Condensation or a leak? How to tell
Condensation appears on cold surfaces, spreads evenly, and is worst in corners, on external walls and around window reveals. It gets better in summer.
A leak produces a defined patch that does not move with the weather, often with staining that is darker at the centre. It gets worse when a specific fitting is used.
Mould along the top edge of a bath or shower tray usually means the seal has failed, which is a leak, not condensation.
Damp low down on a wall that is worse after rain points at something structural rather than anything happening in the bathroom.
If the pattern does not fit any of the above, do not guess. It is worth having someone look before you re-tile over the top of it.
The fixes, ranked by cost
Work down this list. A surprising number of persistent mould problems are solved by the first three lines.
| Fix | Typical cost | Effort | Works when |
|---|---|---|---|
| Longer fan run-on, door closed | £0 | None | Fan is adequate but under-used |
| Replace failed silicone | £80-£180 | Half day | Mould is on seals only |
| Upgrade to quiet timer fan | £200-£320 | Half day | Existing fan is weak or noisy |
| Rigid, insulated ducting | £150-£400 | Half day | Fan is fine, airflow is not |
| Humidity-sensing continuous fan | £400-£700 | 1 day | Heavy use, internal room |
| Add or upgrade heating | £350-£900 | 1 day | Room is persistently cold |
| Insulate cold external wall | £600-£1,500 | 2-3 days | Mould tracks the outside wall |
| Strip and re-tile with proper backer | £2,500+ | 1 week | Substrate behind tiles has failed |
The three habits that make the biggest difference
Close the bathroom door while showering and for fifteen minutes afterwards. An open door pushes moist air into cold bedrooms where it condenses out of sight.
Let the fan run on. A fan that stops the moment the light goes off has extracted a fraction of what the shower produced.
Keep the room gently warm rather than heating it hard only when in use. Cold surfaces are what condensation needs, and a cold towel rail is a mould magnet.
What we find behind the tiles
Plasterboard used as a shower substrate without a waterproof membrane. It works for a while, then it does not.
Tile adhesive dabbed rather than fully bedded, leaving voids that hold water behind the tile face.
Silicone applied over old silicone, which traps moisture and grows mould from underneath where you cannot clean it.
Ducting that was never connected to the outside, so every shower for years has been venting into the ceiling void.
We tank shower areas properly as standard because these are the failures we get called out to fix in other people's work.
When re-tiling is the honest answer
If grout is soft, tiles sound hollow when tapped, or a section of wall flexes under hand pressure, the substrate has taken on water and cosmetic repair will not hold.
If mould returns within weeks of a full clean and the ventilation has already been upgraded, something is wet inside the wall.
Where that is the case, we strip back to a sound substrate, install tile backer board and a tanking system, then tile. It costs more than a patch and it stops the cycle.
We survey free of charge and will tell you if a cheaper fix will do. Not every mould problem needs a refurbishment.
Frequently asked questions
Why does mould keep coming back after I clean it?
Because cleaning removes the growth but not the moisture that feeds it. Until ventilation, heating or the failed seal is addressed, it returns to the same spot.
Is black mould in a bathroom dangerous?
Persistent mould is a health concern, particularly for anyone with asthma or allergies, and it is a clear signal of an underlying damp problem worth fixing properly.
Will an extractor fan alone stop condensation?
Often it will, if the fan is adequate and correctly ducted. If the room is also cold, you usually need to address heating as well.
Should I use anti-mould paint?
It helps on ceilings as part of a wider fix, but on its own it is a delaying tactic rather than a solution.
Can you tell me whether it is a leak without ripping anything out?
Usually yes. A survey plus the pattern of the staining tells us a great deal, and our surveys are free and no-obligation.