The short answer
Mastic cracks because the joint moves more than the sealant can stretch, and it turns black because moisture has got behind or into it and mould has colonised it. Both are symptoms of the same thing: the seal has lost its bond to one of the two surfaces.
Why sealant joints fail
A bath joint is one of the hardest-working seals in a house. An empty acrylic bath and a full one with a person in it can differ by well over 100kg, and the bath physically drops and rises with every use. Sealant has to absorb that movement thousands of times.
When the bead is too thin, applied over an existing bead, or bonded to a dirty or damp surface, it cannot cope. It splits along one edge, water wicks into the gap behind, and the dark staining follows within weeks.
The usual causes
- Sealing an empty bath, so the joint opens every time it is filled.
- New sealant applied over old, which never bonds properly to the substrate.
- The wrong product — decorator's caulk or a non-sanitary silicone in a permanently wet area.
- A damp or dusty surface at the time of application, so the bond fails from day one.
- A gap too wide for a single bead to bridge without support.
- An unventilated bathroom, where constant surface condensation feeds mould growth.
- A flexing shower tray or an unsupported bath that moves far more than it should.
Why cleaning it or re-sealing over the top does not work
Bleach and mould sprays kill what is on the surface, but the black staining that keeps coming back lives inside and behind the silicone where nothing you spray can reach. Once mould is in the bead, that bead is finished.
Applying a fresh bead over a failed one is the most common shortcut and the reason so many bathrooms have three layers of increasingly thick, increasingly grey sealant. The new silicone bonds to old silicone, not to the bath or the tile, and the water that is already trapped behind stays there — which is how a cosmetic problem quietly becomes a rotten floor or a stained ceiling in the room below.
A seal that lasts depends on complete removal, a properly dried and prepared surface, the right sanitary-grade product, correct joint width and loading the bath before sealing. Every one of those steps is a reason the DIY version fails within a year.
The hidden risk
Persistent black mastic around a bath or shower is worth taking seriously, because the water is going somewhere. Chipboard flooring under a bath swells and softens once it gets repeatedly wet, and the first visible sign is often a stain on the ceiling downstairs rather than anything in the bathroom itself.
How we put it right
Mastic & caulking, from £45
Full removal, proper preparation and sanitary-grade sealant applied with the bath loaded so the joint holds.
Serving Leamington Spa, Warwick, Kenilworth, Whitnash, Cubbington, Southam and surrounding Warwickshire.
Common questions
How often should bathroom sealant be replaced?
A properly applied sanitary silicone in a ventilated bathroom should last around five years or more. Sealant failing within a year points to preparation, product choice or excessive movement rather than normal wear.
Why does mould keep coming back on new sealant?
Either the old bead was left underneath, the surface was damp when the new bead went on, or the room's ventilation is leaving surfaces wet for hours after every shower. The third one keeps recurring until the ventilation is addressed.
How much does re-sealing a bathroom cost?
Mastic and caulking work starts from £45, including stair stringers and kitchen worktop joints as well as bathrooms.