Ask anyone who has lived through a St. Louis July and the January that follows it, and they will tell you the weather here does not do subtle. Dew points in the low 70s in August. Single digits in February. Your hardwood floor feels every bit of that. Wood is a moisture sponge with a grain direction, and in this climate it spends the whole year quietly getting a little wider or a little narrower. Knowing what is normal here saves a lot of worry, and it tells you when something actually needs a look.
Why Wood Moves So Much Around Here
Missouri sits in a humid continental climate, which is a polite way of saying we get the full menu. Summer air carries a lot of water. Winter air, once your furnace has heated it, can fall below 20 percent relative humidity indoors, which is drier than plenty of desert climates. Red and white oak, which is what is hiding under the carpet in an enormous number of homes in this region, holds moisture in balance with the air around it. Damp air in, boards swell across the grain. Dry air in, boards shrink.
Across one 2 1/4 inch oak strip that movement is a few thousandths of an inch. Nobody notices a few thousandths. Across a 15 foot wide living room in a Webster Groves Cape Cod, those thousandths add up over 80 boards into something you can see from the doorway. Wide planks show it sooner, because each board is moving more on its own. A 7 inch plank simply has more width to give back in January than a strip floor does.
This is also why engineered hardwood behaves differently. Its core is built from cross-layered plies, so the layers fight each other instead of all shrinking the same direction. It still moves, just less, which is why it goes into basements and over slabs when solid wood should not.
The Number That Matters: Indoor Relative Humidity
Forget the outdoor forecast. The only humidity your floor cares about is the air in the room it lives in. Most hardwood manufacturers, including the lines we stock from Hallmark Floors and Portercraft, want that air between 35 and 55 percent relative humidity all year. Hold that band and seasonal movement mostly disappears. Let the house ride from 15 percent in February to 65 percent in August and the floor will show you the difference.
| Season | What the air is doing | What the floor does | What helps |
|---|---|---|---|
| Winter (Dec - Feb) | Cold outside, furnace drying the house out | Boards shrink, hairline gaps open at the seams | Humidifier, aim for 35 - 45 percent |
| Spring (Mar - May) | Big swings, wet weeks, warm weeks | Gaps start closing back up | Watch the gauge, run the AC on muggy days |
| Summer (Jun - Aug) | Dew points in the 70s, humid basements | Boards swell, seams tighten, edges can cup | Air conditioning plus a basement dehumidifier, 45 - 55 percent |
| Fall (Sep - Nov) | Cooling off and drying out | Floor settles back toward neutral | Usually nothing |
A digital hygrometer costs about as much as lunch. Buy two. Put one on the main floor and one in the basement, and check them the way you check the thermostat. In most St. Louis homes the basement runs 10 to 15 points wetter than the living room in July, and that moisture does not stay downstairs. It comes up through the joists into the back of your floor boards, which is the part nobody thinks to protect.
Acclimation: Why the Wood Sits in Your House First
Acclimation is not a stalling tactic. It is the step that decides whether your floor looks right next August. Wood delivered off a truck in humid weather, or out of a cold warehouse in January, is not at the moisture content your house will hold it at. If it goes down that day, it will finish acclimating after it is nailed, and then it has nowhere to move but against itself.
So we stack the material in the actual room it will live in, with the HVAC running the way you normally run it, and we let it sit. How long depends on the product and the season, not on a number somebody read online. What tells us it is ready is a meter: we take readings on the flooring and on the subfloor and want them close, generally within two to four percent of each other depending on plank width. If the subfloor is wet, nothing goes down that day. That is a conversation we would rather have before the job than after.
Expansion Gaps and the Edges You Never See
Every wood floor needs somewhere to grow into. That space is the perimeter gap, hidden under the baseboard and shoe molding, and it exists at every wall, every hearth, every door jamb, and around every pipe. Long runs and wide open floor plans need more of it than a small bedroom does. Transitions in doorways are there for the same reason: they let one room's floor move independently of the next room's.
Most of the buckled floors we get called out to look at are not defective floors. They are floors that were pinned. A baseboard nailed down through the boards instead of into the wall. A heavy kitchen island set on top of the finished floor. Cabinets installed over the field. Caulk packed solid into the perimeter gap. When the floor swells in July and the edges have nowhere to go, the middle lifts. If you are dealing with lifting or hollow spots now, that is floor repair territory and worth diagnosing before it spreads.
When Gapping Is Normal and When It Is Not
Here is the plain version, the same thing Erik would tell you standing in your living room.
- Normal: thin gaps you notice in February and cannot find in June. A business card slides in, a nickel does not.
- Normal: gaps that show up in the same few boards every winter. Some boards are just cut closer to the pith and move more.
- Worth a look: gaps that stay open all summer, or that got wider this winter than last winter.
- Worth a look: cupping, meaning board edges sitting higher than their centers. That is moisture coming from underneath.
- Call us: crowning, buckling, boards lifting off the subfloor, or dark staining spreading along the seams. Something is actively wet.
Not sure if what you are seeing is seasonal or a real problem?
Book a free in-home lookCupping, Crowning, and Water From Below
Cupping means the edges of your boards are wetter than their faces, so they have swelled and risen. The water almost never comes from the top. It comes from a humid basement, an unsealed crawlspace, a slab without a vapor barrier, a slow dishwasher or refrigerator line, or a sump pump that quit during a storm. Fix the water first. If you sand a cupped floor while it is still wet, the raised edges get cut flat, and when the wood finally dries the centers rise and you own a crowned floor instead.
We would rather come out, take readings, and tell you to run a dehumidifier for three weeks than sell you a refinish that fails. Once the floor is dry and flat again, hardwood refinishing runs $3.50 to $6 per square foot for sand, stain and two to three coats, and an original oak floor usually comes back better than the homeowner expected.
Living With a Wood Floor in St. Louis
A wood floor in this climate is not a fragile thing. It is a natural material doing what natural materials do, in a place with real weather. Keep the house in the 35 to 55 percent band, keep the basement dry, wipe up winter salt and slush before it sits, and your floor will outlast the furniture on top of it.
If your boards are doing something you have not seen before, we will come look at it. We install and refinish hardwood across St. Louis and the whole metro, and the showroom at 149 Weldon Pkwy in Maryland Heights is open Monday through Friday, 8 to 5, if you would rather bring photos in and talk it through. Call (314) 897-4383 or set up a free in-home estimate and we will give you a straight answer either way.
