Mould keeps coming back. Why, and what actually stops it
Mould that returns is not a cleaning failure. It is a moisture supply that was never removed. Cleaning takes the growth off the surface; the water that fed it is still arriving, from the air, from a leak, or through the building fabric. This guide helps you work out which, using the timing, the location and the pattern, and tells you what to record so the next conversation with a landlord or contractor is about the cause.
In short
Mould comes back because the moisture that feeds it is still there. Match the pattern to the source: mould that returns in cold weather on cold surfaces (corners, behind furniture, window reveals) is condensation and needs heat, ventilation or insulation; mould in a fixed patch that grows after rain is water getting in from outside; mould near pipes or below bathrooms that appears regardless of weather is a leak. Record each episode with dated photographs from the same spot so the pattern is provable, then ask for the cause to be fixed, not the stain.
- Tenants and homeowners with mould that returns after cleaning or repainting
- Landlords trying to work out why a treatment did not hold
- Not emergencies: standing water, sagging ceilings or electrical faults need immediate action
- Note when it returns: season, weather, time since the last clean
- Note where: cold external corners, behind furniture, around windows, below pipes or bathrooms
- Photograph each episode from the same position, dated
- Check the obvious moisture sources: extractor fans, trickle vents, gutters, seals, overflows
- Ask for the cause to be identified in writing before any further treatment
- UK-wide. The building science is the same everywhere; who must fix it depends on tenure and nation.
- Photographs and timing narrow the cause; they do not prove it. Some cases need a moisture survey or a leak trace
- Mould-resistant paint and fungicidal wash hide symptoms; they do not remove a moisture source
- Health effects are for a doctor to assess, not for this guide
Record the same location over time so recurrence is visible as a timeline rather than a memory.
Record the recurrence freeThe three moisture sources
Rising damp is a fourth possibility but a rare one: a band of damp and salts along the bottom of ground-floor walls, seldom above skirting height, rarely with black mould. Most "rising damp" turns out to be one of the three above.
| Source | Where it shows | When it shows | What stops it |
|---|---|---|---|
| Condensation | Cold surfaces: external corners, behind wardrobes, window reveals, north-facing walls, ceilings under cold lofts | Cold months; worse after cooking, showering or drying clothes; improves in summer | Warmer surfaces and drier air: insulation, working extraction, background ventilation, steady heating |
| Penetrating damp | A fixed patch on an external wall or chimney breast, often high up or below a defect outside | After rain; follows the weather, not the season | Fixing the defect: gutters, downpipes, pointing, flashing, cracked render, failed seals |
| Leak | Near pipes, under bathrooms and kitchens, around radiators, on ceilings below plumbing | Any weather; may track with use of a bath, shower or appliance | Finding and repairing the leak; the mould stops once the material dries |
Use the timing
Keep a simple log: the date it reappeared, what the weather had been doing, whether the heating had been on, and how long since the last clean. Condensation mould has a season. Penetrating damp has a rainfall pattern. A leak has neither. Three episodes with dates usually make the source obvious even before anyone visits.
Use the location
- Corners and behind furniture on an external wall: cold spots where air does not move. Condensation.
- Around windows, on the reveals and the sill: cold bridging at the frame, or condensation running down the glass. Usually condensation; a stain that spreads sideways after rain suggests a failed seal.
- A ceiling patch that grows after rain: roof, flashing or gutter. A ceiling patch under a bathroom: a leak.
- A patch that follows a pipe run or sits under a radiator valve: a leak.
- A horizontal band along the bottom of a ground-floor wall with white salts: possible rising damp or bridged damp-proof course, or ground levels outside that are too high.
Why the last treatment did not hold
Fungicidal wash, anti-mould paint and a dehumidifier all treat the symptom. They can be part of the answer, but if the surface is still cold or the water is still arriving, growth returns within a season. A contractor who cleans and paints without stating the cause has not fixed anything, and the invoice is not evidence that the problem was addressed.
Ask, in writing, for the cause to be identified before further treatment is agreed. The answer should name the source and the work that removes it.
Checks you can do yourself
- Extractor fans: hold a sheet of toilet paper to the grille with the fan running; it should stick. If not, the fan, the duct or the outside grille is blocked or failed.
- Trickle vents on windows: open them and leave them open in cold weather.
- Gutters and downpipes: after rain, look for overflow, drips at joints, and green staining on the wall below.
- Ground levels: soil, paving or a path against the wall higher than the internal floor bridges the damp-proof course.
- Overflow pipes: a dripping overflow outside means a cistern or tank valve has failed.
- Humidity: an inexpensive hygrometer shows whether indoor humidity stays high overnight, which points to condensation.
What to record between episodes
Photograph each recurrence from the same position, on the same day of the month if you can, and keep the log next to the photographs. A dated series that shows growth returning in the same place after a "fix" is the evidence that the fix was cosmetic. Stop The Mould keeps the series, the log and every landlord response on one timeline so the pattern is visible at a glance.
What to ask for
From a landlord: an inspection that identifies the cause in writing, and work that addresses that cause, not another wipe-down. From a contractor: a written statement of the suspected source, what they will do about it, and what you should expect to see over the following months. From an independent surveyor, if the sums or the stakes are high: a report you can hand to whoever has to pay.
Frequently asked questions
It can reduce condensation while the real fix is arranged, and it will tell you something if the tank fills fast. It does not fix a leak, a cold wall or a failed gutter, and it costs money to run. Treat it as a stopgap and a diagnostic, not a solution.
It adds moisture to the air, which matters in a home that cannot vent it. If the mould returns only in a cold, unventilated corner, drying clothes may contribute; a building that needs its occupants to never dry clothes is still a building with a ventilation problem.
Photograph it first, report it, then clean small areas with a suitable product if you need to. Do not paint over it before it has been seen.
Escalate on the pattern, not the count. Two episodes in the same place after a claimed fix is enough to ask, in writing, for the cause to be identified.
From first photo to clear action.
Upload photos, record what's happened and build one clear damp and mould case file you can use with your landlord, council, adviser or for your own records.