Hardwood floor cupping

Symptoms

• Boards feel higher at the edges than the middle
• Floor looks slightly wavy when viewed at a low angle
• Often appears after a spill, flood, or humid season
• Worse near exterior walls, bathrooms, or kitchens

Likely causes

• CommonExcess moisture in the wood from below (crawlspace or basement humidity)Wood absorbs moisture unevenly, expanding more at the bottom of the board than the top, which cups the edges up.
• CommonA water leak or spill that wasn't dried quicklyLocalized water intrusion causes localized cupping.
• PossibleHigh indoor humidity without adequate ventilationPersistent humidity swings stress the wood's moisture balance.
• PossibleMissing or inadequate vapor barrier under the floorWithout a barrier, ground moisture wicks up into the wood continuously.

What to do

1. Identify and fix the moisture source first; cupping won't resolve while the wood keeps absorbing water.
2. Check basements or crawlspaces below the cupped area for standing water, high humidity, or a missing vapor barrier.
3. Use a moisture meter on the wood if you have one, or note how long the cupping has been present.
4. Run fans and, if needed, a dehumidifier in the room to bring indoor humidity into a normal range.
5. For a recent spill, dry the area thoroughly and monitor over the following weeks; mild cupping from a one-time event often flattens back out as the wood dries evenly.
6. Avoid sanding a cupped floor while it's still elevated in moisture, since sanding it flat while wet can leave it too thin once it dries and shrinks.
7. If cupping doesn't improve after the moisture source is fixed and the wood has had weeks to dry, consult a flooring pro about refinishing or board replacement.

Tools and parts

• Moisture meter (optional)
• Fans
• Dehumidifier

When to call a pro

Call a flooring professional to assess before sanding a cupped floor, and a moisture/waterproofing contractor if the source is a basement or crawlspace issue.

Safety