How to Fix Grout That Is Moldy (Kill It and Keep It Gone)

Moldy grout is more than an aesthetic problem — mold in grout lines releases spores into the air and can cause respiratory issues, especially in enclosed spaces like bathrooms. The black, green, or pink discoloration you see in grout joints is a sign that moisture has been sitting there long enough for mold to take hold.

The good news: moldy grout can be treated effectively at home, and with the right approach, you can keep it from coming back.

Why Does Grout Get Moldy?

• Persistent moisture — bathrooms, showers, and kitchens are prime environments for mold growth
• Poor ventilation — without airflow, moisture lingers on surfaces after use
• Unsealed grout — porous grout absorbs water and provides a perfect environment for mold spores to grow
• Grout cracks — water gets into cracks and sits in the dark, damp interior of the joint
• Soap and product residue — organic residue in grout lines feeds mold growth

What You'll Need

• 30 Seconds Mold & Mildew Stain Remover — for surface mold treatment
• RMR-86 Instant Mold Stain Remover — for heavy or deep mold staining
• Stiff grout brush
• Rubber gloves and eye protection
• Good ventilation (open windows or run exhaust fan)
• Grout sealer (for after treatment)

How to Fix Moldy Grout: Step-by-Step

Step 1: Ventilate the area.
Open windows and run the exhaust fan before you start. Mold removers release fumes, and disturbing mold releases spores — good airflow protects you during the process.

Step 2: Apply mold remover.
For surface mold and mild staining, spray 30 Seconds Mold & Mildew Stain Remover directly onto the affected grout lines. Allow it to sit for 5–10 minutes.

For heavy black mold staining that has penetrated deep into the grout, use RMR-86 Instant Mold Stain Remover — it works faster and penetrates deeper for severe cases.

Step 3: Scrub the grout lines.
Use a stiff grout brush to scrub along the grout lines. Work in short strokes along the joint rather than across it. Rinse and repeat if needed for stubborn staining.

Step 4: Rinse thoroughly.
Rinse the treated area with clean water and wipe dry with a clean cloth. Don't leave cleaning product residue on the surface.

Step 5: Allow to dry completely.
Let the grout dry fully — at least a few hours — before assessing the result. Mold staining often looks worse when wet and improves significantly once dry.

Step 6: Seal the grout.
Once the grout is completely dry and clean, apply a grout sealer. This is the most important step for preventing mold from returning — sealed grout doesn't absorb moisture, so mold has nowhere to grow.

When to Regrout Instead of Clean

If mold has penetrated so deeply into the grout that cleaning doesn't restore the color, or if the grout is crumbling and cracked (which allows mold to grow inside the joint), regrouting is the better solution. Remove all the affected grout, treat the subfloor and tile edges with mold remover, allow to dry fully, then apply fresh mold-resistant grout.

How to Prevent Mold from Coming Back

• Ventilate after every shower or bath — run the exhaust fan for at least 15–20 minutes after use
• Squeegee tile walls after showering — removing surface water dramatically reduces mold growth
• Seal grout annually in wet areas
• Fix leaks immediately — any persistent moisture source will cause mold to return
• Use a daily shower spray — a light spray of diluted mold inhibitor after each use keeps mold from establishing

Frequently Asked Questions

Is moldy grout dangerous?
Yes — mold in grout releases spores that can cause respiratory irritation, allergic reactions, and in sensitive individuals, more serious health effects. Fix moldy grout promptly, especially in enclosed spaces like bathrooms.

Can I use bleach on moldy grout?
Bleach kills surface mold but doesn't penetrate deeply and can degrade grout over time with repeated use. A dedicated mold remover like RMR-86 is more effective and safer for grout.

Why does mold keep coming back to my grout?
If mold returns quickly after treatment, the grout is likely unsealed and absorbing moisture. Sealing the grout after cleaning is essential to break the cycle.

What's the difference between mold and mildew in grout?
Mildew is a surface fungus that appears white or grey and is easier to remove. Mold is typically black or green, penetrates deeper, and requires stronger treatment. Both should be addressed promptly.