Change in width
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Shop estimate. Measure your stock and cut a test on scrap.
How to use it
- Pick the species. Names follow the handbook. Sugar maple is the hard maple. Commercial red oak and white oak are separate, because they move differently.
- Choose the direction. Across a flat-sawn board, use tangential. Across a quarter-sawn board, use radial. Mixed is the average, for rings that run on a diagonal. It is a guess, and it is labeled as one.
- Enter the current width and the two moisture contents.
- Read the change and the finished width. A negative change is shrinkage.
- If either moisture content is outside 6% to 14%, the arithmetic still runs and the card says the coefficient was not built for that band.
The formula
Change = dimension × coefficient × (ending moisture − starting moisture).
- C. Dimensional change coefficient per 1% moisture, based on the size near 10% moisture content.
- Tangential. Around the rings. This is the width of a flat-sawn board.
- Radial. Across the rings. This is the width of a quarter-sawn board, and roughly the thickness of a flat-sawn one.
Worked example
This is the Wood Handbook’s own white-fir check. A flat-sawn width of 9.15 inches at 8% moisture content, moving to 11%, changes by 0.067 inch. The tangential coefficient is 0.00245. The same board’s thickness, read radially, uses 0.00112.
A 200 mm flat-sawn white-oak panel from 12% down to 8% shrinks 2.92 mm across the width. Leave that much room, or more, if the panel is trapped. The coefficient is only tabulated for 6% to 14%. Outside that band the page still does the arithmetic and tells you the band was left.
What it will not decide
- It does not predict cup, twist, or bow. Those depend on how the rings sit in the board, not only on the average width.
- Movement along the grain is small for furniture lengths and is not computed.
- It is not a structural rating and not a kiln schedule.
Questions
Where do the coefficients come from?
The Wood Handbook, published by the U.S. Forest Products Laboratory, a federal public-domain source. The white-fir worked example on this page is the handbook’s own check: 9.15 inches, tangential, from 8% to 11%.
What if my shop swings from 5% in January to 13% in August?
Thirteen is inside the band. Five is just outside it. The page will flag that. The estimate is still a useful size for a gap, and it is still an estimate. Leave a little more than the number if the panel is trapped in a frame.
Which oak do I pick?
White oak and red oak are both in the list, with different tangential coefficients. If you do not know which you bought, measure a sample through a season rather than averaging them and calling it exact.