The short answer
Wood takes on moisture in humid summers and gives it up in dry, heated winters, so a hardwood floor moves twice a year. Small winter gaps that close in spring are normal. Keeping the house between 35 and 45 percent relative humidity all year prevents most of the rest.
Why wood moves
Wood swells when it takes on moisture and shrinks when it dries. Oklahoma pushes it both ways: humid summers swell the boards, and a winter of forced-air heat dries them back out. The gap between July and January is a long one.
A floor can only stay flat and tight if the house stays reasonably steady. That is why we measure moisture before installing anything, and why new wood sits in the house to acclimate before a single board goes down.
Gaps in winter
Thin gaps between boards in the driest part of winter are the floor shrinking, and they usually close up as humidity comes back in spring. Do not fill them: filler gets crushed or pushed up when the boards swell again in summer.
Gaps that stay open all year, or get wider every winter, point to a house that runs too dry — or to wood that was installed before it acclimated.
Cupping in summer
Cupping is when the edges of each board sit higher than the middle. It means moisture is coming from below: a crawl space that runs damp, a slab that wicks, a leak under a sink or a dishwasher.
The fix is the source, not the floor. Once the moisture stops and the house holds 35 to 45 percent relative humidity, most cupped floors flatten back out over a season.
Crowning: the mistake after cupping
Sanding a cupped floor flat before it dries takes wood off the high edges. When the boards finally dry and settle, the middles end up higher than the edges — a crowned floor. Sanding a cupped floor too early is what turns a fixable floor into a replacement.
Squeaks
A squeak is movement: a loose board over a joist, or subfloor that was never fastened properly, often louder in the dry months when the boards shrink. From above, the board is screwed and plugged through its face. From a basement or crawl space, it is shimmed from underneath and nothing shows at all.
Keeping it steady
Most of it is the house, not the floor:
- Hold indoor relative humidity between 35 and 45 percent all year
- In winter, a humidifier offsets what forced-air heat takes out
- In summer, keep the air conditioning running instead of opening the house to the humidity
- Keep crawl spaces dry — vapor barrier on the ground and working ventilation
- Test moisture before installing, and let new wood acclimate in the house first
Keep reading
- Service Hardwood Installation in Tulsa
- Service Repairs & Remodels in Tulsa
- Guide Refinish or replace hardwood floors? How to tell
- Guide How long hardwood floor refinishing takes, and how to get ready
- Guide Luxury vinyl plank vs. hardwood: which floor for which room
- Guide Solid vs. engineered hardwood: how to choose