This is when the edges of a board are higher than its center, due to moisture which causes the wood to expand. This can occur after water spills onto the floor and is absorbed by the wood. However, usually high RH is the culprit. As the wood expands, compression can result as the boards are crushed together, deforming the boards at the edges. Cupping is caused by a moisture imbalance through the thickness of the wood. The wood is wetter on the bottom of the board than on the top surface, which dries quicker than the bottom. Cupping most often appears after the floor has been installed and may or may not be an installation issue.