Underlayment and or subfloor that is too wet at the time of installation will cause wood floor cupping.
Wet hardwood floor cupping.
As you can see moisture and wood don t go well together.
When the moisture increases the wood swells and then when it decreases the wood shrinks.
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.
The paper will curl up away from the water.
It ends up looking a little like an accordion.
Wood is hygroscopic in nature and thereby tends to absorb moisture from its surroundings causing problems in hardwood floors.
When moisture interacts with a hardwood floor the side of the boards closest to the moisture expands.
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.
The general effect is easy to demonstrate by putting a small strip of paper onto a small drop of water.
The simplest way to think of wood floor cupping is to imagine the edges of each plank sticking up higher than the centers.