Why Hardwood Floors Gap in Winter (and When a Gap Is a Problem)

Hairline gaps between hardwood boards in winter are usually seasonal, not a defect. Heating dries the air, wood gives up moisture and shrinks, and the gaps tend to close again when humidity returns. Gaps that stay open through the humid months are a different story and are worth a call to your installer.

What causes winter gaps

Brett Miller explains in Hardwood Floors magazine that when the air in a building is heated, relative humidity drops. As the air dries, wood boards begin to shrink, and he says nearly every wood floor will show some of this. In a Wisconsin house that runs its furnace for months, that is the ordinary pattern: the furnace dries the air and the boards contract.

Are the gaps normal?

Seasonal gaps should close once the heating system is off for the season or once humidity is added to the home. Miller draws a line between those and abnormal gaps, which, in his words, will likely never completely close back up in the humid months. So the useful test is time: look again in late spring or summer. If the gaps are still there, the cause is probably something other than the weather.

Do not fill seasonal gaps

It is tempting to push filler into a visible gap. Miller advises against filling normal seasonal gaps, because filler can damage the edges of the boards when they swell back in humid weather. If a gap bothers you, the better fix is steadier humidity, not a patch.

Keep indoor humidity in range

Hardwood Floors magazine reports that the National Wood Flooring Association (NWFA) recommends keeping a home at 60 to 80 degrees and 30 to 50 percent relative humidity. Two practical steps follow from that:

• Put an inexpensive humidity gauge (a hygrometer) in the room with the most wood flooring and check it through the winter.
• If the reading falls below the range, add humidity with an adequate humidification system. Miller notes this also benefits the rest of the home, not only the floor.

Engineered hardwood has its own limits. Miller says problems show up when relative humidity stays below the manufacturer's recommendation for long enough, so read the manufacturer's range for your product and treat it as the target.

What matters at installation time

Gaps are partly decided before the first board goes down. Matt Thrane writes in Hardwood Floors magazine that the building owner should maintain normal living conditions during acclimation, the period when the wood adjusts to the home. He also says there should be no more than a 4 percent difference in moisture between a wooden subfloor and the flooring.

Roger Barker, also writing in Hardwood Floors magazine, gives the width-based version. Solid strip flooring under 3 inches wide should be within 4 percentage points of the wood subfloor's moisture content. Solid plank flooring 3 inches or wider should be within 2 percentage points.

This is why installers test the subfloor and why we treat subfloor preparation as its own step. Rushing it is how gaps and other problems get built in.

When to call someone

Call your installer if gaps remain open after the heating season, if they look uneven from one area to the next, or if boards seem to be moving in other ways. If your floor was installed by Illustrious Flooring, our workmanship warranty covers installation-related issues, including gap issues from improper installation, so call us and we will look at the floor. We are available at (262) 683-5301, Monday to Friday, 9 AM to 5 PM.

For ongoing upkeep, our care and maintenance guide has seasonal tips for hardwood, LVP and laminate.

Sources

• 'Tis the Season: Understanding Seasonal Related Issues - Brett Miller, Hardwood Floors magazine, October 2, 2017 (page updated March 6, 2026).
• Acclimation: Environmental Conditions Before, During, and After the Installation - Matt Thrane, Hardwood Floors magazine, October 9, 2023 (page updated March 6, 2026).
• Subfloor Focus: Minimizing Moisture - Roger Barker, Hardwood Floors magazine, January 30, 2017.