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Hardwood Cupping and Gaps: Why Wood Floors Move With the Seasons in Ontario, What Is Normal, What Is a Moisture Problem, What Is a Claim, and the Humidifier That Fixes Most of It
Flooring & Tile
Hardwood Cupping and Gaps: The Floor Is Not Broken, It Is Breathing, Until It Is Broken, and Here Is How You Tell the Difference
Every January I get a call about gaps in a hardwood floor I installed in the summer, and every August I get one about a floor that has gone slightly wavy. Both callers are describing the same thing: wood takes moisture from the air and gives it back, and as it does it gets wider and narrower. In an Ontario house that runs 20% relative humidity in a cold snap and 65% in a humid week in August, a solid oak floor can move almost 1/16 in. per board. Most of what people worry about is that. Some of it is a wet subfloor, a leak or an installer who skipped the acclimation, and that is a different conversation. This page tells you which one you have.
What wood does with water
Kiln-dried flooring leaves the mill at 6 to 9% moisture content. In a house it chases the air: the drier the room, the drier the wood, and the narrower the board. The relationship is steady enough to put in a table, and installers and inspectors use the same one.
| Indoor relative humidity at 20 C | Wood settles at about | Where you see it in Ontario |
|---|---|---|
| 20% | 4.5% MC | A forced-air house in a January cold snap with no humidifier. Gaps you can lose a dime in. |
| 30% | 6% MC | A typical winter house with a humidifier set low, or a new ICF house, which hold humidity better. Hairline gaps. |
| 35 to 55% | 7 to 10% MC | The window every flooring manufacturer writes into the warranty. The floor was milled for this. |
| 65% | 12% MC | A humid August week with the windows open, or a basement without a dehumidifier. Boards tight, edges lifting. |
| 80%+ | 16% MC and up | A leak, a wet slab, a flooded crawlspace. Not weather. Cupping you can see from the doorway. |
Flatsawn red oak changes width about 0.3% for every point of moisture content. A 5-in. board moving from 10% to 6% shrinks about 1/16 in.; across a 12-ft room of thirty boards, that is nearly 2 in. of total shrinkage shared out as gaps. A 2-1/4 in. strip floor moves the same total but splits it across sixty-four joints, which is why old strip floors look better in winter than new wide plank.
Gaps
Gaps open in winter and close in summer. If they do that, they are seasonal, and the only cure is humidity. Gaps that are there in August too, or that have dirt and finish down in them from day one, were there at installation: the boards were not driven tight, or they went down wet from a trailer and dried out in the house. Look at the pattern. Seasonal gaps are even across the whole floor, a hair between every board. Installation gaps are random, some boards tight and one with a 1/8 in. slot beside it.
One wide gap every few feet with the boards tight between is something else again: the boards came in dry, the house was damp, they swelled, the whole floor pushed, and when it dried back the boards stayed stuck to each other in groups and the movement all opened in one place. Finishers call it panelization. It happens most with site-finished floors where the finish glued the edges together.
Cupping and crowning
Cupping is when the edges of each board are higher than the middle, so the floor feels like a washboard across the grain. The bottom of the board is wetter than the top. In summer a little cupping is normal on wide plank, and it flattens in the fall. Cupping that does not flatten, or that is in one area, means water from below: a damp basement or crawlspace under a main floor, a slab that was not tested, a dishwasher line, an ice-maker, a plant that gets watered on the floor. Find the water before anybody touches the floor.
Crowning is the opposite, the middle of each board higher than the edges. It almost always comes from sanding a cupped floor flat while it was still wet. The floor dried later, the edges shrank back below the freshly flattened middle, and now every board has a hump. The fix is another sanding after the floor has been dry for a season, which is why the refinishing page says to fix the moisture first.
What is normal and what is a claim
| What you see | Call it | Who owns it |
|---|---|---|
| Hairline gaps in winter that close by June, whole floor evenly | Normal | You. Run a humidifier; keep the house at 35% or better in winter. |
| Gaps up to about 1/16 in. in a 20% RH house in January | Normal for the conditions | You. The warranty says 35 to 55%; the house was at 20%. |
| Gaps that are the same in August as in January | Installation | Installer, if raised within the first year. He drove the boards loose or installed them wet. Warranty problems. |
| Slight cupping on 5-in. or wider boards in August that is gone in October | Normal | You. A dehumidifier in the basement helps more than people think. |
| Cupping in one area, any season | Water from below | Whoever owns the water: the plumber, the basement, the appliance. Not the floor. |
| Cupping across the whole floor the first summer | Acclimation or subfloor moisture | Installer, if he did not check the subfloor and the boards with a meter. What he should have done. |
| Boards lifting off a slab, hollow sound, adhesive failed | Slab moisture | Installer, if there is no moisture test on paper. You, if the test passed and the slab got wet later. |
| Finish checking or peeling between boards, no movement | Product | Manufacturer, with the dealer handling the claim. |
In a new house the first-year Tarion warranty covers defects in work and materials, and seasonal movement within the manufacturer’s humidity range is not a defect. The workmanship inspection page covers what to document at the pre-delivery walk-through so a real defect does not get called “seasonal” in month eleven.
The humidifier
Most seasonal gapping in Ontario is a humidity problem that costs a few hundred dollars to fix. A whole-house humidifier on the furnace, set to hold 35% in January, keeps the wood within a couple of points of where it was milled, and most of the gaps never open. It helps your sinuses and your piano too. In a house with no ductwork, two console humidifiers do the same job with more refilling. The flip side is the dehumidifier in the basement from May to September, which keeps the slab side of the house under 55% and stops the summer cupping. A $30 hygrometer on the kitchen counter tells you whether either is working.
The houses that need the least of this are tight, well-insulated ones, because a leaky house pumps dry outdoor air through itself all winter and takes the humidity with it. The ICF houses we build sit near 35 to 40% in winter with the humidifier barely running, which is one of the quieter reasons the floors in them stay tight. Radiant heat dries the floor from below as well, so the humidity window matters even more over a heated slab.
Where homeowners go wrong
- Filling winter gaps with filler. The boards swell in June and the filler pops out in crumbs, or worse, it holds and the boards crush their own edges. Leave them; they close.
- Running the house at 20% all winter and calling the installer in March. He will bring a hygrometer and that will be the end of the claim.
- Sanding a cupped floor in August. Crowning by November. Find the water, let it dry a season, then sand.
- Buying 7-in. solid plank for a drafty house with forced air. It is the floor that moves the most in the house that swings the most. Engineered for wide plank; why.
- Mistaking a leak for the weather. Cupping in one spot is never the weather. Pull the dishwasher.
Questions people ask
Are gaps in hardwood floors normal in winter?
Yes. Hairline to about 1/16 in. gaps in a dry Ontario winter are seasonal and close in summer. Gaps that stay open all year, or one wide gap with tight boards around it, are installation issues.
Why is my hardwood floor cupping?
The bottom of the boards is wetter than the top. In summer on wide plank, that is humidity and it flattens in the fall. In one area, or year round, it is water from below: a damp basement, a slab, an appliance leak. Find the water first.
What humidity should I keep my house at for hardwood floors?
35 to 55% relative humidity, which is what the manufacturers’ warranties say. In practice that means a humidifier holding 35% in January and a basement dehumidifier holding under 55% in August.
Will the gaps in my new floor close?
If they opened in winter, yes, by early summer. If the floor was installed in a humid month and gapped in its first winter, the first year is the worst and the floor settles into a smaller swing after that, especially with a humidifier.
Is cupping covered by warranty?
Not when the cause is moisture outside the 35 to 55% window, and the manufacturer will ask for humidity readings. It is the installer’s problem if the subfloor or the boards were wet at installation and he has no meter readings to show otherwise, and the manufacturer’s if the boards themselves were defective. Document early.
Pick the floor that moves least for your house
The flooring cost calculator prices solid and engineered side by side so the humidity question has a dollar figure next to it. If the floor is already down and the quote to fix it is on the table, the $99 review tells you whether the fix matches the cause.
Sources and further reading
- National Wood Flooring Association: the installation guidelines every hardwood warranty points to: subfloor flatness, moisture limits, acclimation, fastening schedules and the radiant-heat chapter.
- Addressing moisture and mould in your home: Health Canada: what mould under a floor means, when it can be cleaned and when the floor comes out.
- Tarion warranty coverage: what a new-home warranty covers for flooring gaps, cupping, cracked tile and squeaks in year one and year two, and the Construction Performance Guidelines it measures them by.
