What does cupping in hardwood floors refer to?

Cupping in hardwood floors refers to the phenomenon where the edges of the boards rise higher than the center, creating a concave shape. This typically occurs as a response to changes in moisture content within the wood. When the edges of the boards absorb moisture, they expand more than the center of the boards, leading to this distinctive shape. Cupping can be an indicator of excessive moisture in the environment or issues with the installation process, which makes understanding this term crucial for maintenance and restoration of hardwood floors.

The other options describe different issues unrelated to cupping. For instance, while reduction in size due to heat, formation of bubbles in the finish, and increase in thickness due to moisture can all affect hardwood flooring in various ways, they do not accurately represent the specific issue of cupping.