What's Under the Floor: Mold Beneath Carpet, Laminate and Wood
Water that gets under a floor stays longer than water on top of it, so the floor covering usually isn't the problem; what's under it is. The EPA says wet carpet must be dried within 24 to 48 hours and the subfloor under any flooring must be cleaned and dried too. Carpet pad rarely gets saved. Laminate with a swollen core usually comes up. Solid hardwood can often be dried slowly in place. A moisture meter on the subfloor, or the slab, tells you which you're dealing with.
“The carpet smells like mildew after it rains.” I hear some version of that a lot, and it almost always means the same thing: water is getting under the floor and sitting there. The floor covering is just the lid. The EPA says wet carpet has to be dried within 24 to 48 hours, and that the subfloor under carpet or any other flooring has to be cleaned and dried too. Miss the layer underneath and the smell comes back with the next rain.
Why water under a floor is the slow kind
Water on top of a floor evaporates into the room. Water under the floor is trapped between the covering and the subfloor or slab, with almost no air moving across it. Pad soaks it up, seams and edges pull it in, and a slab or plywood holds it against the underside of whatever is on top. That’s why a spill that looked cleaned up in an hour can still read wet two weeks later.
Floor by floor
| Floor | What usually happens after real water gets under it |
|---|---|
| Wall-to-wall carpet | Carpet can sometimes be dried if it was clean water and fast. The pad almost never is. Tack strip often gets replaced. |
| Laminate | The core is usually pressed wood fiber. Once it swells at the seams, it doesn’t go back, so it generally comes up. |
| Vinyl plank or sheet vinyl | Sheds water on top, traps it underneath. Often has to be lifted to dry what’s under it. |
| Solid hardwood | Often dried in place, slowly, if caught early. The EPA cautions against too much heat on hardwood floors. |
| Engineered wood | In between. Depends on the core and how long it sat wet. |
| Ceramic tile | The tile is fine. Grout lines and what’s under the tile are the questions. |
The EPA’s table treats carpet and wood differently for a reason. Wood holds water in its fibers and has to give it up slowly, and forcing it with heat can cup or crack the boards. Carpet backing and pad are porous and hold water in a way that’s hard to get back out. If growth is already on a porous material, the EPA’s general rule is that it may have to be thrown away, and the CDC says the same for anything that can’t be cleaned and dried.
How to find out what’s going on underneath
- Pull back a corner of carpet near the problem. Look at the pad, the tack strip and the floor itself.
- Pop off a piece of baseboard or shoe molding at the edge of laminate or vinyl and look at the edge of the flooring and the subfloor.
- Press on the floor. Soft, springy or crunchy spots near a toilet, tub or dishwasher are worth a closer look.
- Use a moisture meter. I check the subfloor or slab, not just the surface, and compare against a dry area of the same floor. More on reading that is in moisture meter readings explained.
If the subfloor or joists turn out to be stained or growing, that’s covered in mold on joists, subfloor and framing.
Rain smell in a Gettysburg basement or ground floor
By NOAA’s normals for the Eisenhower National Historic Site station, Gettysburg gets precipitation every month of the year, with June and September the wettest. A carpet that smells after rain is usually on a basement slab or a first floor next to an outside wall, and the water is coming in from outside: grading, a gutter, a crack, a window well. Fix that, or the next floor goes the way of the last one. Finished basements and hidden mold covers the basement version in detail.
The Census Bureau’s latest estimates put a little over half of borough households as renters. In a rental, ask whether anyone checked the slab before the last floor went down. The college area rentals page has more on that.
Before the new floor goes down
Here’s a tip that saves money: get a dry reading on the subfloor or slab before you pay for new flooring, and write it down. The best floor in the store won’t fix water coming up from underneath.
Mold Remediation Gettysburg PA is my company, and I’m John Calhoun. If a floor smells or feels wrong, I’ll check what’s under it and tell you what actually has to come up. What I do is on the mold services page, and how a job runs is in the overview of mold remediation in Gettysburg, PA. House age and rental figures for the area are on the Gettysburg housing data page.
Sources
- EPA, Mold Remediation in Schools and Commercial Buildings, Chapter 4 (Table 1, water damage cleanup)
- EPA, A Brief Guide to Mold, Moisture and Your Home
- CDC, Mold Clean Up Guidelines and Recommendations
- NOAA NCEI, U.S. Climate Normals 1991-2020 (station USC00362537, Eisenhower NHS)
- U.S. Census Bureau, ACS 2020-2024 5-year, B25003 Tenure
Questions about wet and moldy floors
The carpet smells musty but looks fine. What's going on?
Usually the pad or the floor under it is staying damp. Carpet hides that well because the top dries first. Pull back a corner near the smell, often along an outside wall or near a bathroom, and look at the pad, the tack strip and the floor. Dark staining, a damp feel or a musty smell right at the floor means water is getting in from somewhere. A moisture meter confirms it, and then the job is finding where the water comes from.
Can I put new flooring down over an old water stain?
Only after you know the stain is old and dry. Take a moisture reading of the subfloor or slab first. If it reads normal and the leak that caused the stain was fixed long ago, a clean, dry, sound subfloor can take new flooring. If it reads wet, or the wood is soft or has growth on it, new flooring just covers the problem, and you'll be pulling it up again.
Is vinyl plank waterproof enough to skip all this?
The planks themselves shed water well, which is part of the problem. Water from a leak runs through the seams or in at the edges and sits underneath, where the plank keeps it from drying. Over a concrete basement slab, vinyl plank can also hold moisture coming up through the slab. Waterproof on top doesn't mean dry underneath, so check the edges and the slab with a meter after any spill that reached the seams.
Have John check what's under it
(717) 837-3115Request a mold assessment- Locally owned in Gettysburg
- Four years in business
- Independent, not a franchise