Gaps in Hardwood Floors in Winter: Normal, or Time to Repair? (Ottawa)
By Victor Solomon, founder of Solomon Flooring. Victor started the company in Ottawa in 2017 and has grown it from one person into a local crew whose installers hold Ontario’s Floor Covering Installer Certificate of Qualification (Skilled Trades Ontario 448A).
If thin gaps opened between your hardwood boards after the furnace came on, and most of them are thinner than a dime, your floor is fine. Dry winter air pulls moisture out of the wood, the boards shrink across their width, and the gaps usually close again by late spring. You need a repair when one gap is much wider than the rest, when gaps stay open through the summer, or when boards near the gaps are loose or lifting. Solomon Flooring’s floor repairs start at $150, and board replacement starts at $200.
Key takeaways
- Hairline gaps across the whole floor from January to March are normal wood movement, not a defect.
- One wide gap every few boards, with tight boards in between, means the finish glued the boards into panels. That needs a different fix.
- Don’t fill seasonal gaps in winter. The boards swell back in summer and crush or push out the filler.
- In an Ottawa winter, aim for 30 to 35% indoor humidity. Higher than that frosts the windows.
Why do hardwood floors get gaps in winter?
Wood trades moisture with the air around it, and heated Ottawa air in January is very dry. As the boards lose moisture they shrink, almost entirely across their width and very little along their length.
You can put numbers on it. The U.S. Forest Products Laboratory’s Wood Handbook gives flat-sawn red oak a dimensional change coefficient of 0.00369: for every 1% of moisture it loses, a board shrinks by 0.369% of its width. A typical house sits around 9% wood moisture in late summer and drops to about 6% by late winter. That 3-point drop works out to:
- 2¼-inch strip flooring: 3 × 0.00369 × 2.25 in = 0.025 in, about 0.6 mm per board
- 5-inch plank: 3 × 0.00369 × 5 in = 0.055 in, about 1.4 mm per board
That’s why wide-plank floors show wider winter gaps. Each inch of wood shrinks the same amount, and a wide board simply has more inches.
Radiator-heated homes get even drier. A Russian flooring guide, written for apartments on central radiator heat, puts indoor humidity at 50 to 55% in autumn and 20 to 25% by January. Older radiator homes in the Glebe and Old Ottawa South tend to follow the same pattern.
How wide is too wide for a gap in a hardwood floor?
Use a dime. A Canadian dime is 1.22 mm thick. On a standard 2¼-inch strip floor, winter gaps you can’t push a dime into are normal. On 5-inch and wider planks, gaps up to about the thickness of a dime are still within normal movement.
Call a flooring contractor when you see any of these:
- One wide gap every 5 to 10 boards, with the boards between them tight together
- Gaps that are still open in July and August
- Gaps together with cupped edges, raised boards, soft spots or new squeaks
- Gaps wide enough that the tongue of the board is visible from above
Why is there one big gap instead of many small ones?
The floor finish has glued the boards together. When a floor is finished on site, some finish runs into the joints and bonds boards edge to edge. In winter, a block of glued boards shrinks as one panel and all of that shrinkage lands on the weakest seam.
Holzforschung Austria, Austria’s wood research institute, tested this. Solvent-based polyurethane finishes bonded the board edges weakly, at about 0.3 to 0.5 N/mm². Water-based finishes bonded them at 0.5 to 2.0 N/mm², and two-component waterborne systems and thin waterborne primers glued the boards the hardest. Their worked example is an oak strip floor in a room 5 m wide where the wood drops from 10% to 8% moisture: 26 mm of total shrinkage across the room if the boards shrink freely, about one inch. Spread across every seam, that’s hairlines. Collected into a few seams, it’s a crack you can lose a coin in.
We fix this by filling those few wide seams with thin slivers of matching wood, glued on one side only so the boards aren’t locked together again, and by using a low-bonding primer at the next refinish.
Should you fill gaps in hardwood floors in winter?
No. Seasonal gaps close on their own, and anything you pack into them in January gets squeezed when the boards swell in June. Rigid putty cracks and pops out, or the board edges push up.
Fill only gaps that stay open year-round, and fill them in late summer, when the wood is at its fullest size. A simple test: in February, draw a pencil tick on each side of a few gaps. Check them again in July. If the ticks moved closer, the floor is still moving and the gap is seasonal.
For permanent gaps, the method depends on width:
- Under about 3 mm: trowel-filled during a sand and refinish
- Wider than about 3 mm: glued slivers of the same species, planed flush and finished with the floor
- Loose, split or badly gapped boards: the board comes out and a new one goes in
What humidity should an Ottawa home be at in winter?
Aim for about 35% when it’s mild outside and about 30% once it drops below -10°C. That keeps gaps small without frosting your windows.
European flooring standards ask for more. Holzforschung Austria recommends about 50% through the heating season, and German finish maker Loba says to avoid long stretches below 45%. That works in Vienna. At -25°C in Orleans or Kanata, 45% indoors means water running down the windows and moisture in the exterior walls. Small winter gaps are the right trade for dry walls.
A cheap digital hygrometer tells you where you are. If your house sits at 20% all winter, a furnace-mounted humidifier from an HVAC contractor usually runs $400 to $1,500 installed.
Do hardwood floors gap less as they get older?
Yes, a little. A study by García Esteban, Gril and colleagues (arXiv, 2005) put seven wood species, including an oak, through five wet-dry cycles. Every species came out less responsive. The wood took up less moisture and swelled less for the same change in humidity, with the strongest effect at high humidity. That is likely part of why the first winter or two after installation shows the widest gaps.
How a board is cut matters more than age. A 2014 ETH Zurich wood model by Hassani and colleagues uses swelling coefficients for European beech of 0.00462 along the growth rings against 0.00191 across them, so flat-sawn beech moves about 2.4 times as much as quarter-sawn. The Wood Handbook shows the same for red oak: 0.00369 flat-sawn, 0.00158 quarter-sawn. If you’re planning a new floor and gaps bother you, quarter-sawn boards or engineered hardwood move far less.
If you’re weighing a full redo, our guide on whether to refinish or replace hardwood floors walks through the decision.
How much does hardwood gap repair cost in Ottawa?
Most gap repairs are a single floor repair visit at $150 to $700. Solomon Flooring’s pricing:
- Filling a few wide seams with slivers of matching wood: $150 to $700 per job
- Replacing a loose, split or badly gapped board: $200 to $500
- Sand and refinish, with permanent gaps across the room filled as part of the job: from $3 per sq. ft., up to about $6. A 200 sq. ft. living room comes to roughly $600 to $1,200.
Every quote is free and in writing, and under Ontario rules the final price can’t exceed a written estimate by more than 10% without your approval.
FAQ
Does winter gapping mean my floor was installed badly? Usually not. Boards that were installed too wet do show bigger gaps the first winter, which is why we check moisture before we fit a single board.
What if the gaps come with cupping or buckling? That’s the opposite problem: too much moisture, not too little. Read our guide on water-damaged hardwood before you fill anything.
Get the gaps checked before you fill them
Most winter gaps need nothing but patience and a hygrometer. If you have one wide crack, gaps that never close, or boards moving around them, Solomon Flooring’s floor repair team can tell you which kind you have and fix it so it stays fixed. Send us a photo with a coin in the gap through our contact page and we’ll tell you whether it’s seasonal or a repair.
How we put this together
The shrinkage figures are our own calculations, using the dimensional change coefficients published in the USDA Wood Handbook. The research on side bonding, humidity cycling and wood swelling comes from the Austrian, Spanish, French and Swiss studies listed below. The winter humidity targets are what we use on Ottawa jobs. Prices are Solomon Flooring’s current ranges for 2026.
Sources
Research and technical references behind the figures in this article:
- García Esteban, L., Gril, J., de Palacios, P. and Guindeo Casasús, A. (2005). Reduction of wood hygroscopicity and associated dimensional response by repeated humidity cycles. arXiv:physics/0503174.
- Hassani, M. M., Wittel, F. K., Hering, S. and Herrmann, H. J. (2014). Rheological Model for Wood. ETH Zurich. arXiv:1411.1358.
- Forest Products Laboratory, USDA Forest Service (2010). Wood Handbook: Wood as an Engineering Material. General Technical Report FPL-GTR-190, Chapter 13: Dimensional changes.
- Holzforschung Austria (2009). Merkblatt zur Vermeidung der Seitenverleimung durch Versiegelungslacke bei Holzfußböden (fact sheet on side bonding of wood floors by finishes). Vienna.
- Loba GmbH. Pflegeanweisung: Manuell lackiertes Massivparkett gemäß DIN 18356 (care instructions for lacquered solid parquet).
- Hardwood Floors Magazine. Quick Tips: Determining Dimensional Stability in Wood.
- Sostav.ru. Почему зимой между планками пола появляются щели? (Why do gaps appear between floorboards in winter?).