Cup (Lumber Defect)
Cup is a warp across the width of a board, causing the face to curve from edge to edge like a shallow trough. It is most common in flat-sawn lumber where the growth rings are tangent to the wide face. Cup is measured as the height of the arc from a straight edge laid across the board's width.
In-Depth Explanation
Cupping occurs because wood shrinks approximately twice as much along the growth rings (tangentially) as it does across them (radially). In flat-sawn boards, this differential shrinkage causes the face closest to the bark side to become concave as the board dries. Wider boards are more prone to cupping because the tangential shrinkage has more material to act upon. Quarter-sawn lumber resists cupping because the growth rings run perpendicular to the face, equalizing shrinkage across the width. To minimize cupping in service, woodworkers often rip wide boards into narrower strips and re-glue them with alternating grain orientations, or they use quarter-sawn stock for tabletops and panels.
Usage Example
The wide pine boards cupped significantly after installation because they were flat-sawn and not acclimated to the indoor humidity before being fastened down.