Mold under a floor, carpet, or laminate.

Lift a corner and look at the back of the covering and at the subfloor underneath, not at the surface. A floor is the one place where the visible problem is reliably the smallest part of the problem, because water that gets under a covering spreads sideways and has no route out.

If the underside is stained or smells, the wet area extends well past what you lifted.

Why floors hide it so well

Floating floors sit on underlay over a subfloor, with no adhesive and a sealed surface. Water that gets to that layer travels along it, and the vapor cannot rise through a vinyl or laminate wear layer, so it stays.

Carpet dries at the surface within a day while the pad underneath holds water for much longer, and the subfloor under the pad longer still. Carpet that feels dry underfoot proves nothing.

Tile looks impervious and is not: grout takes water, and what is under the tile, usually a wooden subfloor or a backer board, absorbs it.

Solid wood cups and crowns when it takes water from below, and by the time the shape has changed the subfloor has been wet for a while.

The consequence is the same in each case: the mold clock started when the material got wet, which is usually well before anyone noticed anything.

The signs before you lift anything

• A musty smell at floor level, strongest in one part of the room.
• Cupping or crowning in wood, or a lifted seam in laminate.
• Springiness or a soft spot underfoot.
• Discoloration spreading from a wall or a doorway.
• A patch that is cooler to the touch than the rest of the floor.
• Staining on a baseboard at the bottom edge.
• A carpet that smells worse in humid weather.

A moisture meter across the floor, compared against an area you know is dry, tells you more than any of those.

Lifting a corner properly

Choose the edge nearest the suspected source, or a doorway.

1. Kill the circuit for the room if there is any chance of wiring near the floor or if the area is damp.
2. Lift a small corner: carpet off the gripper, or a laminate board out of a row, or a vinyl tile at an edge.
3. Look at three things: the back of the covering, the underlay or pad, and the subfloor itself.
4. Photograph all three before you decide anything.
5. Meter the subfloor if you can.

What can be saved

Carpet pad: no. It is a sponge, and it is cheap to replace relative to the labor of trying.

Carpet: sometimes, if the water was clean and it is dealt with quickly. Not if it sat, and not if the water was contaminated, in which case the rule is removal rather than drying.

Laminate: almost never. The core is compressed fiberboard. Once it swells it does not return, and the joints will never close properly again.

Engineered wood: sometimes, depending on the thickness of the wear layer and how long it was wet.

Solid wood: often, with patience. Cupped boards frequently flatten as they dry evenly from both sides, which can take weeks, and sanding them before they have dried is how a floor gets ruined.

Vinyl sheet: sometimes, but it has to come up so the subfloor can dry, and it rarely goes back down well.

Tile: usually, if it is on a sound substrate. The question is what is under it.

Subfloor: usually, if it dries. Plywood recovers reasonably well. Particleboard and chipboard subfloor does not: it swells, loses strength, and gets replaced.

Drying it, which is the actual job

The covering coming up is not the fix. The subfloor and the joists underneath have to reach a normal moisture content before anything new goes down, and that takes equipment and days rather than a fan and a weekend.

Ask for, or take yourself, a moisture reading before new flooring is laid, and compare it against an unaffected part of the same floor. New flooring over a wet subfloor traps the moisture and you will be doing this twice.

If the floor is on a slab, the slab holds water for a long time and releases it upward for weeks. A slab leak is its own problem, and flooring laid over a slab that is still releasing moisture will fail.

Where the water came from

Fix that first or none of this matters. Common sources under floors: a dishwasher or fridge line, a toilet seal, a shower tray, a slow supply leak, a past flood, ground water through a slab, and condensation under a floor over an unventilated crawl space.

If it is not obvious, work it out from when it appears.

The short version

Look at the underside and the subfloor, never the surface. Pad and laminate are usually finished, solid wood usually is not, particleboard subfloor gets replaced. Meter the subfloor and compare it against a dry area before anything new is laid, because new flooring over a wet subfloor is the same job twice.