Where this starts
A sealant joint is a flexible bridge between two things that move relative to each other, and it has a limited amount of movement it can take: a percentage of its own width, stated by the manufacturer. Everything on this page is about whether the joint is inside that budget.
THE PRESS TEST IS THE WHOLE FIRST PASS. Have somebody stand in the empty bath or on the shower tray, or press hard on the base, while you watch the sealant line at the wall. If the gap visibly opens and closes, the joint is moving more than a thin bead can survive, and the sealant is being asked to do something no sealant does. If nothing moves, the bead itself or its preparation is the problem.
WHY BATHS AND TRAYS MOVE. An acrylic bath flexes under a person's weight unless it has been fully supported, and a bath full of water weighs a great deal more than the bath — so the joint at the wall opens when the bath is filled and closes when it drains, twice a day. A shower tray on adjustable feet without a solid bed does the same under one person. That is why the correct sequence is to fit the bath, fill it with water, and seal it while it is full and loaded.
MOULD IN A SOUND BEAD IS A DIFFERENT PROBLEM. Black staining growing in silicone that is still bonded and still intact is surface growth on a damp surface — the same question as the bathroom's ventilation, not a failure of the joint. The distinction matters because one is a five-minute cleaning job and the other is taking the bath out.
THE THIRD FAMILY IS PREPARATION, and it produces a bead that looks perfect and lifts cleanly in one piece: sealant applied over old sealant residue, onto a wet or dusty surface, or over a joint so narrow that the bead is thinner than it needs to be to flex at all.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
The bath or tray moves under load
Most commonThe joint visibly opening when somebody stands in or presses on the base, failure returning within months of each reseal, often on an acrylic bath or a tray on adjustable feet.
How to check: Press down hard and watch the line. Movement you can see is far more than any bead can take.
Applied over old sealant or a contaminated surface
Most commonA bead that lifts off cleanly in one continuous piece with no residue on the surface beneath it, and old sealant visible underneath or in the corners.
How to check: Look at the back of a piece you have removed. Clean and unbonded means it never stuck.
Mould growing in an otherwise sound bead
CommonBlack speckling or staining within silicone that is still firmly bonded, worst where water stands, in a room that stays humid after showering.
How to check: Whether the bead is still attached. Growth on an intact joint is a ventilation question, not a sealant one.
The joint is too narrow for the sealant to flex
CommonA very fine bead in a tight gap, splitting along its length rather than peeling from a face, on a tiled edge sitting hard against the bath rim.
How to check: How wide the gap is. A sealant accommodates a percentage of its own width, so a hairline joint has almost no movement budget.
Water getting behind the joint from elsewhere
CommonFailure concentrated at one point, often a corner or below a shower mixer, with the wall behind soft or the tile grout failing in the same area.
How to check: Whether the surrounding grout and wall are sound. A joint being fed from behind fails from behind.
The wrong sealant for the location
Less commonRapid mould growth or poor adhesion on a product not intended for sanitary use, or a sealant applied to a surface it does not bond to such as some plastics.
How to check: What the tube said. Sanitary-grade silicone contains a fungicide and is formulated for constant wetting; general-purpose sealant is not.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It is the whole first pass. A joint you can see open is outside what any sealant accommodates, and everything else on this page is irrelevant until that is fixed.
- 2
Clean and unbonded means it never stuck — a preparation failure. Torn, with material left on the surface, means it stuck and was pulled apart by movement.
- 3
Growth in an intact bead is about the room's humidity and is cleaned; a bead that has let go is a joint problem and cleaning it changes nothing.
- 4
Sealant accommodates a percentage of its own width, so a hairline gap has almost no movement budget and will split however well it was applied.
- 5
It is the cause behind the commonest cause. A bath on its feet alone, or a tray without a solid bed, will keep moving no matter how many times the joint is redone.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the bath or tray moves under load, since the remedy is to support or re-bed it and that usually means taking it out.
- Where the wall behind the joint is soft, stained through or the tiles sound hollow, which means water has been getting in for some time.
- Where the failure is at a shower tray on an upper floor, because water passing that joint goes into the floor structure and appears on a ceiling below.
- Where resealing has been done repeatedly and keeps failing, which means the movement has never been diagnosed and another bead will not change that.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.