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Wood moves with changes in relative humidity. Wood expands/contracts, even changes shape, as the relative humidity increases/decreases. Most of the change is tangential to the growth rings (width and/or thickness), the lengthwise change is insignificant. Flat sawn wood (typical) moves mostly in width, e.g. a 3% increase/decrease in relative humidity will cause a 24" wide flat-sawn pine board to grow/shrink 3/16".
Moisture Content and Movement (image source)
How to Calculate Wood Shrinkage and Expansion (pine example source)
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Wood Movement
Wood moves with changes in relative humidity. Wood expands/contracts, even changes shape, as the relative humidity increases/decreases. Most of the change is tangential to the growth rings (width and/or thickness), the lengthwise change is insignificant. Flat sawn wood (typical) moves mostly in width, e.g. a 3% increase/decrease in relative humidity will cause a 24" wide flat-sawn pine board to grow/shrink 3/16".Moisture Content and Movement (image source)
How to Calculate Wood Shrinkage and Expansion (pine example source)