Gaps in Hardwood Floors: Normal or Not?

Almost every hardwood floor opens up a little between boards in the dry months of fall and winter, and most of those gaps close again when humid weather returns. The harder part is telling that normal cycle from a real problem. Here is what the National Wood Flooring Association (NWFA) publishes, how much movement to expect, and the signs that call for an inspection.

Are gaps between hardwood floorboards normal?

Usually, yes. Brett Miller, NWFA's vice president of education and certification, writes in Hardwood Floors magazine that nearly every wood floor will show some seasonal separation between boards in fall and winter. Solid floors should be expected to have seasonal gaps in dry months without humidification, and most of those gaps disappear when humidity returns. NWFA's consumer page agrees: gaps are normal if they appear and disappear with the seasons, and worth a call to a professional if they are large or do not close in humid months.

We found no single number in NWFA guidance for how wide a normal gap can be. Wood moves in proportion to its width, so each joint in a wide-plank floor can open further than in narrow strips.

Why gaps open in winter and close in summer

Wood trades moisture with the air around it, and it changes size as it does. In heating season the indoor air dries out, boards lose moisture and shrink across their width, and the joints between them open. When humid weather returns, the boards take moisture back in and swell toward each other again.

NWFA's equilibrium moisture content table shows the range. At 70°F, wood in 20 percent relative humidity settles at about 4.5 percent moisture content. At 30 percent it settles at 6.2, at 50 percent at 9.2, and at 65 percent at 12.0. Each point a board gains or loses is a small change in width, and those changes are where gaps come from.

How much movement to expect

NWFA's installation guidelines include a method for estimating how much a board changes width. For red oak, the dimensional change coefficient is .00369 per percentage point of moisture content. Multiply it by the board width in inches and by the change in moisture content to get the change in inches. A 5-inch plank whose moisture content changes by 4 points moves about .0738 inch. Applying that method to NWFA's moisture content table gives the figures below. This is our arithmetic, not a table NWFA publishes.

How to read it: red oak only, at 70°F. Going from 30 to 50 percent relative humidity is a 3.0-point change in moisture content (6.2 to 9.2), and 20 to 65 percent is a 7.5-point change (4.5 to 12.0). Each number is how much one board's width changes, not a guaranteed gap. Other species have their own coefficients, so treat the table as a sense of scale, not a prediction for your floor.

Width matters: Miller notes that an 8-inch plainsawn white oak plank shrinks twice as much as a 4-inch one. So does the size of the swing: the wide range moves a board about two and a half times as far as 30 to 50 percent does. Holding the house inside NWFA's range keeps movement small, and Miller adds that the only way to completely remove seasonal fluctuation is to control the conditions in the home.

Seasonal gap or a problem?

The test is timing and company. A seasonal gap opens in the dry months, closes when humidity returns, and shows up alone. An abnormal gap does not close in humid months, and it may come with other signs.

NWFA's list of causes behind abnormal gaps: structural movement or settlement, improper fasteners or fastening schedule, an inadequate subfloor, improper adhesive, improper acclimation, edge crush from earlier moisture, and a poorly insulated duct below the floor. Installing before the subfloor has dried is another: NWFA's guidelines say it leaves gaps along the joints and noisy floors.

To sort one from the other, NWFA says to check room temperature, relative humidity and the floor's moisture content against the season. Its inspection form also records the average gap, any cupping, and whether the floor has been through a full cycle of seasons. One January reading says little; a floor still open after a humid summer says a lot.

Solid versus engineered flooring

Miller notes that seasonal gaps are more prominent in solid hardwood than in engineered, because solid wood moves more with humidity. That is part of why engineered flooring is the better call in humid homes and over radiant heat.

Engineered is not gap-proof or cup-proof, though. Miller notes it can dry-cup when relative humidity stays below the manufacturer's range, so follow the manufacturer's humidity range for engineered floors. Dry-cupped engineered flooring often settles once humidity is back in range.

Do not confuse that with cupping from moisture below. Dry cupping in engineered flooring follows low humidity and often eases once humidity is back in range. Moisture cupping can mean water has reached the floor or subfloor, a different problem with a different fix, covered in our guide to water-damaged hardwood.

What actually fixes gaps

It depends on which kind of gap you have.

• Seasonal gaps: wait, or humidify. NWFA says normal seasonal gaps need no repair. Bringing indoor humidity into range, or waiting for humid weather, is the remedy.
• Leave filler out of them. Fillers can crush board edges, and gaps built in on purpose, such as washer rows, are not filled at all.
• Abnormal gaps: repair them. NWFA lists matching filler followed by a recoat, wood slivers for larger gaps, and sometimes board replacement. A humidifier will not fix these, because dry air is not the cause.
• Dry-cupped engineered floors: be patient. They often settle once humidity is back in range.
• Whole-floor replacement: rarely the answer. Miller calls replacing a floor only because of gaps "usually both unnecessary and self-defeating."

So filling is not always wrong: it is the wrong move for a gap that closes by itself, and a reasonable one for a gap that does not.

Humidity and temperature targets

NWFA's general rule for solid wood is indoor relative humidity between 30 and 50 percent and a temperature between 60 and 80°F, year-round. Some climates can differ, and factory-finished and engineered floors follow the manufacturer's range. NWFA's guidelines say to use a humidifier in winter, a dehumidifier in summer, or both as the house requires.

A basic hygrometer in the room that matters most tells you where you stand. Our posts on flooring and humidity at the Jersey shore and hardwood maintenance cover the rest. A humidifier reduces the seasonal kind of gap. It does not repair a gap that has another cause.

When to call a pro

• Gaps that stay open through humid months. NWFA's consumer page says to call a professional if gaps are large or do not close then.
• Gaps with loose boards or squeaks. A loose board can squeak, and our squeaky floor guide explains the usual causes.
• Gaps with crushed board edges. Edge crush from earlier moisture is on NWFA's list of causes.
• Gaps with cupping after a leak or a wet season. That points to moisture, not dry air.

Gaps that stay wide year-round can point to a moisture or subfloor issue, one of the warning signs in our refinishing guide. Replacement is for structural settling or subfloor failure, as our replace versus refinish comparison explains.

Jersey Concept installs and refinishes hardwood in Monmouth and Ocean County and offers a free in-home assessment. If your gaps have not closed after a humid stretch, see our hardwood flooring page or sanding & refinishing page, or request a free in-home assessment.