MoldHelpUSA
Mold on a subfloor is almost always found by accident: during a bathroom remodel, when a soft spot by the toilet finally gets opened up, or when old flooring comes out. It grows on the plywood or OSB under the finished floor, where water collects and has little chance to dry. The same thing happens under vinyl plank flooring, which is water resistant on top but can trap moisture underneath.
This guide covers how to check for subfloor mold, when the wood can be cleaned and when it must go, and what is different about vinyl plank on a concrete slab.
Where subfloor mold starts
- Around toilets: a failed wax ring lets water seep under the flooring every time the toilet is flushed.
- Tubs and showers: cracked grout, failed caulk, or a leaking drain.
- Kitchens and laundry rooms: dishwasher, refrigerator water line, sink, and washing machine leaks.
- Exterior doors and walls: rain getting under a threshold or through the bottom of a wall.
- Crawl spaces: humid air and ground moisture under raised floors, which can wet the subfloor from below. See mold in the crawl space.
- Flooding: water that soaked through the floor covering.
Signs of mold on a subfloor
Mold on a subfloor usually looks like gray, black, green, or white spots and patches on the plywood or OSB, heaviest near a toilet, tub, appliance, or exterior wall, often with darker staining around the wet area. Black or white mold on a subfloor is the same problem: color does not change the fix.
- A musty smell near the floor, especially in a bathroom or kitchen
- Soft, spongy, or bouncy spots
- Flooring that is cupped, lifting, or has gaps opening up
- Tiles that crack or grout that keeps failing in one area
- Stains or dark lines along the bottom of baseboards
- From a crawl space: dark spots on the underside of the subfloor and on joists
How to check
- Find an edge. A floor vent, a transition strip, or the area around the toilet often gives you a look at the subfloor without tearing anything out. From a crawl space, inspect the underside.
- Wear protection. An N-95 respirator, gloves, and goggles.
- Look for color and staining. Mold on wood often shows as gray, black, green, or white patches.
- Do the screwdriver test. Push a screwdriver into the wood near the stain. Firm wood that resists is likely surface growth. Wood that is soft, stringy, or crumbles has decay and needs to be replaced.
- Check moisture. A moisture meter tells you whether the wood is still wet, which means the source is still active.
Can a moldy subfloor be saved?
| What you find | Usual fix |
|---|---|
| Light surface growth, wood firm, now dry | Scrub with detergent and water, dry fully, and seal |
| Growth with staining, wood firm but still damp | Fix the leak, dry with fans and a dehumidifier, then clean and seal |
| Wood swollen, delaminated, or soft | Cut out and replace the affected section |
| Rot around the toilet flange | Replace the section and the flange; check the joists below |
| Subfloor soaked by floodwater | Remove the flooring and inspect; replace sections that stayed wet |
Sealing primer, sometimes sold as a mold encapsulant, is only for wood that has been cleaned and dried. It does not fix wood that is still wet or decayed, and painting over active mold is something the EPA advises against.
Bleach, vinegar, and other products
The EPA does not recommend bleach or other biocides as a routine part of mold cleanup. Mold on wood reaches below the surface, and no spray reliably gets into porous material. Dead mold can still cause allergic reactions, so the steps that work are physical: clean, dry, and remove what cannot be cleaned. Never mix bleach with ammonia or other cleaners.
Mold under vinyl plank flooring
Vinyl plank, often called LVP, is popular because the planks do not absorb water. The trouble is what happens underneath. Water that gets under the planks from a leak, a pet accident, a wet mop that seeps through the seams, or moisture rising through a slab can stay trapped there, because the vinyl itself does not let it evaporate.
Signs of mold under vinyl plank:
- A musty smell near the floor with nothing visible
- Planks that feel soft, lift, or separate at the seams
- Dark staining that shows at the edges or seams
- Baseboards that are stained or swollen
To check, pull up a few planks near the suspected area. Most click-lock vinyl plank can be removed and reinstalled by starting at the nearest wall. Look at the bottom of the planks, the underlayment, and the floor below.
Vinyl plank on a concrete slab
Many homes in Florida and Texas sit on concrete slabs with no wood subfloor at all. Concrete does not grow mold itself, but it can pass moisture up from the ground, and a floor covering on top can trap it. That is why flooring manufacturers typically call for a moisture test of the slab and a vapor barrier or underlayment before installation; check the installation guide for the product in your home.
If you find mold under vinyl plank on a slab:
- Find the water. A leak, an AC condensate overflow, an exterior door, or slab moisture.
- Remove the affected planks and underlayment. Foam underlayment with mold growth is thrown away.
- Clean the slab with detergent and water and let it dry completely.
- Clean or replace the planks. Hard vinyl planks can often be cleaned; planks with mold in the attached pad are usually replaced.
- Reinstall only when the slab is dry, with a vapor barrier if the product calls for one.
This is a common call in slab-on-grade homes in Orlando, Tampa, Houston, and Dallas. Older raised homes in Jacksonville and Panama City are more likely to have wood subfloors over a crawl space.
When to call a professional
- More than about 10 square feet of the floor is affected
- The subfloor or joists are soft or decayed
- The mold extends under walls, cabinets, or a tub
- The water came from a flood or sewage
- The floor sits over a crawl space with widespread growth
- Someone in the home has asthma, allergies, or a weakened immune system
If any of these apply and you own the home, one free call connects you with an independent local remediation company that can check the floor.
A remediation company can contain the area, remove damaged flooring and subfloor, dry the structure, and check moisture before anything is rebuilt. See what mold remediation involves and mold under carpet for the carpeted rooms.
Not sure how far it spreads under the floor? A local inspector can map it with a moisture meter, and the call to find one is free.
What it costs
Removing mold from a small area typically runs about $500 to $1,500, according to HomeGuide's cost data. Subfloor replacement and reinstalling the finished floor are separate costs that often make up much of the total. See our mold remediation cost guide.
Health
The CDC says mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes, or skin rash in sensitive people, with stronger reactions in people with asthma or mold allergies. If anyone has symptoms, talk to a doctor.
Next step
If you own your home and found mold on the subfloor, soft spots in the floor, or mold under vinyl plank that covers more than a small area, call the number on this page to be connected with an independent local mold remediation company.