Defects
Cup (Lumber Defect)
A warp across the width of a board that causes the face to curve inward, forming a concave shape from edge to edge.
In-Depth Explanation
Cupping occurs when one face of a board gains or loses moisture faster than the other, causing differential shrinkage or expansion across the width. Flat-sawn boards are especially susceptible because the growth rings run roughly parallel to the face. Cupping can often be corrected by jointing one face flat and then planing the opposite side to uniform thickness. Proper storage with even airflow on both faces helps prevent cupping. In severe cases, ripping a wide cupped board into narrower strips and re-gluing them with alternating grain orientation can produce a flat panel.
Usage Example
The wide poplar boards had developed a slight cup after sitting in the shop, so we ran them through the jointer and planer before glue-up.
Related Terms
bow
flat-sawn
moisture-content