Flooring Problem
Gaps Between Hardwood Floor Boards
Why are there gaps between my hardwood floorboards — and should I worry?
The Short Answer
What Causes Gapping?
The most useful thing to know about gaps is that most of them are normal. Wood is hygroscopic: as indoor air dries out in winter, boards lose moisture and shrink across their width, opening gaps; when humidity rises in summer, they swell and the gaps close. NWFA considers gaps and splits that appear and disappear with the seasons to be normal, and wider boards show proportionally larger gaps.
The gaps to worry about are the ones that are large, uneven, or do not close when the humidity comes back up. Those usually trace to wood installed too dry, poor acclimation, or a moisture problem — a real issue rather than a seasonal one. So the honest first step is not to fill anything, but to watch whether the gaps track the seasons.
How to Recognize Gapping
Visible spaces between board edges, typically widest in the dry winter months. Seasonal gaps are thin and consistent across the floor; abnormal gaps are larger, uneven, or persistent through humid months.
Root Causes
Why Gapping Happens
Normal seasonal contraction
Winter heating dries indoor air, boards give up moisture and shrink across the grain, and thin gaps open — then close again in summer. This is expected hygroscopic behavior, not a fault.
Installed too dry
If the wood went in below its in-service equilibrium moisture content, it can only swell and then permanently separate, leaving gaps that never fully close.
Poor acclimation
Flooring that was not allowed to reach the home's actual conditions before installation moves more than it should afterward, producing chronic or uneven gaps.
A moisture event that dried unevenly
Localized abnormal gapping can be the aftermath of a leak that wet and then dried part of the floor differently from the rest.
What You Can Check
- Watch the seasons: a gap that opens in the dry months and closes in the humid months is normal and needs nothing.
- Compare your interior temperature, relative humidity, and the flooring's moisture content against the season — gaps that track the humidity swing are seasonal by definition.
- Flag as abnormal any gaps that are large, uneven across the room, or that do not respond to a humidity change; those point to an installation or moisture cause.
- Do not fill seasonal gaps — filler will either pop out when the boards swell or hold the gap open the following season.
Can It Be Saved? Repair vs. Replace
Seasonal gaps: do nothing. They are the floor breathing with the weather, and forcing them shut causes more damage than the gap.
Permanent gaps: options run from filler strips or slivers (best done in the dry season on a stable floor), to board-by-board replacement, to re-laying the floor if the installation moisture content was fundamentally wrong.
Adding whole-home humidification in winter often shrinks chronic dry-season gaps without touching the floor at all — the cheapest fix when the cause is simply too-dry indoor air.
Prevention
How to Prevent Gapping
- Acclimate flooring to the home's in-service conditions before installation — the single most important step for preventing gaps — and install only when the flooring and subfloor moisture contents are within NWFA's differential.
- Get the installation moisture content right for the local climate rather than to a generic number.
- Hold the home at 30–50% relative humidity year-round: a humidifier in winter, air conditioning or a dehumidifier in summer, so the boards never dry far below their installed moisture content.
Good to Know
Frequently Asked Questions
Are gaps in my hardwood floor in winter normal?
Usually, yes. Winter heating dries the indoor air, the boards lose moisture and shrink, and thin gaps open — then close again when summer humidity returns. NWFA treats gaps that come and go with the seasons as normal. The gaps to worry about are large, uneven, or ones that never close.
Should I fill the gaps in my floor?
Not the seasonal ones. If you fill a gap that is going to close in summer, the filler pops out or props the gap open next winter. Only stable, year-round gaps are candidates for filler strips — and those are best addressed in the dry season, or by fixing the underlying humidity or installation problem.
Why does one room have big gaps and the rest is fine?
Localized, uneven gapping usually means something specific to that area — a spot that was installed too dry, a moisture event that dried unevenly, or a humidity difference in that room. That is worth a look, because it is behaving differently from normal seasonal movement across the whole floor.
Will a humidifier close the gaps?
Often, for winter dry-season gaps, yes. Raising indoor humidity back into the 30–50% range lets the boards swell and the seasonal gaps shrink — frequently enough to solve chronic winter gapping without any work on the floor itself.
Think You Have Gapping?
We'll inspect your floor, find the moisture or structural source, and tell you honestly whether it's a repair or a replacement. Call (916) 749-0272 for an assessment.