What causes gaps opening between boards in winter?

Normal seasonal shrinkage. They open in January and close by May, every year, and they are reasonably even across the floor. This is wood behaving exactly as wood behaves. Indoor humidity running too low. Gaps noticeably wider than previous winters, alongside static shocks, cracking trim, and dry sinuses. Forced air without a humidifier in a Toronto February routinely drops a house below 20% RH. Installed too wet, now drying to equilibrium. A newer floor with gaps that opened in the first heating season and did not close again in spring. A few very wide gaps rather than many small ones. Board groups moving together, usually a fastening problem — the floor is anchored where it should float.

What should I do about it myself?

Put a cheap hygrometer on the floor in the worst room and read it for a week. Below 30% is the problem, and it is fixable. Run a humidifier through the heating season and aim for 35–50%. Wait until late summer before judging. Measure the same gap in February and August. Do not fill seasonal gaps with wood filler, rope, or caulk. It will be pushed out or will split the board edge come summer.

Watch it · Not urgent. Worth understanding.

Gaps opening between boards in winter

Also searched as: cracks between floorboards, gaps in winter, floor separating

What it looks like

Thin dark lines between boards that appear as the heating season sets in and close again by late spring. A credit card or a coin will slip into the worst of them.

What causes it

In rough order of likelihood. Each one has a tell — the observation that separates it from the others.

Normal seasonal shrinkage
How to tell: They open in January and close by May, every year, and they are reasonably even across the floor. This is wood behaving exactly as wood behaves.
Indoor humidity running too low
How to tell: Gaps noticeably wider than previous winters, alongside static shocks, cracking trim, and dry sinuses. Forced air without a humidifier in a Toronto February routinely drops a house below 20% RH.
Installed too wet, now drying to equilibrium
How to tell: A newer floor with gaps that opened in the first heating season and did not close again in spring.
A few very wide gaps rather than many small ones
How to tell: Board groups moving together, usually a fastening problem — the floor is anchored where it should float.

What it actually means

Solid hardwood in Ontario will always move a little between seasons. Gaps that open and close are the floor working. Gaps that stay open through summer, or a handful of very wide ones, are a different conversation.

Can it be fixed?

Usually repairable

Seasonal gaps need no repair at all — filling them in winter causes damage in summer when the boards expand into filler that will not compress. Permanent gaps can be filled or, on a wide one, splined with a matched strip.

What you can do yourself

Put a cheap hygrometer on the floor in the worst room and read it for a week. Below 30% is the problem, and it is fixable.
Run a humidifier through the heating season and aim for 35–50%.
Wait until late summer before judging. Measure the same gap in February and August.
Do not fill seasonal gaps with wood filler, rope, or caulk. It will be pushed out or will split the board edge come summer.

When to call someone

Gaps stay open through a humid August, one gap is wide enough to catch a heel, or boards have started to move as groups.

(416) 847-3366 Book a site visit

The work involved

Hardwood Repairs & Restoration

Board replacement, cupping correction, gap filling, structural sistering, and stain matching on heritage and contemporary floors. Emergency water-damage response across the GTA.

From $650 minimum · $18–$35 / sq ft affected

Read further

Often confused with

Written from 15+ years of GTA callouts, not from a manufacturer's brochure. Every diagnosis here still ends with someone looking at the actual floor — a photograph and a moisture reading beat any web page, including this one.