Wet Subfloor: Will It Dry on Its Own, and When It Has to Come Out

A subfloor is the layer of plywood or OSB between your joists and your finished floor. It usually gets wet from above, from a toilet, a tub, an appliance or a leak that ran across the floor and found the seams. It is hard to see and slow to dry, because the finished floor on top of it seals the water in. Whether it dries or comes out depends on what it is made of, how long it has been wet, and what kind of water it was.

What to Do Now

1. 1Stop the water and pull up what you can. Close the fixture's valve or the main. Take up any rug, carpet or loose floating floor over the wet area, because the subfloor cannot dry through them.
2. 2Find the wet edge with your hand and your eyes. Look for cupped or lifted flooring, soft spots and stains on the ceiling below. The wet area is usually wider than the puddle was.
3. 3Get air moving and the humidity down. Fans across the exposed floor and a dehumidifier running in a closed room. Open the basement or crawl space below if you can reach the underside.
4. 4Have it measured before you put the floor back. A moisture meter reading against a dry part of the same floor is the only way to know it is dry.

Will a Wet Subfloor Dry on Its Own?

Sometimes, if it was wet briefly, with clean water, and the top is exposed. Plywood holds up to water better than OSB, which swells at the edges and can stay swollen after it dries. Covered by vinyl, tile or laminate, a subfloor dries so slowly that mold usually wins. Wet wood starts growing in about 24–48 hours.

Common Wet Subfloor Spots

• Under a toilet. A failed wax ring leaks a little on every flush. The floor around the base goes soft over months. The toilet leaking at the base page covers the seal.
• Under tile. Tile and grout hide water well. Cracked grout lines, hollow-sounding tiles and a stain on the ceiling below are the signs.
• Under vinyl or laminate. Both trap water on top of the subfloor. Laminate that has warped usually means the subfloor under it is wet. See warped laminate floor.
• Under a dishwasher or refrigerator. Slow leaks from a supply line run under the appliance, where nobody looks.

How a Restoration Company Handles It

A certified crew maps the wet area with a meter, then decides what to keep. Plywood that is structurally sound is dried in place, often with a mat system that pulls moisture out of the floor under a sealed sheet, or with air sent into the joist bay from below. Swollen OSB, soft sections and anything soaked with contaminated water are cut out between joists and replaced. Clean-water drying runs around $3–$7 per square foot, and dirty water costs more because more is removed.

The expensive mistake is laying new flooring over a subfloor that still reads wet. The new floor cups or lifts, and the mold that was starting keeps going underneath.

If the floor is soft or the flooring has lifted, (888) 840-6512 is answered 24/7 and connects you with a certified restoration company that serves your area. The company gives you an arrival window for your address on the call, and there is no charge for the referral.