Black Mould On Bathroom Sealant And Grout

What's happening?

Black mould in bathroom silicone is growing on the soap, skin oils and moisture film that collects in and behind the bead. There are two distinct situations that look identical. If the growth is on the surface of the sealant and in the grout, a proper clean removes it and it will stay away as long as the bathroom is dried and ventilated. If the growth is under the silicone (visible through it, and unaffected by anything applied to the top) then water has got behind the bead, the mould is in the gap, and it cannot be cleaned out. That one needs the sealant cut out and replaced. Telling them apart before you buy products is the whole decision, and a cleaner will do it by eye.

What usually causes it?

• Soap scum, shampoo film and skin oils feeding mould in the sealant and grout
• No extractor fan, or a fan that runs but discharges into a loft rather than outside
• Not opening a window or running the fan long enough after showering
• Failed or split silicone letting water behind the bead, where mould grows out of reach
• Grout that has eroded or cracked, so water sits in the joint
• Wet towels, bath mats and shower curtains left in the room to dry
• A leaking shower tray, bath seal or waste keeping the substrate permanently damp

Is it dangerous?

Mould in the home is a health issue, not only an appearance one: damp and mould are linked to respiratory infections and to allergy and asthma symptoms, with greater risk for young children, older people and anyone with a lung or immune condition. The immediate danger in a bathroom is chemical: mould sprays are usually bleach-based, and mixing bleach with a limescale remover or any acidic cleaner produces chlorine gas in a small, poorly ventilated room. Do not combine products.

Can I fix it myself?

Surface growth, yes. Clean the soap film off first (mould spray applied over scum barely works) then apply a bathroom mould treatment, respect the stated dwell time, and rinse. Ventilate the room while you work and afterwards. Then change the conditions: run the extractor during and for a while after showering, squeegee the screen, and dry the sealant line. Do not mix products, do not dry-brush mould, and do not silicone over existing mouldy sealant: the growth continues underneath and reappears within weeks.

When should you call a cleaner?

Call a cleaner when the soap scum and grout need a proper deep clean, when it has spread across a whole bathroom, or when a tenancy inspection is coming. They will also tell you honestly which parts are cleanable. If the mould is under the silicone, or the grout has eroded through, the answer is resealing or regrouting rather than cleaning, and if the room is persistently damp with mould returning on walls and ceiling too, the problem is ventilation or a leak, and worth raising with your landlord if you rent.

What will the cleaner actually do?

• Strip the soap and body-oil film first so any mould treatment can actually reach the growth
• Deep clean the grout lines, sealant beads, screen channel, waste and tray edges
• Descale the screen, tiles and fittings that hold the film mould feeds on
• Clean the extractor grille and, where accessible, the fan cover restricting airflow
• Ventilate through the work and leave the surfaces dry rather than damp
• Identify mould under the sealant, eroded grout or a leaking seal as a repair, not a cleaning failure

Who fixes this

• One-off deep clean

Questions people ask

Sources & References

1. NHS: Can damp and mould affect my health?
2. HSE, COSHH: hazardous substances
3. Approved Document F: Ventilation
4. GOV.UK, Private renting: repairs