Fill gaps between hardwood floorboards
Solid hardwood floors open small gaps between boards in winter and close them again in summer, and those seasonal gaps are normal and best left alone. Gaps that stay open year-round can be filled: with floor filler for narrow cracks, or with thin wood strips or rope for wider ones. Packing a seasonal gap too tightly can make boards push up when they swell. Give it a full year before you decide which kind of gap you have. What looks like a problem in January is often gone by July.
Why hardwood gaps open
Wood shrinks as it dries and swells as it takes on moisture. In heated homes, winter air is dry, so boards shrink and gaps open, then usually close by summer. Keeping indoor humidity steadier reduces that movement.
If gaps stay open all year, they can be filled. Wide gaps take thin slivers of wood glued to one board only, or cotton rope stained to match, both of which can move a little with the floor.
What causes gaps
Boards shrink and gaps open seasonally.
Wider planks move more with humidity.
Boards that dried after installation leave permanent gaps.
Hot, dry spots near vents and heaters.
| Gap | Likely cause | What to do |
|---|---|---|
| Opens in winter, closes in summer | Seasonal movement | Leave it; steady the humidity. |
| Narrow, open all year | Permanent shrinkage | Floor filler or putty. |
| Wide, open all year | Shrinkage or installation | Wood slivers or rope. |
| Gaps with cupped boards | Moisture | Find the moisture source first. |
Fill the gaps, step by step
- Check in the right seasonLook at the gaps in late summer, when humidity is highest. If a gap closes then, it's seasonal and best left alone.
- Clean out the gapScrape out grit and old filler with a putty knife, and vacuum the crack so the filler or strip can bond.
- Fill narrow gapsPress floor filler or putty into narrow gaps with a putty knife, slightly overfilled, and scrape it flush. Let it dry as the label says.
- Fill wide gapsFor wider gaps, spread glue on one side of a thin wood sliver only, push it into the gap, and tap it down with a hammer and scrap block until it's snug. Or press in cotton rope stained to match.
- Sand and finishSand the repair flush by hand with fine sandpaper, wipe off the dust, and brush on a little matching finish to seal it.
Watch it done
- The filled gap is flush with the boards on each side
- The repair is sealed with finish that matches the floor
- Filler or sliver stays in place with no cracking
Filler cracking out? Use a sliver glued to one board, or rope.
If the filler cracks out
If the boards are scratched, there's a separate guide for fixing scratches in a hardwood floor.
If filler cracks and falls out, the gap is still moving with the seasons. A sliver glued to one board only, or rope, lets the boards move.
If gaps come with cupped or crowned boards, moisture is the likely cause. Deal with that first.
What a pro typically charges (a rough estimate; prices vary by area): A flooring pro typically charges $150–$500 to fill gaps across a room, often done as part of a refinish.
Find a flooring contractor near you