Hardwood floors in St. Louis homes go through a seasonal cycle. Humid summers push moisture into the wood, and dry winters with indoor heat pull it back out. Most floors handle that cycle without trouble. When the edges of boards rise and the middle dips, that shape is called cupping, and it is worth finding out why.
What cupping looks like
Cupping is a curve across the width of a board. The edges sit higher than the center, so the board looks like a shallow trough. You may feel a ridge along the seams underfoot, or notice that light reflects unevenly across the floor. Crowning is the opposite, with the center higher than the edges. Both are signs that moisture in the wood is uneven.
Cupping usually means the bottom of the board is wetter than the top. The wood swells on the wet side and bends. The moisture may come from below the floor, from the room air, or from water that reached the surface.
Check the usual sources first
- Look for recent water events, such as a leaking appliance, a pet accident, or a mop that was too wet.
- Check the space under the floor if you can reach it. In basements, look at the slab and the outside walls for dampness. In crawl spaces, check the ground cover and the vents.
- Look near exterior doors and along the bottom of walls for water that wicks in from outside.
- Inspect nearby plumbing, including supply lines and drain connections, for slow drips.
Test the room air
A simple hygrometer placed in the room, away from vents and exterior walls, shows the air conditions over several days. Log readings at different times of day. If the room stays humid for long stretches, especially in summer, the air itself may be the driver. If the air is steady and the cupping appeared after a specific event, look for that event.
A moisture meter, a common tool for flooring checks, can show whether boards hold more moisture than the surrounding wood. If you do not own one, a flooring professional can take readings for you. Reading the results takes experience, so treat them as one piece of the picture.
Watch the seasons
St. Louis winters bring heating and dry indoor air, so boards may shrink and gaps may open between them. Summers bring humid air that can close those gaps or cause cupping. Some seasonal movement is normal. Keep indoor conditions as steady as your home allows. A room humidifier or dehumidifier, used together with a hygrometer, can help avoid large swings, but it will not fix a leak.
What to avoid
- Do not sand or refinish a cupped floor right away. Sanding while moisture is high can lock in the problem.
- Do not flood the floor with cleaner or water while it is cupped.
- Do not cover the damp area with a heavy rug, since trapped moisture can keep the wood wet.
When to get help
If the cupping followed a single wet event and the boards flatten as the room dries, the floor may recover on its own. If it persists after the air and the source are corrected, or if you see dark staining, soft spots, or a musty smell, the cause may be below the floor. At that point, have a qualified flooring or building professional examine the subfloor and the moisture source. Get more than one opinion before agreeing to major work.
Plan the next step
- Find and stop any water source.
- Log the room humidity for a few days, and note the season.
- Check the space under the floor and the outside walls.
- Wait for the wood to dry and see whether the boards recover.
- If cupping persists, get a professional assessment before sanding or replacing boards.