Cupping. I have previously discussed cupping of wood floors, which oftentimes is caused by an imbalance of moisture throughout the wood (see chart). Typically, this happens when the wood is wetter at the bottom and the top, when subjected to heat and low humidity, shrinks and thereby produces a cupping effect. This is the most common cause, usually due to moisture in the form of vapor traveling through a concrete slab on grade.
However, a less common and less understood cupping effect is the result of topical moisture (due to spills) or high humidity. When exposed to either or both of these moisture sources, the wood flooring can expand and compress with cupping then occurring.