Black Mould in Shower Silicone: Why It Keeps Coming Back and When to Reseal

Every Bangkok bathroom eventually grows the same line of black along the shower tray, the bath edge or the bottom of the glass screen. You scrub it with the strongest bathroom cleaner in the shop. It fades to grey. Two weeks later it is black again.

This is the single most common thing we are asked about bathrooms, and the answer is almost always the same: the mould is not on the silicone, it is in it.

Why cleaning stops working

Silicone sealant is soft and slightly porous. Mould starts as spores on the surface, feeding on soap residue and skin oils in a film of water. Given enough time and moisture, the mould grows root-like threads down into the silicone itself.

At that point, cleaning treats only the top. Bleach can lighten the visible colour, which is why it looks better for a while, but the growth inside the bead is untouched and resurfaces. No spray, gel or scrubbing changes that. The bead has to come out.

The quick test: surface or inside?

Clean a short section of the black line with a diluted bleach solution, following the product label, and rinse it.

• The black disappears completely: surface mould. Keep it clean and dry and fix the cause below.
• It turns grey or patchy but does not go: the mould is inside the silicone. Reseal.
• There is black behind clear silicone, or under the edge: water has got behind the bead. Find out why before resealing.

A safety note. Never mix bleach with ammonia-based cleaners or acidic cleaners such as some descalers and toilet cleaners. The mixture releases toxic gas. Use one product at a time, rinse between them, and keep the bathroom ventilated while you work.

Why it grows so fast in Bangkok

Mould needs moisture and food. A Bangkok bathroom supplies both.

The air never dries out. Outdoor humidity is high for most of the year, and many condo bathrooms have no window. The US EPA's guidance is to keep indoor humidity below 60 percent, ideally between 30 and 50 (EPA, A Brief Guide to Mold, Moisture and Your Home). A bathroom after a shower is far above that, and without a strong extractor fan it stays there for hours.

Water sits in the joint. A bead that is flat, concave the wrong way, or left with a ridge holds a line of water after every shower. That line is where mould starts.

The wrong silicone. Clear general-purpose silicone from the hardware store is not designed for showers. Sanitary silicone contains a fungicide that slows mould growth for years.

Silicone over old silicone. The fastest-failing reseal is new silicone smeared over the old bead. It does not bond, it seals the old mould in underneath, and the black shows through within weeks.

No time to cure. Silicone needs about 24 hours before it gets wet. Showering on it sooner washes it out of shape and weakens the bond.

How a proper reseal is done

The difference between a reseal that lasts years and one that lasts weeks is entirely in the preparation.

1. Cut out all the old silicone. Every bead is cut along both edges with a blade and pulled out, then the joint is scraped clean of residue. Any silicone left behind stops the new bead bonding.
2. Kill the mould in the joint. The cleaned joint and the surrounding tile and grout are washed with a fungicidal solution and rinsed.
3. Dry it completely. The joint must be bone dry. In a humid bathroom this is the step people rush. Fans and time, not a quick wipe.
4. Tape both edges. Masking tape on the tile and tray gives a crisp, even line.
5. Apply sanitary anti-mould silicone in one continuous bead. No stops and starts, which leave weak points.
6. Tool it smooth. The bead is shaped so water runs off it, not along it.
7. Pull the tape while the silicone is wet, then leave it to cure for 24 hours before the shower is used.

A tip for baths: fill the bath with water before sealing the joint between bath and wall, and leave it full while the silicone cures. A full bath sits at its lowest point, so the seal is not stretched every time someone gets in.

Stopping it coming back

• Run the extractor fan during the shower and for 20–30 minutes after. If the fan barely moves a tissue, it needs cleaning or replacing.
• Squeegee the screen and tray after showering. Thirty seconds removes most of the water mould needs.
• Leave the door open after showering if the bathroom has no window.
• Rinse soap off the joints. Soap scum is food.
• Check the joint once a year, and reseal at the first sign of mould inside the bead or edges lifting.

When it is not the silicone

Sometimes the silicone is only the visible part of a bigger problem.

• Grout that is black and crumbling. If grout is intact but stained, deep cleaning and sealing brings it back. If it is cracked, missing or soft, it needs cutting out and replacing. Our guide to regrouting vs retiling explains how to tell.
• Mould on the ceiling or behind the air-conditioner. That usually points to condensation, a blocked air-conditioner drain or a cold spot, not the shower. The cause needs fixing, not just the stain. See prepping damp walls before painting.
• Water stains on the wall outside the bathroom, or on the ceiling below. That suggests water getting through the joint at the tray, the grout, or the waterproofing behind the tiles. Fix that before resealing.

For larger mould problems, the EPA notes that areas bigger than about 10 square feet (roughly a 1 m × 1 m patch) or mould following significant water damage are better handled by professionals.

What it costs

RenoRova's sealing and anti-mould service starts from ฿1,900 per bathroom. That covers:

• every silicone joint cut out and resealed with sanitary anti-mould silicone
• the grout deep-cleaned and sealed with a penetrating sealer
• a fungicidal wash and anti-mould treatment on affected walls and ceilings
• a moisture reading where mould keeps returning, with the likely cause noted in the report

The shower is usable the next day, and the seal carries a two-year guarantee. If the grout needs replacing rather than cleaning, regrouting starts from ฿4,900 per bathroom and includes a full reseal.

Landlords get both inside the HostReady between-tenant reset. Send a close-up photo of the joint on LINE and we will tell you whether it needs cleaning, resealing or regrouting before you book.