Wet Subfloor: How to Dry It and Know If It Is Ruined

A wet subfloor needs to be found, dried and tested before any new flooring goes over it. Plywood often dries and recovers if it is only damp and you act fast. OSB swells at the edges and may not. Remove the flooring to dry the deck, check it with a moisture meter, and replace boards that are soft, rotten, delaminated or mouldy.

Wet subfloor: why it matters

The subfloor is the structural layer under your flooring. When it stays wet, it swells, warps, loses strength, grows mold and rots. It can also trap moisture beneath hardwood, laminate, vinyl and tile, so problems appear later as cupping, buckling or loose tiles. Dry it properly before covering it up.

Quick facts: wet subfloor
Common materials	Plywood, OSB (oriented strand board), boards on older homes
Drying time	Days with open airflow; longer if layers are trapped
Test	Moisture meter, compared to a known dry area
Replace if	Soft, crumbly, delaminated, rotten or mouldy

Plywood vs OSB

Feature	Plywood	OSB
Layers	Cross-laminated veneers	Compressed wood strands
Reaction to water	Tends to dry and recover if not soaked long	Edges swell and may not shrink back
Rot risk	Moderate if kept wet	Moderate to high if kept wet
Typical decision	Often dried and kept	Replace if swollen or soft

How to find the moisture

1. Look for the source: plumbing leak, appliance failure, roof leak, or high humidity from a crawl space.
2. Check from below if there is a basement or crawl space: stains, wet insulation, dripping.
3. Use a moisture meter on the floor and compare to a dry area.
4. Feel for soft spots by pressing with a screwdriver tip.
5. Check for mold smell under carpet and pads.

Drying a wet subfloor step by step

1. Stop the water and fix the leak.
2. Remove flooring and padding that cannot dry, such as carpet and pad, vinyl or laminate.
3. Remove baseboards to let the edges breathe.
4. Run fans across the deck and a dehumidifier in the room. If possible, ventilate the space below.
5. Check moisture daily until readings are close to a dry reference.
6. Treat surface mold as advised, or remove mouldy boards.
7. Wait, then lay new flooring.

When to replace

Replace sections that are soft, crumbly, delaminated, dark with rot or mould that has gone through. Cut back to the joists so new boards bear on solid framing. Treat or replace damaged joists. If water was sewage or flood water, remove the affected materials.

Tests and tools you can use

A pin-type moisture meter pushed into the subfloor gives a direct reading, while a pinless meter scans a wider area. A simple plastic sheet test works too: tape a square of clear plastic to the deck for a day and see whether moisture collects underneath. Check the underside from the basement or crawl space with a flashlight for dark stains, rust on nails and sagging insulation. Record the readings and the date, so you can see the trend as the floor dries.

Fixing the cause underneath

If the wetness comes from below, a damp crawl space or basement will keep re-wetting the new floor. Improve drainage, add a ground vapor barrier and run a dehumidifier before you rebuild.

Protecting the new floor

Install new flooring only when the subfloor is dry and flat. Some flooring makers specify maximum moisture readings, and a vapor barrier or underlayment may be required. Installing over a wet subfloor risks cupping, mold and voided warranties.

Moisture from below

A wet crawl space or damp basement can keep a wet subfloor wet from beneath. Address drainage, a vapor barrier on the ground and ventilation or dehumidification.

When to call a professional

Call a restoration company for large areas, water of uncertain quality or mold, and a contractor for structural repairs.

Wet subfloor: when to act

A wet subfloor can rot quickly once it stays damp for more than a few days. Press on the floor to find soft or spongy areas, and check the underside from a crawl space or basement if you can. Dry the area with fans and dehumidifiers, and remove any wet pad or flooring that will not dry. Plywood that delaminates or swells usually needs replacement, while a subfloor that dries within a few days may be saved. Fix the moisture source, such as a leaking supply line, before laying new flooring.

Related reading

• Hardwood Floor Water Damage: Cupping, Buckling and Recovery
• How to Dry a Wet Carpet: Fast Steps That Prevent Mold
• Moisture Meter for Walls: How to Use One and Read the Numbers
• Washing Machine Hose Burst: What to Do in the First Minutes

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