Dry cupping is the term given to the advent of dips and divots in wood, and it can make a formerly beautiful wooden floor look like a wagon worn trail, which it seems no amount of wood floor restoration will be able to put right. Even if cupping does not have quite such an aesthetically catastrophic effect, however, mild cupping can still make the installation of new hardwood floors extremely difficult, particularly if working with feature strips and borders.
The cell walls in wood are made up primarily from cellulose, a very strong substance that water molecules tend to cling to. When the humidity of or around the wood increases, the moisture in the air results in a lot of water/cellulose bonding and this is what causes swelling, which ends up with the wood expanding.
Cupping is the flipside of this, which happens when the moisture level of the wood actually decreases. Generally speaking, the solid wood itself does not actually shrink, however, but the layered construction of engineered wooden flooring. Engineered hardwood flooring tends to consist of a thin strip of sawn, solid wood that is glued to the top of a piece of plywood. When cupping occurs, the plywood itself is a lot more stable in terms of the effect moisture has on it, and remains relatively unchanged but the top layer effectively shrinks, ultimately curling or even peeling away. This tends to affect wooden floors that are shipped from one climate to another that has significantly lower humidity levels.
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