Four causes, and how to tell them apart
| Pattern | Cause | What it needs |
|---|---|---|
| Even spotting across the whole ceiling | Room humidity and poor ventilation | Extraction and air movement |
| A band around the edges where ceiling meets wall | Cold bridge at the wall plate | Insulation and ventilation |
| A defined patch with a brown or yellow stain | Roof leak above | Trace and repair the roof |
| Directly under a bathroom on the floor above | Failed shower or bath waterproofing | Strip and re-waterproof the wet area |
| Around a downlight or ceiling penetration | Warm air escaping into a cold roof space | Seal the penetration, check roof ventilation |
| Worst in one corner of an upstairs room | Cold bridge plus still air | Insulation and air movement |
Stain or no stain — the fastest test
If there is a brown, yellow or tea-coloured stain under or around the growth, water has arrived from above as liquid. That is a leak, and the mould is a consequence.
If the growth is black spotting with no staining at all, no water arrived — the moisture condensed out of the room's own air onto a cold surface. That is a ventilation and insulation problem.
Stained means roof leak repair in Cape Town if the roof is above, or shower waterproofing if a bathroom is. Unstained means extraction, air movement and dealing with the cold surface.
Why ceilings are worse than walls
- Warm moist air rises and reaches the ceiling first
- It is usually the coldest surface in the room, particularly under an uninsulated roof space
- There is no furniture against it and no air movement across it
- Ceiling boards and their paper facing are readily colonised
- Downlights and vents puncture the ceiling and let warm air into a cold void, where it condenses on the back of the board
Growth around downlights is a specific and common signature. It usually means warm humid air is escaping into the roof space through the fittings and condensing on the cold side of the ceiling.
If the growth comes with a stain rather than on its own, water is arriving from above and the ceiling is the last place it shows. That is roof leak repair in Cape Town.
Related reading
Start with Mould on Walls for the overview, or:
- How to Remove Mould from a Bathroom
- Is Black Mould Dangerous?
- Mould-Resistant Paint
- How to Remove Mould From Walls
Common questions
Does black mould on the ceiling mean my roof is leaking?
Only if there is staining with it. A brown or yellow stain means liquid water arrived from above. Black spotting with no stain at all is condensation — the moisture came out of the room's air, not through the roof.
Why is the mould only in one corner of the ceiling?
Corners are colder than the middle — two external walls meeting creates a cold bridge — and air moves least there. Coldest plus stillest is where condensation forms first.
Why is there mould around my downlights?
Warm humid air escapes through the fittings into the cold roof space, condenses on the back of the ceiling board, and growth follows. Sealed fittings and better roof ventilation address it.