Hardwood floor cupping

Why hardwood cups

Wood takes on and gives off moisture with the air and anything it touches. When the underside of a solid or engineered hardwood floor gets more moisture than the top, from a leak, a damp crawlspace or basement, a wet concrete slab or very humid air, the bottom of each board swells more than the top. The edges rise and the centers dip, which is cupping.

Crowning is the opposite shape, with the centers higher than the edges, and it often shows up when a cupped floor is sanded flat before it dries. Buckling, where boards lift right off the subfloor, is more serious. A moisture meter, used the same way on cupped and normal areas, tells you how wet the floor is and when it's dry enough to decide what's next.

Common sources of the moisture

Let's fix it

1. Confirm it's cuppingLook across the floor with a flashlight held low. Cupped boards have raised edges and slightly lower centers, like a shallow U. Centers higher than the edges is crowning, and boards lifting off the floor is buckling, which needs a flooring pro. Note where the cupping is worst, since it usually points toward the moisture.
   Done

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2. Look for a leakCheck around anything with water near the cupped area: the dishwasher, the fridge water line, sinks, a bathroom, an exterior door or a window. Look for stains, soft spots or musty smells. Fix any leak first; check our guide for a leak under the sink.
   Done

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3. Check below the floorGo to the basement or crawlspace under the cupped area. Look for standing water, damp soil without a vapor barrier, sweating ducts or pipes, and a musty smell. Check the humidity there with a hygrometer. A damp crawlspace is a common cause of cupping across a whole room. In a crawlspace, watch for exposed wiring and pests, and wear a mask if there's mold.
   Done

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4. Measure the moistureUse a moisture meter on several cupped boards and on boards in a normal area of the same floor, and write down the readings and spots. Higher readings in the cupped area confirm moisture. Check the meter's settings for your wood species if it has them.
   Done

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5. Control the indoor humidityPut a hygrometer in the room. Indoor humidity in the range of about 30 to 50 percent is commonly recommended for hardwood floors. Run the air conditioning or a dehumidifier to bring it down, and use bathroom and kitchen exhaust fans.
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6. Dry it slowlyOnce the moisture source is stopped, run fans and a dehumidifier to help the floor dry. Avoid blasting heat at it, which can dry the top too fast and cause cracks. Drying can take weeks or longer. Take readings from the same spots every week.
   Done

   Something went wrong?

   • Readings aren't dropping There's still a moisture source. Recheck under the floor and around appliances.

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7. Wait before sandingDon't sand until the cupped boards read about the same as the normal areas and the readings have held steady for a few weeks. Many floors flatten on their own as they dry. Sanding a floor that's still wet leaves it flat for now, but the edges shrink as it dries and the boards end up crowned.
   Done

   Something went wrong?

   • It's flat but readings are still high Keep waiting; sanding now can crown the boards later.

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8. Sand, refinish or replace if neededIf the floor stays slightly cupped once it's fully dry, sanding it flat and refinishing is usually the fix, and it's best done by a flooring pro. Boards that are cracked, stained black or loose may need replacing; there's a separate guide for a loose hardwood floorboard.
   Done

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Watch it done

What Causes Wood Floor Cupping & What To DoBy Cali Floors on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it's a different floor problem

If one board is loose or raised, we have a guide for a loose hardwood floorboard.

If a laminate or vinyl plank floor is peaking, see the guide for a laminate floor that's peaking or buckling.

Cupped hardwood questions