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 centers — 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 centers 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. |
How to be sure it is this
Gaps in a timber floor are a moisture reading you can see. Whether they need action depends on which kind of movement you are looking at.
-
Do the gaps close again in summer?
Seasonal gapping opens in the heating season and closes when humidity comes back up in spring. That is normal behavior for solid timber and needs a stable humidity rather than joinery. Gaps that never close indicate the boards have taken a permanent set, usually after a long period of very low humidity.
-
What does the hygrometer read in that room?
Sustained readings below 30% in winter explain gapping entirely. Readings in the forties do not, and point instead at an installation problem — boards laid without an expansion allowance, or laid at a moisture content far from the room they now live in.
-
Are the gaps even or concentrated?
Even gapping across the whole floor is humidity. A single large gap, or cupping and crowning in one area, points at a local moisture difference — a leak, a damp subfloor, a bathroom doorway, or a heating pipe running under the boards.
-
Are the boards cupped or crowned?
Edges higher than the center means the underside is wetter than the top, which is a subfloor moisture problem rather than a dry-room problem. Center higher than the edges means the opposite. Flat boards with gaps are simple shrinkage.
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 equalize, 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 happens if you leave it
The first winter of very dry air produces gaps that close again in spring, and nothing has been harmed. This is what solid timber does, and a floor that has been doing it for fifty years will keep doing it.
Repeated hard cycling is what causes lasting damage. Boards that shrink and swell through a large range every year loosen their fixings, develop squeaks, and open joints in the finish along the board edges. Engineered and laminate floors handle it worse than solid timber, because the layers move against each other.
At the extreme — sustained readings in the low twenties over several winters — boards split, ends lift, and the finish cracks along the movement lines. A floor kept between 35 and 50% year-round avoids essentially all of this, which is the practical reason to run a humidifier in a house with a timber floor rather than a purely comfort-based one.
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.
Should I fill the gaps in a wooden floor?
Only if they no longer close seasonally, and then with something flexible. Filling a gap that is going to close in spring pushes the boards apart when they swell, which either crushes the fibers at the edges or lifts the filler straight out. Deal with the humidity for a full year first and see what the floor does before committing to a repair.
What humidity should I keep a house with hardwood floors at?
Between about 35 and 50% year-round, with slow changes rather than sudden ones. The exact figure matters less than the stability: timber tolerates a wide band comfortably and objects strongly to swinging across it twice a year. Aim to soften the winter trough with a humidifier rather than to hit a precise number.
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