Cleaning mold off concrete takes a stiff brush, water and detergent, plus a bleach mix capped at 1 cup per gallon — and the moisture source fixed first.
Mold on a garage floor or basement slab is a moisture problem wearing a disguise. Scrub the fuzz off and skip the leak, condensation, or poor drainage behind it, and growth returns within weeks. The CDC’s cleanup guidance for hard surfaces covers concrete well, and the job runs on supplies you probably already own.
What You Need Before You Start Scrubbing
Safety gear comes first because dry mold spores go airborne the moment you disturb them. The CDC recommends at least a NIOSH-approved N95 respirator, non-latex gloves (vinyl, nitrile, or rubber), and goggles that seal around the eyes.
Vent-hole safety glasses let fine dust slip past the edges, so full eye protection matters more here than on most cleaning jobs. Open windows and doors for airflow, but save fans and dehumidifiers until you know the electrical system is safe — never run equipment that took on flood water.
- Respirator: NIOSH-approved N95 or better
- Gloves: non-latex, waterproof
- Eye protection: enclosed goggles, not vented glasses
- Scrub brush: stiff-bristled, dedicated to this job
- Detergent: dish soap or a general-purpose cleaner
- Bleach: regular unscented, 5%–9% sodium hypochlorite
How to Clean Mold on Concrete: The Step Order That Works
Wet cleaning in the right sequence removes the mold and keeps it gone: fix the moisture, clear debris, wash with detergent, treat with diluted bleach if needed, then rinse and dry fast.
1. Find and fix the water source
Trace where the dampness comes from — a leaking pipe, a cracked foundation, a downspout dumping water against the wall, or condensation from humid air. Cleaning without correcting that lets mold return, and no cleaner changes that.
2. Clear the loose stuff first
Mud, dirt, and debris block a cleaner from reaching the mold. Sweep or wet-vacuum, and avoid dry-brushing aggressively — that launches spores into the air you’re breathing.
3. Wash and scrub with detergent
Mix water with dish detergent and work it into the concrete with a stiff brush, focusing on rough, pitted spots where mold anchors. Keep the surface wet rather than scrubbing a dry patch.
4. Apply the bleach solution if mold persists
The CDC permits a mix of no more than 1 cup of household bleach per 1 gallon of water on hard surfaces. Stronger is not better — follow the label and mix in a ventilated space.
5. Rinse and dry within 24–48 hours
Rinse with clean water and air-dry or speed it up with safe equipment. The CDC’s water-damage guidance calls for drying within 24–48 hours, because lingering damp feeds a second round of growth.
How Much Bleach Is Safe on Concrete?
One cup of bleach per gallon of water is the ceiling, and unscented household bleach at 5%–9% sodium hypochlorite is what the sanitizing guidance assumes. Never mix bleach with ammonia, vinegar, acids, or other cleaners — the fumes can turn toxic fast.
Painted, sealed, or coated concrete may react to bleach or detergent differently than bare slab. Test any solution on an inconspicuous patch first and follow the coating maker’s directions. Concrete is tough; the finish on top may not be.
Once the mold is gone, pick a dedicated cleaner from a tested mold killer for concrete roundup for the next spot.
| Job | The CDC-Supported Rule | Why It Matters |
|---|---|---|
| Bleach mix | 1 cup per 1 gallon of water, max | Stronger solutions don’t clean better and fume more |
| Bleach strength | 5%–9% sodium hypochlorite | Matches the label the sanitizing guidance assumes |
| Respiratory gear | NIOSH-approved N95 or better | Spores and bleach fumes both need filtering |
| Eye protection | Fully enclosed goggles | Vented safety glasses let particles reach the eye |
| Dry time | 24–48 hours after water exposure | Fast drying stops mold from restarting |
| DIY vs. pro | Under 10 square feet, DIY is fine | Larger areas need professional remediation |
| Never mix | No ammonia, vinegar, or acids with bleach | Toxic gas forms immediately |
Keeping Mold From Coming Back on Concrete
Mold returns whenever the slab stays damp, so prevention is a moisture-control routine rather than a yearly scrubbing ritual. Repair leaks promptly, fix drainage that sends water toward the slab, and keep indoor humidity down with ventilation or a dehumidifier.
Do not paint, seal, or caulk over active mold — covering it doesn’t kill it, and growth resurfaces through or under the coating. Reinspect a few weeks after cleaning. Fresh growth means the moisture problem never got fixed, which sends you back to step one.
Common Questions
The CDC permits 1 cup per gallon of water for hard surfaces, followed by rinsing and drying. If the slab stays damp, mold returns no matter how thoroughly you scrub.
Should I use a pressure washer on moldy concrete?
A pressure washer can knock mold off a slab, but it also blasts spores and dirty water into the surrounding air. The CDC recommends wet cleaning and gentle removal instead of aggressive disturbance. If you use one, wear full protective gear and keep the area well ventilated.
When should a professional handle concrete mold?
Tight corners, hidden wall cavities, and any flooded HVAC system are also pro jobs. Small spots on a concrete floor are normally a homeowner’s own cleanup.
Sources
- Centers for Disease Control and Prevention. “Mold Clean-Up Guidance for Homeowners and Renters” Source for the bleach concentration limits, protective gear, and drying window.
- Centers for Disease Control and Prevention. “8 Tips to Clean Mold” Source for the cleanup sequence and homeowner guidance.
- Centers for Disease Control and Prevention. “Mold Cleanup” (PDF) Source for remediation thresholds and drying times.