Why Hardwood Floors Cup, Crown, and Gap in the Puget Sound

Short answer: hardwood floors cup, crown, and gap because wood is hygroscopic — it constantly takes on and gives off moisture to match the air around it. In the Puget Sound, the air swings from damp and cool in winter to noticeably drier in summer, and your floor moves with it. Cupping means the boards are wetter underneath than on top. Gapping means they've dried out and shrunk. After 15+ years sanding and refinishing floors across Seattle, Tacoma, and the rest of King and Pierce County, I can tell you the vast majority of these calls trace back to one thing: where the moisture is, and where it isn't.

Wood never stops moving — it's hygroscopic

A hardwood floor is not a finished, sealed-off object. Even years after it's installed and coated, the wood underneath keeps absorbing moisture from humid air and releasing it back into dry air until it reaches what's called equilibrium moisture content — basically, a balance with whatever the surrounding air is doing. When the air is damp, the boards pick up moisture and swell slightly. When the air dries out, they give it back and shrink. This movement happens mostly across the width of each board, not its length, which is why you see it at the seams between boards.

That seasonal movement is normal and expected. A properly installed floor is built with that movement in mind. Problems start when the moisture coming and going is uneven — wet on one face and dry on the other — or when the swings are far bigger than the floor was ever meant to handle.

The Puget Sound climate is the whole story

Our marine climate is mild but wet. Seattle's relative humidity averages roughly the mid-80s percent in the dead of winter and drops to the mid-60s by July — and indoors, that swing can be even wider once heating systems get involved. From October through April we're in the wet season: rain, gray skies, damp ground, and water tracked in on boots. Then summer arrives and the air dries out fast. Your floor feels both extremes.

Here's the catch most homeowners miss: the outdoor numbers aren't what your floor feels. What matters is the indoor relative humidity right at floor level, and the moisture conditions underneath the floor in the crawlspace or basement. A home can be soaking wet under the floor in February and bone-dry above it in a heated living room — and that mismatch is exactly what makes wood cup.

Cupping, crowning, and gapping — what each one is telling you

• Cupping — the edges of each board sit higher than the center, like a shallow U. This means the underside is wetter than the top surface. The bottom swells, the dry top stays put, and the edges push up. In our area this almost always points to moisture coming from below — a damp crawlspace, a wet subfloor, or standing humidity under the house.
• Crowning — the opposite: the center of the board is higher than the edges. Most often crowning shows up after cupping, when the moisture finally leaves but the floor was sanded flat while it was still cupped. As the boards dry and try to return to flat, the over-sanded edges end up low and the centers stand proud.
• Gapping — visible spaces open up between boards. This is the dry-season story: the wood has lost moisture and shrunk across its width, usually during the winter heating season when indoor air gets very dry. Gaps that open in winter and close back up in summer are normal seasonal movement.

The humidity range that keeps wood floors flat

Keep your indoor relative humidity in roughly the 30 to 50 percent range, year-round, with the room temperature somewhere between 60 and 80 degrees. That's the window the National Wood Flooring Association points to, and most flooring manufacturers ask for the same (some specify 35 to 55 percent). It isn't about hitting an exact number — it's about staying stable and avoiding the extremes at both ends.

Local homes drift out of that window in two predictable directions. In winter, forced-air furnaces and heat pumps pull the indoor humidity well below 30 percent — the warm, dry air pulls moisture out of the wood and gaps open up. In the wet months, an unsealed crawlspace, a leaky basement, or constant tracked-in rain can push the moisture under and around the floor far above 50 percent, and the boards cup.

The real local culprits we see under Puget Sound homes

• Crawlspaces and daylight basements with bare dirt or no vapor barrier — ground moisture evaporates straight up into the subfloor and into the underside of your hardwood. This is the number-one cupping cause we find in older King and Pierce County homes.
• Unvented or poorly vented bathrooms — daily showers dump humid air into the house. Run the fan, and make sure it actually vents outside, not just into the attic.
• Drying firewood indoors — a fresh cord stacked by the woodstove can release a surprising amount of moisture into a room as it dries. Season it outside under cover, not in the living room.
• Wet-season tracking — months of rain coming in on shoes and pets keeps the floor surface damp, especially at entries. A mat and a quick wipe go a long way.
• Winter forced-air heat — the same furnace keeping you warm is drying the air out and shrinking your boards. This is what opens those winter gaps.

How to prevent it

• Acclimate before installing — let new wood sit in the room it's going into until it matches the home's normal conditions. Skipping this is one of the most common reasons a brand-new floor cups or gaps within its first year.
• Seal the crawlspace — a proper vapor barrier (heavy poly sheeting over the dirt, run up the walls) blocks ground moisture from ever reaching your subfloor. On a damp crawlspace this is the single highest-impact fix for cupping.
• Control indoor humidity — add moisture in the dry winter months with a whole-home or room humidifier, and pull it out in damp conditions with a dehumidifier. The goal is to hold that 30 to 50 percent range across the seasons.
• Fix leaks immediately — a slow dishwasher, fridge line, or toilet leak will cup a floor from below before you ever see water. The faster you catch it, the more floor you save.
• Don't panic over seasonal gaps — gaps that appear in dry winter weather and close again when the humidity comes back up are normal. The floor is doing exactly what wood is supposed to do.

Cosmetic movement vs. a real moisture problem

Not every bit of movement is a disaster. Mild seasonal gapping in winter that closes by summer, or very slight, even cupping that comes and goes with the weather, is usually cosmetic — the floor breathing with the seasons. You don't need to do anything but keep your indoor humidity reasonable.

What you should act on: cupping that keeps getting worse or doesn't reverse, any soft or spongy spots, a musty smell, dark staining, or boards lifting or buckling. Those point to an active moisture source — a leak, a wet crawlspace, a failed barrier — that will keep damaging the floor until it's found and fixed. The mistake we see most often is sanding a cupped floor flat while it's still wet. Fix the moisture first, let the wood dry and settle, and only then refinish. Sand too soon and you trade cupping for permanent crowning.

"When someone calls about a cupped floor, my first question is never about the wood — it's 'what does your crawlspace look like?' Solve the moisture and the floor usually tells you what it needs."

If your floor is cupping, gapping badly, or you're just not sure whether what you're seeing is normal, we're happy to come take a look. We'll check the conditions above and below the floor, tell you straight whether it's cosmetic or a moisture problem to chase down, and give you a free estimate on the fix. We've been doing this across the Puget Sound for over 15 years — there's very little we haven't seen come up out of a King or Pierce County crawlspace.