I have mold showing up on my shower grout lines. What can I do to fix it?

Mold on shower grout is typically caused by poor ventilation and failing grout sealer, and while surface mold can often be cleaned, recurring mold usually indicates the grout needs to be removed and replaced. The good news is that catching it early means you can likely address this before it becomes a bigger moisture problem behind your tiles.

Surface Cleaning and Assessment
Start by determining whether this is surface mold or deeper penetration. Mix a solution of 1 part bleach to 10 parts water and apply it to the affected grout lines. Let it sit for 10-15 minutes, then scrub with an old toothbrush and rinse thoroughly. If the mold disappears completely and doesn't return within a few weeks, you're dealing with surface mold. However, if it returns quickly or you can't get it completely clean, the mold has likely penetrated into the grout itself.

For Ottawa homes, this is especially common in older properties (pre-1990s) where bathroom ventilation wasn't prioritized in the original design. The combination of our humid summers and the fact that many Ottawa homes have limited natural ventilation in bathrooms creates perfect conditions for mold growth.

When Grout Replacement is Necessary
If cleaning doesn't solve the problem, you'll need to remove and replace the affected grout. This involves scraping out the old grout with a manual grout removal tool or oscillating multi-tool, cleaning the area with an antifungal solution, and applying new grout. After the grout cures (typically 24-72 hours depending on humidity), apply a high-quality grout sealer. In Ottawa's climate, choose a sealer rated for high-moisture environments.

Preventing Future Mold Growth
The root cause is almost always inadequate ventilation. Ensure your bathroom fan is properly sized (1 CFM per square foot of bathroom space) and runs during and for 30 minutes after each shower. Many Ottawa homes have undersized or improperly ducted fans that exhaust into the attic instead of outside - this creates moisture problems throughout the house. Also, squeegee shower walls after use and keep the bathroom door open when possible to promote air circulation.

Professional Guidance and Safety
While grout replacement is a DIY-friendly project, there are important safety considerations. Always wear a respirator when removing moldy grout, ensure adequate ventilation, and contain the work area with plastic sheeting. If you discover mold covering more than 10 square feet, or if it's spread to the drywall behind the tiles, this becomes a more serious remediation project requiring professional assessment.

If the mold keeps returning despite proper cleaning and regrouting, you may have a moisture issue behind the shower walls that requires opening up the wall to address properly. For a comprehensive assessment of your shower's condition and ventilation needs, contact Justyn Rook Contracting for a free consultation.