Mould in the shower is not a cleanliness problem. It is a moisture problem. Shower walls stay wet for hours, the fan is undersized or never used, and mould spores that exist in every home find the perfect place to grow: grout lines, silicone caulk, the ceiling above the shower head and the bottom of the curtain. This guide tells you which cleaner to use on each surface (they are different), when scrubbing is pointless and the caulk has to come out, and the ventilation fix that stops it coming back.
Key takeaways
- Grout: hydrogen peroxide or a bleach-and-baking-soda paste, 15–30 minutes, scrub with a grout brush.
- Silicone caulk with black spots inside it: cannot be cleaned. Cut it out and re-caulk with mould-resistant silicone.
- Ceiling and painted walls: vinegar or diluted bleach on a sponge, then repaint with a bathroom (mould-inhibiting) paint.
- Never mix bleach with vinegar or ammonia. Use one product, rinse, then another if needed.
- The permanent fix is ventilation: a fan that actually moves 50+ CFM, run for 20–30 minutes after every shower, plus a squeegee.
Identify what you are dealing with
| Looks like | Probably | Notes |
|---|---|---|
| Pink or orange slime in corners, on the curtain, in the drain | Serratia marcescens bacteria | Not mould, but treated the same. Easy to remove, comes back fast without drying. |
| Black or dark-green speckles on grout lines | Surface mould (Cladosporium, Aspergillus) | Cleans off. If it returns within weeks in the same spot, the grout is porous and needs sealing. |
| Black spots inside clear or white silicone | Mould growing under the caulk surface | Cannot be cleaned. Replace the caulk. |
| Fuzzy grey or black patches on the ceiling above the shower | Mould on paint from condensation | Clean, then repaint. Indicates the fan is not doing its job. |
| Soft, crumbling grout or tiles that sound hollow; musty smell | Water behind the tile; mould in the wall | Not a cleaning job. The wall needs to be opened and the shower rebuilt. |
Safety first
- Open a window and run the fan. Wear rubber gloves and, for bleach or heavy mould, a mask (N95) and eye protection.
- One product at a time. Bleach + vinegar makes chlorine gas. Bleach + ammonia (in many glass cleaners) makes chloramine gas. Rinse thoroughly between products.
- Anyone with asthma or a compromised immune system should leave the cleaning to someone else.
- Mould patches larger than about 1 m², or any mould inside walls, are a job for a remediation professional.
Grout lines
Grout is porous cement. Mould grows on it and slightly into it, so you need a cleaner that penetrates plus mechanical scrubbing.
Option A: Hydrogen peroxide (gentler, no fumes)
- Pour 3% hydrogen peroxide (drugstore strength) into a spray bottle.
- Spray the grout until wet. Wait 15–30 minutes. It will fizz on mould.
- Scrub with a stiff grout brush or an old toothbrush.
- Rinse and dry. Repeat for stubborn spots.
Option B: Bleach and baking soda paste (stronger)
- Mix baking soda with enough household bleach to make a thick paste.
- Spread it along the grout lines with an old toothbrush and leave it 15–20 minutes. The paste keeps the bleach in contact with vertical grout instead of running off.
- Scrub, rinse thoroughly, and ventilate.
- Keep bleach off natural stone, coloured grout (it can lighten it) and metal fixtures.
Option C: Commercial grout mould cleaner
Gel-type mould removers (the ones sold for silicone and grout) cling to vertical surfaces for hours and work well on stains that peroxide does not lift. Follow the label and ventilate.
Silicone caulk
Surface mould on caulk wipes off with peroxide or diluted bleach. But if the black spots are inside the silicone, no cleaner will reach them. The caulk has to be replaced, and it is a 1–2 hour job.
- Cut along both edges of the old bead with a utility knife and pull it out. A plastic caulk-removal tool helps in corners.
- Scrape off residue and wipe the joint with rubbing alcohol. Let it dry completely (a hair dryer speeds this up).
- Treat the joint with peroxide or diluted bleach to kill any remaining spores, then dry again.
- Apply a new bead of 100% silicone labelled "mould and mildew resistant" for kitchen and bath. Tool it smooth with a caulk tool or a wet finger.
- Let it cure 24 hours before showering. Our bathtub caulking guide has the full technique.
Ceiling and painted walls
- Mix 1 part bleach to 10 parts water (or use undiluted white vinegar if you prefer no fumes; it is slightly less effective but safer).
- Sponge it onto the mould. Do not saturate drywall. Wait 10–15 minutes.
- Wipe with a clean damp cloth, then dry with a towel.
- Once fully dry, prime with a stain-blocking primer and repaint with a bathroom paint that contains a mildewcide. Satin or semi-gloss sheds moisture far better than flat paint.
Mould on the ceiling above the shower is the clearest sign that the exhaust fan is not removing steam. Fix the fan (below) or the paint will go black again within a year.
Shower curtain and liner
- Fabric curtain: wash in the machine on warm with regular detergent plus 1/2 cup of baking soda. Add 1/2 cup of vinegar in the rinse. Hang to dry.
- Plastic or PEVA liner: machine wash on gentle with two towels (they scrub the liner) and detergent plus baking soda. Hang immediately; do not put it in the dryer.
- Heavy mould at the hem: liners cost $10–$20. Replace it and buy one with magnets or weights at the bottom so it dries flat.
- After every shower, pull the curtain closed so it dries fully extended rather than in folds.
Shower head, drain and door tracks
- Shower head nozzles: black rings around the nozzles are biofilm. Scrub with dish soap and a toothbrush, then do a vinegar soak (see our shower head cleaning guide).
- Drain cover: remove it, scrub with peroxide, and pour a cup of peroxide or diluted bleach down the drain to kill growth in the trap.
- Glass door tracks: vacuum debris, fill with peroxide for 20 minutes, scrub with a toothbrush and cotton swabs, rinse.
Cleaner comparison
| Cleaner | Kills mould? | Removes stain? | Safe on | Avoid on |
|---|---|---|---|---|
| Hydrogen peroxide 3% | Yes | Good | Grout, tile, caulk, fixtures | Dark coloured grout (may lighten) |
| Bleach 1:10 | Yes, on hard surfaces | Excellent | Ceramic tile, white grout, tub | Stone, metal, coloured grout, porous drywall |
| White vinegar | Yes, most species | Fair | Tile, glass, chrome, curtains | Natural stone, unsealed grout (long contact) |
| Baking soda | Mildly | Good (abrasive) | Grout, tubs | Coated glass, brushed metal |
| Tea tree oil (1 tsp per cup water) | Yes, slowly | Poor | Everything | Nothing, but it is a preventive spray, not a stain remover |
The permanent fix: ventilation and drying
- Check the fan. Hold a square of toilet paper to the grille with the fan running. If it does not stick, the fan is too weak, the duct is blocked, or the damper is stuck. Ontario building code calls for at least 50 CFM for a bathroom fan; most builder-grade fans are 50 CFM on paper and far less in practice.
- Run it long enough. 20–30 minutes after every shower. A $25 timer switch or a humidity-sensing switch makes this automatic.
- Upgrade the fan to 80–110 CFM if the bathroom is over 8 m² or the shower is a steam machine. Insulated rigid ducting to the outside, not into the attic.
- Squeegee the walls and glass after each shower. Removing standing water removes the mould's food supply.
- Leave the door or curtain open so air moves through the shower.
- Seal the grout annually and re-caulk every 3–5 years.
- Weekly preventive spray: vinegar, peroxide or a daily shower spray on the grout and corners. Ten seconds.
When to stop cleaning and start renovating
If tiles sound hollow when tapped, grout is crumbling, the tub or shower base has separated from the wall, or there is a musty smell that cleaning does not remove, water has been getting behind the tile for a long time. Mould in the wall cavity cannot be cleaned from the front. The shower needs to be opened up, dried, and rebuilt with a proper waterproofing membrane. It is expensive, but it is also the moment to get the shower you actually want: a walk-in with a linear drain, a thermostatic system, and glass that stays clear. Our walk-in shower planning guide and bathroom renovation cost guide cover what to expect.
Rebuilding the shower?
Shower systems, freestanding tubs, vanities and toilets, delivered across Ontario from our warehouse. Free delivery on orders over $600 within Ontario.
Frequently asked questions
What kills mould in the shower permanently?
Nothing kills it permanently while the surfaces stay wet. Peroxide or bleach removes what is there; a working exhaust fan, a squeegee and sealed grout keep it from returning.
Is vinegar or bleach better for shower mould?
Bleach removes stains faster on hard, non-porous surfaces. Vinegar and hydrogen peroxide penetrate porous grout better and have no fumes. Many people use peroxide for grout and diluted bleach for the tub. Never combine them.
How do I get black mould out of shower caulk?
If it is inside the silicone, you cannot. Cut out the caulk, clean and dry the joint, and re-caulk with mould-resistant 100% silicone.
Is the pink stuff in my shower mould?
No, it is a bacterium called Serratia marcescens. It is harmless to most people, cleans off with any disinfectant, and comes back wherever water sits. Drying the shower is the cure.
Why does mould keep coming back on my shower ceiling?
Steam is condensing on a cold ceiling because the fan is not removing it. Run the fan 20–30 minutes after each shower, upgrade it if it is weak, and repaint with a mildew-resistant bathroom paint.
When should I call a professional for shower mould?
When the affected area is larger than about 1 m², when tiles are loose or the wall is soft, when there is a persistent musty smell, or when anyone in the home has respiratory issues that worsen in the bathroom.
