How to Remove Mildew and Stains from Shower Grout
Shower grout turns dark for two distinct reasons — mildew (a surface-level fungal growth) and deep staining from soap scum and mineral deposits. These require different approaches. Here's the full method for each.
Identifying What You're Dealing With
- Mildew/mould: Black or dark green discolouration, often in a spotty or spreading pattern. Typically worse in corners, the bottom of the shower, and grout lines that stay wet longest. Mildew is a surface growth.
- Soap scum and mineral staining: White, grey, or beige discolouration. Uniform across grout lines, particularly heavy in areas with hard water (much of southern Ontario has moderately hard water). This is a mineral deposit, not a growth.
Removing Mildew from Grout
Method 1: Bleach-Based Grout Cleaner (Most Effective)
- Apply a bleach-based grout cleaner or a mixture of household bleach and water (1:1 ratio) to a stiff-bristled grout brush
- Scrub the affected grout lines
- Allow to sit 5–10 minutes (don't let it dry on the surface)
- Scrub again with the grout brush
- Rinse thoroughly with water
- Ventilate the bathroom during and after — bleach fumes accumulate in enclosed spaces
Method 2: Hydrogen Peroxide Paste (For Coloured or Sensitive Grout)
- Mix hydrogen peroxide (3%) with baking soda to form a thick paste
- Apply to grout lines with an old toothbrush
- Allow 15–20 minutes dwell time
- Scrub and rinse
- Less effective than bleach on severe mildew, but won't discolour the grout or adjacent tiles
Removing Mineral Staining and Soap Scum from Grout
- Apply undiluted white vinegar or a commercial calcium/lime/rust remover to the stained grout
- Allow 10 minutes dwell time
- Scrub with a stiff grout brush
- Rinse thoroughly
- Note: Do NOT use vinegar on natural stone tile (marble, travertine, limestone) — the acidity etches the stone. Use a stone-safe pH-neutral cleaner instead.
After Cleaning: Grout Sealer
Once grout is clean, apply a grout sealer. Sealed grout resists staining and mildew significantly better than unsealed grout. Apply with a small brush or the applicator tip included with most grout sealers. Reapply annually in high-use showers.
Preventing Mildew Regrowth
- Squeegee the shower walls and floor after every use — mildew requires moisture to grow
- Keep the bathroom fan running during and for 20 minutes after showering
- Leave the shower door or curtain open after use to allow drying
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