Gaps in a hardwood floor are almost always the wood shrinking as it dries — and in Western Washington that happens in winter, when your furnace dries the house out. Local floors typically open around Thanksgiving and close again by May. Keeping indoor humidity at 35–50% shrinks the swing. Seasonal hairline gaps are normal and shouldn't be filled; gaps that stay open all summer, keep widening, or come with cupped boards deserve a professional look.
Every year, starting a week or two after the furnaces kick on, our phone starts ringing with the same worried question: "gaps just appeared in my hardwood floor — is it failing?" Almost always, no. We're OC Flooring — 1,000+ floors across King and Snohomish County since 2013 — and this is our definitive local answer to the gap question: why boards open here in winter (yes, the opposite of what national articles say), which gaps are harmless, which ones mean something, and what actually fixes each kind.
The Physics, in One Minute
Wood never stops being wood. Long after it's sawn, dried, and nailed down, it keeps exchanging moisture with the air around it — absorbing when the air is humid, releasing when it's dry. As its moisture content drops, each board shrinks across its width; hundreds of boards each shrinking a hair adds up to visible lines between them. That's the whole story. Gaps aren't the floor pulling apart — they're the boards themselves getting slightly narrower, right where they sit.
a wood floor's equilibrium moisture content at 30% versus 60% indoor relative humidity. Your floor physically re-sizes itself between those two numbers every year.
gap season in Western Washington. Boards open when furnace season dries the house and quietly close again when heating stops in late spring.
how much more a 7-inch plank moves than a 2¼-inch strip — board movement is proportional to width, which is why wide-plank floors show bolder seasonal gaps.
Why Winter? (Yes, Winter — Not Summer)
If you read national flooring articles, you'll see "gaps appear in dry summer heat." Not here. Western Washington summers are mild and our unconditioned homes stay relatively humid; almost nobody runs whole-house air conditioning long enough to dry a floor out. What dries local houses is heating season: from roughly November through April, furnaces and heat pumps warm cold outdoor air, and warming air without adding moisture crashes its relative humidity — a heated living room in January can sit near 30% RH while the same room in October floated near 60%. The floor tracks that swing, which is why the pattern is so predictable that we schedule around it: gaps open around Thanksgiving and close by May. If your gaps follow that calendar, your floor isn't failing — it's breathing. You'll notice gaps show up first and widest near heat sources: above furnace runs, along baseboard heaters, in front of fireplaces, and over floor vents.
Normal Gaps vs. Problem Gaps
- Hairline to credit-card width, spread fairly evenly across the floor.
- Follows the calendar — opens in late fall, closes in late spring.
- Boards stay flat — no cupping, crowning, or movement underfoot.
- Worse near heat sources — vents, fireplaces, baseboard heaters.
- Gaps that never close, even in a damp June — the wood has permanently shrunk or the floor has shifted.
- A few large gaps instead of many small ones — boards may have slid or the subfloor moved.
- Gaps plus cupped or crowned boards — that's a moisture imbalance, often from a crawlspace below.
- Movement, squeaks, or flexing at the gap — fastening or subfloor issue, not humidity.
One more local wrinkle for older homes: in pre-1950 Seattle, Everett, and Snohomish houses, some "floor gaps" are really the structure's history — decades of joist and subfloor shrinkage, or plank subfloors that moved before modern plywood existed. Those gaps are usually permanent, character-grade, and harmless; old-growth fir floors have worn them proudly for a century.
How to Keep Gaps Small: Humidity Is the Whole Game
You can't stop wood from moving, but you can shrink the swing it moves through. The target is 35–50% indoor relative humidity, year-round — tight enough that seasonal gaps stay hairline, loose enough that you're not fighting your own house. In practice, for our climate:
- Buy a $20 hygrometer and put it in the main hardwood room. You cannot manage what you're guessing at.
- In heating season, if you're reading below 35%, add moisture: a humidifier (portable or furnace-mounted) sized to the space is the single most effective gap-reducer there is.
- In fall, don't let the house sit sealed and swampy for weeks — run bath fans and ventilation so the floor doesn't load up with moisture it will dump all at once when the furnace starts.
- Keep heat moderate and steady. Blasting a wood stove or cranking heat in one room drives the local RH down hard; the gaps in front of the stove will tell on you.
Should You Fill the Gaps? Usually Not — Here's the Rule
The most common gap mistake we undo: filling seasonal gaps. If you pack filler into a winter gap, the boards swell back in spring with nowhere to go — the filler either gets crushed and squeezed out, cracks into crumbs, or worse, the pressure contributes to edge damage. The rule: never fill a gap that closes on its own each year. Filling is only for permanent gaps — ones still open in a humid June — and even then the method matters: flexible filler or wood slivers for wide fixed gaps, done board-by-board, or filler applied floor-wide during a sand-and-refinish, which is when trowel-filling actually holds. If individual boards are damaged, loose, or badly gapped, targeted board repair beats fighting the whole floor.
What Fixes What
| Gap situation | The fix | Who does it |
|---|---|---|
| Hairline gaps, open Nov–May, closed all summer | Nothing + humidity control (35–50%) | You + a hygrometer |
| Gaps wider than they used to be each winter | Add humidification in heating season | You / HVAC |
| Permanent gaps, boards flat and sound | Fill during a refinish, or sliver-repair the worst | Pro |
| Gaps + cupping or crowning | Find the moisture source (often crawlspace) first — then refinish once dry | Pro + moisture meter |
| Loose, moving, or split boards at gaps | Board-level repair or replacement | Pro |
| New floor gapping badly in year one | Likely acclimation or subfloor-moisture issue at install — get it assessed | Pro |
That crawlspace row deserves a sentence more: most Western Washington homes sit over vented crawlspaces, and a damp one pushes moisture up into the boards from below. That combination — gaps in winter, cupping in fall — is the signature of a crawlspace problem, and no amount of filler or refinishing outruns it until the moisture source is fixed. It's one of the first things we check at any free in-home estimate, meter in hand.
Hardwood Gap Questions, Answered
Are gaps between hardwood boards normal in winter?
When will the gaps in my floor close up again?
What humidity level prevents hardwood floor gaps?
Is it OK to fill gaps in my hardwood floor with wood filler?
Why are the gaps worst near my heat vents and fireplace?
Do gaps mean my hardwood floor was installed badly?
Do wide-plank floors gap more than strip floors?
When should I actually worry about floor gaps?
Will refinishing my floor get rid of the gaps?
Gaps That Never Close? Let's Take a Look — Free
We'll check your boards, your subfloor, and your crawlspace moisture, then tell you honestly whether it's normal seasonal movement or something worth fixing. Free in-home assessments across King & Snohomish County.
Related reading: Best flooring for the Pacific Northwest climate · 5 ways to protect hardwood flooring · Flooring repair · Hardwood floor refinishing
