How to Fix Wood Floor Swelling

Moisture is bad for all types of solid and engineered wood floors, as well as bamboo floors. Apart from the damage it can do to the surface -- stains and discoloration -- moisture causes wood to swell, and when that happens the floor can be ruined. Reducing humidity in the room may help, but if the moisture is in the subfloor -- which is usually the case -- the solution may not be straightforward. If you can't dry things out, you may end up having to replace the flooring.

Flooring Defects Caused by Moisture

Wood is hygroscopic, meaning it attracts and holds water, and when it does, it swells to make room for the extra fluid. Common moisture defects in solid hardwood flooring reflect the fact that this absorption usually isn't uniform.


When water soaks in through the bottom of a board, the bottom swells, but the face doesn't. This causes the sides of the board to lift higher than the center, and the warping that results is called cupping.


One of the remedies for cupping is to sand the floor flat, but if the wood is finished before it has a chance to dry out, the face might swell and lift higher than the edges -- a condition called crowning.


Severe crowning, cupping or warping that results in part of a board lifting off the subfloor is called buckling. Buckling is caused by excessive moisture and usually requires replacement of part of the floor.

Moisture Control

If your hardwood floor starts cupping, it's a sign that moisture has migrated into the subfloor. It may have gotten there because moisture in the air condensed on the floor and seeped between the boards, or it may come from another source, such as a plumbing leak. It's essential to address the cause of the moisture, either by lowering room humidity, fixing a leak or adding moisture protection underneath the subfloor. You should then use fans or dehumidifiers to ventilate the subfloor and dry it out. When you do this, the boards may shrink back to their normal shape. It's also possible that they won't.

Sanding a Cupped Floor

The failure of the floor to flatten leaves you with two options. If only a few boards are affected, you can . On the other hand, a large section of the floor may be damaged, in which case, sanding and refinishing the floor is the alternative.

During the first pass with the floor sander, a refinishing professional will run the machine in a diagonal direction to flatten the edges more quickly, then do repeated passes parallel to the boards with progressively finer sandpaper to smooth the wood.

Preventing Swelling

Some instances of swelling may be due to poor installation practices. To prevent cupping and buckling, the installer should:

  • Acclimate the flooring by laying it loose around the installation space and leaving it for three to five days before nailing it down.
  • Cover the subfloor with a moisture barrier -- usually resin paper, tar paper or plastic sheeting stapled to the plywood.
  • Provide an extra moisture barrier between the ground and the subfloor on ground-floor installations. In some case, extra ventilation may be required in the crawl space.
  • Use the recommended number of fasteners. Wider planks need more fasteners than narrower ones, and the fasteners should be more closely spaced.

Chris Deziel

Chris Deziel

Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.