Mold After a Flood or Leak: What to Do in the First 48 Hours

Reviewed by Elena Ramirez, Mold Remediation & Drying Specialist, IICRC AMRT #327506, October 2026.

Short answer: Mold can start on wet drywall, carpet pad, insulation and wood within 24 to 48 hours after a flood or leak. The way to stop it is to remove standing water fast, take out materials that can’t be dried, and dry the structure with dehumidifiers and air movers until moisture readings reach dry standard. Clean-water losses dried quickly usually don’t grow mold; slow or partial drying often does.

Hour by hour

• 0 to 6 hours: stop the water, extract standing water, move contents out of the wet area.
• 6 to 24 hours: moisture mapping finds where water traveled; drying equipment goes in; materials that can’t be dried come out.
• 24 to 48 hours: mold can start on materials that are still wet. Daily moisture readings show whether drying is working.
• After 48 hours: anything still wet is a mold risk, and Category 2 water can degrade toward Category 3.

What you can do right now

1. Shut off the water source.
2. Cut power to wet rooms if you can do it safely.
3. Move rugs, furniture and boxes off wet floors.
4. Open interior doors; don’t turn the heat way up, which can spread moisture.
5. Photograph everything for insurance.
6. Call for extraction and drying.

Places mold starts first after a leak

• Behind baseboard and the bottom of drywall
• Carpet pad under a dry-looking carpet
• Insulation inside exterior walls
• Subfloor under vinyl or laminate
• Inside cabinets under sinks and dishwashers

Common questions

Will mold always grow after water damage?

Not if wet materials are dried quickly. Mold needs moisture and time; drying that starts within 24 to 48 hours usually prevents it.

Can I just run fans?

Fans move air but don’t remove moisture from walls, subfloor or insulation. Professional drying uses dehumidifiers and air movers with daily moisture readings until materials reach dry standard.

How do I know if the wall is still wet inside?

You usually can’t tell by touch. A moisture meter and thermal camera can find moisture behind drywall without cutting it open.

What does a mold check cost after water damage?

4Sure’s mold inspections are $99 basic visual, $199 with moisture meter and thermal camera, and $349 with air sampling and lab analysis. On an active water job, moisture readings are part of the drying work.