Remove Mold from Bathroom Grout

Mold grows in bathroom grout because grout is porous, stays damp, and offers all the microscopic food sources mold needs to thrive. The black or greenish staining you see isn't just surface dirt — it's living fungus rooted in the tiny cavities of your grout lines. Left unchecked, it spreads, weakens grout integrity, and can become a respiratory irritant. The good news: mold in grout responds well to the right chemical treatment and mechanical scrubbing, and once cleaned, a proper sealer will keep it from coming back. This is a Saturday morning project that requires ventilation, patience, and some elbow work. You're not replacing grout here — you're restoring it. The goal is clean, uniform grout lines that stay clean because you've addressed both the organism and the environment it needs.

1. Clear the Space. Open windows and turn on the exhaust fan. Remove bath mats, toiletries, and anything stored on the floor or tub edge. You want clear access to all grout lines and good airflow to handle fumes from whatever cleaner you're using.
2. Kill the Mold Root. Spray 3% hydrogen peroxide or a 10:1 water-to-bleach solution directly onto moldy grout. Saturate the lines — don't just mist them. Let it sit for 10-15 minutes so the solution penetrates and kills mold below the surface. For hydrogen peroxide, use it straight from the brown bottle; for bleach, mix fresh and work in sections.
3. Scrub Every Line. Use a grout brush or old toothbrush and scrub along each grout line with firm, consistent strokes. You're dislodging dead mold and surface staining. Work systematically — tile by tile, row by row. Reapply cleaner to stubborn areas and scrub again.
4. Inspect and Repeat. Wipe down all treated areas with a damp sponge or cloth, rinsing frequently. Check your grout lines in good light. If you still see dark spots, repeat the application and scrubbing on those sections. Mold that's been there a long time may need two passes.
5. Dry Completely. Let the grout air-dry for at least 2-3 hours, longer if possible. Run the fan, crack a window, or aim a small fan at the tiled area. Grout must be bone-dry before you seal it, or you'll trap moisture and invite mold back.
6. Seal the Grout. Once dry, apply a penetrating grout sealer with a small foam brush or applicator bottle. Work it into the grout lines, wiping off excess from tile faces immediately. Follow the sealer's cure time before exposing it to water. This step is what prevents mold from recolonizing.
7. Stop Mold Before It Starts. Run your exhaust fan during showers and for 20 minutes after. Squeegee tile and glass after each use if mold has been a recurring problem. Keep the bathroom door or window cracked when not in use to promote air circulation.