Damp, Mould and Your Plumbing
When black mould is a leak, when it is condensation, and how to tell before spending money.
Distinguish condensation from a leak before treating anything. Condensation appears on cold surfaces, in corners and around windows, and is worst in winter. A plumbing leak produces a defined damp patch that does not dry out, often with staining or salts, and it stays wet regardless of ventilation.
Published 3 August 2026 · Written by the Emergency Plumb engineers
Leak or condensation?
Mould in a cold corner of an outside wall or around window reveals is usually condensation. A defined damp patch that grows regardless of season, especially on an internal wall or ceiling, is water from somewhere, and that somewhere is often pipework.
The plumbing culprits
Weeping joints inside voids, failed shower sealant, a slow toilet pan leak, or a heating pipe under a solid floor. The meter test and a moisture map find them without opening anything speculatively.
Why speed matters
Mould spores affect breathing, and structural timber that stays wet rots quietly. If a damp patch survives two weeks of ventilation and heating, stop redecorating over it and get the source found.
Condensation, penetrating or rising: they look alike
Condensation appears on cold surfaces, worst in corners and behind furniture, and comes with black spot mould. Penetrating damp tracks from an external defect and worsens after rain. Rising damp stops at a consistent height with a tide mark. A plumbing leak is none of these, it usually has no seasonal pattern and does not respect the geometry.
The plumbing causes worth ruling out
A slow leak in a buried heating pipe, a failed shower tray seal, a leaking waste in a wall void, or an overflowing gutter soaking a wall. The meter test settles supply leaks. Timing the damp to when a shower is used settles the second. Neither takes equipment, and both are worth doing before anyone quotes for a damp-proof course.
Ventilation is usually the answer
Most household damp is condensation, and most condensation is warm moist air with nowhere to go. Extractors that actually vent outside rather than into a loft, run for fifteen minutes after a shower, plus trickle vents left open, resolve a great deal of it. Chemical treatments applied to a condensation problem achieve nothing at all.
Damp, Mould and Your Plumbing, your questions
How do I tell condensation from a leak?
Can a hidden leak cause mould?
Is black mould dangerous?
Who is responsible for damp in a rented property?
Ten minutes in November that prevent the two most expensive January callouts.
The changes that genuinely cut water and heating costs, ranked by payback.
Fifteen minutes a year that catches the failures before they choose their own timing.
Five minutes at the door that prevents coming home to a swimming pool.