Hardwood floor cupping — edges higher than the middle

Each board's edges sit higher than its centre, so the floor reads as a series of shallow troughs. Low winter light across the floor shows it before your feet do. Run a straightedge across the grain — a cupped board rocks on its edges.

What causes it

In rough order of likelihood. Each one has a tell — the observation that separates it from the others.

1. 1

   Moisture reaching the boards from underneath

   How to tell: The cupping is worst over a crawlspace, an unconditioned basement, a bathroom below, or one end of the room. Underside moisture is the cause in the large majority of cases.

2. 2

   A slow leak nobody has found yet

   How to tell: Cupping concentrated around a dishwasher, fridge line, radiator, or exterior door, spreading over weeks rather than appearing overnight.

3. 3

   Seasonal humidity swing in an unhumidified house

   How to tell: It appears every summer and largely flattens by February. Mild, uniform across the whole floor, and it has done this for years.

4. 4

   Installed before the material acclimated

   How to tell: It showed up within the first few months of a new floor and has never fully settled. This is an installation failure, not a maintenance one.

What it actually means

Wood cups when the underside is wetter than the top. It is a symptom, not a disease — the floor is telling you where water is. Sanding a cupped floor flat while it is still wet is the single most expensive mistake in this whole list, because when it finally dries you get crowning and a floor with no wear layer left to fix it.

Find and stop the moisture, then wait. Mild cupping frequently flattens on its own over weeks or months once the moisture differential is gone. Only then does sanding make sense. If the boards have delaminated, split, or lifted, that section is done.

• Find the water first. Check under sinks, behind the fridge, the dishwasher, and any basement or crawlspace below the affected area.
• Get the room to normal indoor humidity — 35–50% relative humidity — and hold it there. A dehumidifier in the basement often does more than anything done to the floor itself.
• Do not put fans or heat directly on the boards. Drying the top faster than the bottom makes the differential worse, not better.
• Mark the edge of the affected area with tape and photograph it weekly. Whether it is spreading changes the diagnosis completely.
• Do not sand it, and do not let anyone else sand it, until it has been dry and stable for weeks.

• Hardwood floor crowning — middle higher than the edges
• Hardwood buckling or lifting off the subfloor
• Black or grey pet stains on hardwood