Wood movement in design

Wood moves with its moisture content almost entirely across the grain. Along the length the change is so small that furniture can ignore it; across the width of a wide board it is easy to measure.

3 sources agree [source 1, source 2, source 3]

Between about 6% and 14% moisture content the change is close to a straight line, so the Wood Handbook gives each species a coefficient: the change in width per 1% change in moisture content, tangential for flatsawn faces and radial for quartersawn ones.

One source: USDA Forest Products Laboratory, the authority for the wood properties it measured [source 3]

Northern red oak’s coefficients are 0.00304 tangential and 0.00137 radial. A 12 in flatsawn top that goes from 6% to 12% moisture content grows 12 × 0.00304 × 6 = 0.219 in, about 7/32 in; quartersawn, 12 × 0.00137 × 6 = 0.099 in, about 3/32 in. The wood movement calculator works this for any species; the EMC calculator tells you the moisture content wood settles at for a temperature and humidity.

Arithmetic: exact by definition

A solid top fixed rigidly to its base across the grain can’t move, so it cracks or pulls apart. Attach it with wooden buttons, metal clips, or screws in slotted holes, so it can slide across its width.

Two sources: Fine Woodworking (Taunton Press) and Lee Valley Tools [source 1, source 2]

Quartersawn wood moves about half as much across its width as flatsawn wood of the same species, so it is the better choice where movement matters most.

Two sources: Lee Valley Tools and USDA Forest Products Laboratory [source 2, source 3]