Wood absorbs and releases moisture until it is in equilibrium with the air around it, and it changes width as it does. The floor is telling you what your humidity has been doing for the last few weeks.
Wood never stops responding to humidity. A solid oak board takes on moisture in a humid summer and swells across the grain, then gives it back in a heated winter and shrinks. Over a year that movement is normal and reversible.
What is not normal is movement that does not reverse, or that concentrates in one part of the room. Both point at a humidity problem rather than a flooring fault, and both are fixable before the boards are.
Reading the floor
| What you see | What it means |
|---|---|
| Thin gaps that open every winter and close every summer | Normal seasonal movement. Nothing to fix, though holding 35–40% in winter keeps them smaller. |
| Gaps that stay open through the summer | The floor has been over-dried. Sustained winter humidity below 30% shrinks boards past the point they fully recover. |
| Board edges higher than their centres — cupping | Moisture from below. The underside is wetter than the top: a damp crawl space, a slab without a vapor barrier, or a leak. |
| Board centres higher than the edges — crowning | Usually a cupped floor that was sanded flat while still wet, then dried. The sanding removed the high edges and the middle rose. |
| Gaps or cupping in one area only | A local moisture source — a vent blowing on the floor, a radiator, a leak under a dishwasher, or a cold spot over an unheated space. |
What is your reading telling you?
- Dew point
- 53 °F
- Mold risk, cold walls
- Moderate
Still inside the accepted range, but this is where cold surfaces begin to sweat.
What to do, in order
Free actions first. Do not buy anything until these have had two weeks.
Today
Put a hygrometer in the room and read it morning and evening for a week rather than judging on one number.
Look underneath if you can — a crawl space or basement below a cupping floor is where the moisture is coming from.
Move rugs off the affected area so the boards can equalise, and keep furniture from trapping moisture against the surface.
Check for the obvious local sources: dishwasher, fridge line, radiator, a floor vent aimed at the boards.
This week
In winter, hold 35–40% with a humidifier if the reading sits below 30%. This prevents permanent gapping.
In summer, hold under 55% with a dehumidifier if cupping appears.
If the floor sits over a crawl space, that space needs a sealed vapor barrier before anything else will hold.
Give a cupped floor time. Boards frequently flatten on their own over weeks once the moisture difference is removed.
If it has not improved
Never sand a cupped floor while it is still wet. It will crown when it dries and the damage is then permanent.
Fix the source below the floor first: crawl space encapsulation, slab vapor barrier, or the leak.
Once the moisture content has been stable for several weeks and the boards have flattened as far as they will, then consider refinishing.
For gaps that never closed, filling is cosmetic and often cracks. Holding a stable humidity range prevents the next round.
What the floor is telling you about the rest of the house
A cupping floor is one of the clearest signals of a moisture problem underneath a house, and it usually appears before any smell or visible growth. If the boards are cupping, the joists and subfloor below them are damp too.
Winter gapping is the mirror image: if the floor has shrunk enough to open visible gaps, the air has been dry enough for long enough to be affecting the people in the room as well — dry airways, static and cracked skin come from the same conditions.
Common questions
What humidity is best for hardwood floors?
35–55% year-round. Most manufacturers specify something close to that, and staying inside it keeps seasonal movement small and reversible.
Why are there gaps in my wood floor in winter?
Heated air is dry, so the boards give up moisture and shrink across the grain. Thin gaps that close again in summer are normal; holding 35–40% keeps them minor.
What causes a wood floor to cup?
The underside being wetter than the top — a damp crawl space or basement, a slab with no vapor barrier, or a leak. The wetter face swells and the edges lift.
Will a cupped floor flatten on its own?
Often, once the moisture difference is removed, over several weeks. Sanding it flat while still wet is what turns a temporary problem into a permanent one.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions