Gaps in Winter, Cupping in Summer: What Your Floor Is Telling You

Wood is still wood after it becomes a floor. It takes on moisture from humid air and gives it up to dry air, and it changes size doing so — every year, forever. The Mid-Atlantic swings a long way between a soaking July and a dry heated January, so floors here move more visibly than floors in a steadier climate. Most of that movement is normal. Some of it is a warning. Knowing which is which saves a great deal of money.

Gaps in winter: usually normal

In a heated house in February the indoor air is dry, the boards release moisture, and each one shrinks slightly across its width. Multiply a fraction of a millimetre by every board in a room and you get visible lines between them.

What tells you it is normal seasonal movement:

• The gaps are thin and fairly even across the whole floor rather than concentrated in one spot.
• They appear in the cold months and close again by summer.
• The floor is otherwise flat — nothing standing proud, nothing dished.
• Wider boards show it more, which is expected. A 5-inch plank moves more across its width than a 2¼-inch strip.

The important instruction: do not fill seasonal gaps in winter. Filler goes in at the floor's smallest dimension, and when the boards expand in July they have nowhere to go. Best case the filler crumbles out. Worse case the floor pushes against itself and cups or lifts.

Gaps that are not normal

• Gaps that do not close in summer. That is permanent shrinkage, usually from a floor installed without proper acclimation, or one that has spent a long time in an unheated, dried-out house.
• One or two very wide gaps while the rest of the floor is tight. Often a board that has come loose, or a subfloor problem pulling the floor apart in one place — see hardwood floor repair.
• Gaps with movement. If a board rocks or clicks when you step across the gap, it is not seasonal.

Cupping: always worth investigating

Cupping is when the edges of each board sit higher than its center, so the floor develops a fine washboard you can feel through socks and see under raking light. It has exactly one cause: the underside of the boards is wetter than the top.

Where that moisture comes from, in rough order of how often we find it in DMV houses:

• Crawlspace or basement humidity rising into the floor from below. Extremely common in older Maryland and Northern Virginia housing, and often seasonal.
• A slow plumbing leak — a supply line, a dishwasher seal, a refrigerator water line, a toilet flange. Cupping concentrated in one area near a fixture is this until proven otherwise.
• A slab or subfloor that was too wet at installation, or a slab that was never moisture-tested.
• A spill that sat, particularly under a rug where nobody saw it.
• Basement or below-grade wood that should never have been installed there. See waterproof basement flooring.

What to do about a cupped floor, in order

1. Find the source and stop it. Nothing else matters until this is done. Sanding a floor while moisture is still arriving guarantees you will be sanding it again.

2. Let it dry slowly. Bring indoor humidity back into range and give it time — weeks, sometimes months. Do not accelerate it with heat or fans pointed at the floor, because drying the top faster than the bottom can crack the boards.

3. See how far it flattens. A great many cupped floors return close enough to flat that no refinishing is needed at all. This is the step that gets skipped, and skipping it is what turns a free fix into a sanding job.

4. Only then decide about sanding. If the floor has dried and a residual ripple remains, sanding flattens it — provided there is enough wood left. Our guide to refinish or replace covers how to check that.

Sanding a floor that is still holding moisture is the single most expensive mistake in this article. The boards are sanded flat at their swollen dimension; when they finally dry they shrink and go concave — crowned in reverse — and the fix is another sand, which many floors cannot survive.

Crowning, and what it tells you

Crowning is the opposite of cupping — the centers stand higher than the edges. Occasionally it is a drying issue in its own right. Far more often it is a floor that was cupped, got sanded too early, and then dried. If you are looking at a crowned floor, ask when it was last refinished, because the answer is usually "while it was still wet".

The prevention that actually works

• Hold indoor humidity between roughly 35 and 55 percent. A humidifier in winter, a dehumidifier in a Mid-Atlantic summer, and a dehumidifier in the basement more or less permanently.
• Deal with the crawlspace or basement, not just the room. A vapor barrier under a crawlspace does more for the floor above it than anything you can apply to the floor itself.
• Check the usual suspects yearly — dishwasher, fridge line, under-sink supply, toilet base.
• Acclimate new wood properly. The most common cause of a floor that gaps permanently is one installed straight off the truck. See preparing your home for installation.
• Consider engineered where moisture is a known factor. A cross-layered core moves considerably less than solid wood across its width. See engineered hardwood.

Frequently asked questions

If your floor is doing something you are unsure about, it is worth a look before it is worth a quote — and the look is free. Request a free in-home visit and we will tell you whether you are watching normal seasonal movement or a moisture problem worth chasing.