When the moisture increases the wood swells and then when it decreases the wood shrinks.
Wet hardwood floor cupping.
When moisture interacts with a hardwood floor the side of the boards closest to the moisture expands.
If you imagine a picture of a child drawing a boat in the water then the water will give you a good idea of what a cupped floor looks like.
A cupped floor develops as moisture from the underlayment or subfloor moves upward through the hardwood as the back of the wood becomes wetter than the face of the wood.
The general effect is easy to demonstrate by putting a small strip of paper onto a small drop of water.
Underlayment and or subfloor that is too wet at the time of installation will cause wood floor cupping.
Cupping is a result of the changes in moisture in the room.
Wood is hygroscopic in nature and thereby tends to absorb moisture from its surroundings causing problems in hardwood floors.
This expansion can be in the form of cupping the center of the board is lower than the edges or crowning the center of the board is higher than the edges.
Cupping in solid wood floors cupping occurs in solid wood flooring as a result of an elevated mc in the bottom of the flooring compared with the mc of the face.
As you can see moisture and wood don t go well together.