Bathroom, kitchen and wet room leaks and mould
A line of black mould around your bath, shower or worktop, or water tracking behind a shower tray or bath panel, almost always comes back to the same cause: the sealant has failed. It sounds simple, but it's worth understanding why it fails, what the risks are, and why a proper repair takes more than a fresh bead of sealant from a tube.
What causes internal sealant to fail
Internal silicone sealant in wet areas has a harder life than most people realise.
Daily moisture cycling means the sealant is being constantly wetted and dried. It expands slightly when wet and contracts when dry. Over years of daily use, this cyclical stress causes the sealant to lose adhesion at its edges and eventually crack or lift.
Cleaning products are a major factor. Many bathroom cleaners contain bleach or other chemicals that degrade silicone over time. The sealant appears intact but its surface structure breaks down, allowing mould to establish more easily.
Movement between the bath or tray and the wall is more significant than most people expect. A bath filled with water can deflect by several millimetres. A shower tray flexes slightly underfoot. This movement repeatedly stresses the joint, and eventually the sealant cracks or pulls away from one of the surfaces.
Age is the final factor. Internal silicone sealant typically has a reliable lifespan of five to ten years in a regularly used bathroom. Most bathrooms in older properties have not had their sealant replaced.
The health risks of black mould
The black mould that establishes in failed bathroom sealant is typically Cladosporium or Aspergillus/Penicillium species, common surface moulds that grow wherever there is persistent moisture and organic material. For most healthy adults, these cause irritation rather than serious illness - respiratory irritation, allergic responses, and skin reactions.
For children, the elderly, and anyone with respiratory conditions or a compromised immune system, exposure to mould spores carries a more significant health risk. It is not something to leave and monitor.
Mould in sealant cannot be removed permanently by cleaning alone. If the sealant has become mould-contaminated throughout its depth, cleaning will reduce visible surface mould temporarily but it will return because the source - failed, mould-saturated sealant - remains.
Surface mould vs structural damp
It's worth being clear about the distinction.
Surface mould on sealant is caused by failed, contaminated silicone. It's localised to the joint area and is resolved by full removal and replacement of the sealant.
Structural damp from water behind the fitting occurs when sealant has been failing long enough that water has tracked behind the shower tray, bath, or along the floor. You may notice damp patches on the floor on the other side of a partition wall, staining at skirting board level, or a soft or spongy floor. This is a more significant problem that requires addressing the water damage before resealing.
Why DIY resealing often fails
This is one of the most common situations we're called in to correct.
The old sealant isn't fully removed. Applying new sealant over old - even over clean-looking old material - means the new joint bonds to the old sealant rather than to the bath or wall surface. The old material eventually moves or lifts, taking the new layer with it.
Mould isn't treated before resealing. New sealant applied over mould-contaminated surfaces will grow mould again quickly. The substrate needs to be properly treated before any new material goes on.
The wrong product is used. Not all silicone is appropriate for wet rooms. Standard decorator's sealant (often acrylic) is not suitable for areas of constant water contact. Sanitary-grade silicone with fungicide built in is the correct specification. Products that aren't labelled specifically for wet areas have a much shorter useful life in these conditions.
What a professional reseal involves
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All existing sealant is removed completely using cutting tools and careful mechanical removal. Every trace is cleared from both surfaces.
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Surfaces are cleaned thoroughly and any mould on the substrate is treated with an appropriate antimicrobial product.
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Sanitary-grade silicone sealant - with fungicide - is applied in a single continuous bead, pressed firmly into the joint.
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The bead is tooled to a smooth, concave profile that sheds water rather than retaining it.
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The sealant is left to cure for a minimum of 24 hours before contact with water.
The result is a clean, properly bonded joint that will outperform any DIY repair applied over existing material.
See our internal sealant removal and replacement service for full detail.
When the problem isn't the sealant
Sometimes what looks like a sealant problem is actually a plumbing or fitting issue. If water is appearing in unexpected places - below the floor level, in a ceiling void, or at some distance from the fitting - the source may be a waste connection, a supply pipe, or a failed shower valve. We'll tell you honestly if what we're seeing suggests a plumbing issue rather than sealant failure.
Similarly, if a shower tray has cracked or a bath has significant flex, replacing the sealant will not be a lasting fix. Sealing around a cracked tray is a temporary measure at best.
How often should internal sealant be checked?
As a general guide:
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In a regularly used shower or bath, inspect the sealant once a year. Look for any cracks, lifting edges, or discolouration.
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Replace as a precaution every seven to ten years, regardless of apparent condition, in high-use wet rooms.
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Replace immediately if you see lifting, cracking, mould that keeps returning after cleaning, or any sign of water tracking behind the fitting.
Proactive replacement is significantly cheaper than dealing with water damage to floors and walls.
Frequently asked questions
Q1. How serious are the health risks from black mould in a bathroom? Black mould - typically Aspergillus or Cladosporium species - releases spores that can irritate the respiratory system and aggravate conditions such as asthma, particularly in children and elderly occupants. Prolonged exposure in poorly ventilated rooms is associated with more serious respiratory effects. The mould itself is a symptom of a moisture problem; removing it without fixing the source means it returns quickly. Northern Seal & Joint addresses the root cause - failed sealant allowing water ingress - rather than just cleaning the surface. Q2. How often should bathroom sealant be replaced? Bathroom sealant typically lasts between five and ten years, depending on water exposure, cleaning products used and ventilation levels. Showers see more aggressive use than bath edges and may need attention sooner. Rather than waiting for visible mould or cracking, inspect sealant annually: if it is discoloured throughout, pulling away at any point, or has developed a musty smell, it is time to replace it. Northern Seal & Joint removes old sealant completely and replaces it properly - rather than adding a new bead on top. Q3. Can I replace bathroom sealant myself? Yes, for basic DIY jobs on clean, accessible joints. The difficulty is preparation: all of the old sealant must be fully removed and the surfaces must be dry and free of soap residue before new sealant is applied. Applying new sealant over old - even a thin layer - drastically reduces adhesion and the new bead will fail quickly. If the joint has underlying mould growth on the substrate or the grout has also failed, professional removal and treatment gives a significantly more durable result. Q4. What actually causes sealant around a bath or shower to fail? Movement is the primary cause. Baths flex slightly under the weight of water and a person; this cyclical movement stresses the sealant bead over time until it cracks or pulls away from the surface. Poor original application - too thin a bead, applied over a damp surface, or without a backing strip on wider joints - accelerates failure. Hard water deposits and aggressive cleaning products also degrade silicone over time. In wet rooms, constant moisture with insufficient ventilation speeds the process further. Q5. How do I know if it is the sealant failing or the shower tray itself? If water is appearing below the shower tray or in the room below, the issue may be more than sealant alone - a cracked tray, failed waste fitting or leaking pipe connection are possible. Sealant failure typically presents as visible cracks or gaps in the bead, with water staining or mould around the edge. If the tray surface itself is cracked or the tray is moving noticeably, that is a separate plumbing and tiling issue. Northern Seal & Joint will identify on inspection whether it is a sealant job or whether you need to involve a plumber first. Q6. Does the price include mould removal and surface preparation? Northern Seal & Joint's internal sealant replacement includes removal of old sealant, surface preparation and treatment of any mould on the substrate before new sealant is applied. This is the only approach that gives a lasting result - sealing over mould allows it to grow beneath the new bead. The full process, including the time spent on preparation, is covered in our quote. We use fungicidal-grade products where mould is present to reduce the chance of recurrence.
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