Why wood floors gap, cup or buckle, and what humidity has to do with it

Updated November 11, 2026 · KeystoneProGuide editorial team

Why do wood floors change with the seasons?

Because wood trades moisture with the air. Penn State Extension explains that wood is hygroscopic: it gains or loses moisture until it is in balance with the air around it, a point called its equilibrium moisture content. That point depends on temperature and relative humidity. Wood that takes on moisture swells, and wood that loses it shrinks. A floor is hundreds of boards doing this together, so the changes show up as gaps in dry weather and tight, sometimes cupped boards in humid weather.

How big are the swings in a Pennsylvania house?

Penn State's figures give a sense of it:

• At 70 degrees and 40 percent humidity, wood settles at about 7.7 percent moisture.
• At 70 degrees and 77 percent humidity, it settles at about 14.9 percent.
• Outdoors in Pennsylvania, and in an unheated building such as a shed, the equilibrium runs about 11 to 14 percent through the year.
• In a house with forced-air gas heat in winter, the extension's example drops to around 4 percent.

Most hardwood lumber is dried to around 7 to 8 percent. So a floor installed at that level can lose moisture in a dry, heated winter and gain it in a humid summer. In the extension's example of a table, the top lost moisture and warped enough in winter that a drawer would not open, then took moisture back in summer and freed it.

Which gaps are normal?

Seasonal ones. The National Wood Flooring Association (NWFA), the hardwood industry's association, says floors that lose moisture can gap or split, between boards or on the face of a board, and that gaps that appear and disappear with normal seasonal changes are considered normal. Its line for calling a professional is gaps that are large, or that do not close during the more humid months.

What does cupping mean?

That the boards have taken on moisture. The NWFA describes cupping as edges raised higher than the center of each board, across its width, and says it happens when floors gain moisture. Persistent cupping is a sign to find out where the moisture is coming from. A soaked floor is a different problem from seasonal movement; the EPA says wood surfaces should have moisture removed immediately and be dried with dehumidifiers, gentle heat and fans, with caution using heat on hardwood.

If a contractor proposes sanding a cupped floor flat, ask what has been done about the moisture first.

How do you keep a wood floor stable?

Keep the house steady. The NWFA says gaps and cupping can both be minimized by keeping a home between 60 and 80 degrees Fahrenheit and between 30 and 50 percent humidity all year. The U.S. Environmental Protection Agency (EPA) gives the same 30 to 50 percent as the ideal range for limiting mold, with an upper limit of 60 percent, and says a humidity meter costs about $10 to $50 at hardware stores.

In practice that can mean adding moisture in a dry winter and removing it in a humid summer, with dehumidifiers or air conditioning. The EPA notes that higher heat and humidity also speed up formaldehyde emissions from composite wood such as laminate cores, so keeping a home cool and dry in summer helps on both counts.

Does the type of wood floor make a difference?

The NWFA says engineered wood floors, made of several layers with real wood on top, expand and contract less than solid wood, which is why it calls them ideal for basements. Solid wood can be sanded and refinished more times. Both will still respond to humidity, so the same range applies.

Can moisture affect other floors too?

Yes, in different ways.

• Carpet. The Carpet and Rug Institute (CRI), the carpet industry's association, lists high humidity among the causes of buckling, along with improper installation, the wrong cushion and moving furniture. It says restretching should be done with all furniture out of the room.
• Tile. The Ceramic Tile Education Foundation (CTEF), a tile industry education body, says every tile installation moves with temperature and humidity, and that movement joints are needed to accommodate it. Ask a tile installer where they will go.

What about warranties?

Read the conditions. The Federal Trade Commission advises checking whether a warranty covers only problems that happen when a product is maintained and used according to the directions. If your flooring's care or installation instructions give humidity or temperature ranges, keep a copy and a record of the readings. When gaps or cupping appear, check the paperwork before you call.

When should you call a flooring contractor?

When gaps stay open through the humid months, when boards cup and stay cupped, or when the floor has been wet. Other signs and what they point to are in signs your floors need work, the seasonal timing of jobs is in how often floors need refinishing, and below-grade rooms are covered in flooring for a basement or slab. Businesses are listed on the flooring contractors directory page.

Sources

1. Understanding Equilibrium Moisture Content, Penn State Extension
2. Problem Prevention, National Wood Flooring Association (NWFA)
3. Types of Wood Floors, National Wood Flooring Association (NWFA)
4. A Brief Guide to Mold, Moisture and Your Home, U.S. Environmental Protection Agency
5. Mold Remediation in Schools and Commercial Buildings Guide: Chapter 4, U.S. Environmental Protection Agency
6. Protect Against Exposures to Formaldehyde, U.S. Environmental Protection Agency
7. Frequently Asked Questions, The Carpet and Rug Institute (CRI)
8. The Homeowner's Guide to Hiring a Qualified Tile Installer, Ceramic Tile Education Foundation (CTEF)
9. Warranties, Federal Trade Commission