.
Why This Matters
If you know how your boards will distort as they absorb or lose moisture from the air, you can predict how it will affect your project and plan accordingly. That knowledge alone can save a lot of frustration—and a lot of firewood.
If you’d like to learn more about wood movement, including the differences between quarter-sawn, flat-sawn, and other types of cuts, check out this tutorial we made some time back.
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