A. Howard Brickman responds: You’re right — the cupping is a result of excessive moisture from below. You can sand a cupped floor as long as the cupping is actually permanent. Remember that most structures undergo a moisture cycle with the changing seasons. In the winter when outdoor temperatures are below freezing, buildings tend to dry out and wood flooring will shrink. In the summer, with higher temperatures and more humidity, the moisture content of the wood increases and cupping is more pronounced. If you sand a cupped floor that later flattens out as the relative humidity and the moisture content of the wood decrease, then the floor will crown (develop a convex profile).
The secret to sanding any cupped floor is to be absolutely certain the cupping is permanent. I would advise observing the floor for an entire heating season before sanding.
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