Black spotting on a bathroom ceiling is one of the most common household complaints in Sydney, and it appears every winter in the same houses.
Wiping it off works for about three weeks. The reason it comes back is that mould is not the problem — it is the symptom of water being somewhere it should not be, and there are two quite different causes with two quite different fixes.
Telling the two apart
Condensation looks like fine black speckling spread fairly evenly across the ceiling, worst in the corners, around the cornice, and directly above the shower. It is seasonal — bad in winter, better in summer. The ceiling is cold to touch after a shower and the paint is intact.
A leak looks different. Mould concentrated in one area rather than spread out, often with a brown or yellow tide-line staining around it. Paint bubbling, flaking or sagging. It gets worse after rain rather than after a shower, and it is present year round.
The quickest diagnostic: does it correlate with the weather, or with the shower? Run the test over a few weeks and you will know.
If it is condensation
The mechanism is straightforward. A hot shower puts a large volume of water vapour into a small room. That vapour meets the coldest surface available — usually the ceiling, because the roof space above it is cold — and condenses into liquid water. Mould spores are always present in the air, and liquid water on a surface is all they need.
The fixes, in order of effect:
Run the exhaust fan properly. During the shower and for a further 20 to 30 minutes afterwards. Most people turn it off with the light. A timer switch solves this for about $80.
Check where the fan actually vents. This is the one that catches people. A great many Australian bathroom fans discharge straight into the roof cavity rather than outside. The moisture leaves the bathroom and goes into the roof space, where it condenses on the underside of the sarking and comes back down. It should duct to an external vent or an eave vent. Worth putting your head into the roof space to check.
Check the fan has enough capacity. An undersized fan in a large bathroom does very little.
Open the window if there is one. Simple and effective.
Improve roof space ventilation. Whirlybirds and eave vents reduce the temperature differential and help the cavity dry — see whirlybirds and roof vents.
Use a mould-resistant ceiling paint when you repaint. It buys time; it does not fix the cause.
If it is a leak
Then the mould is a messenger and the roof is the problem.
Common sources above a bathroom:
- A blocked or overflowing gutter putting water back into the eaves. By far the most common. See why gutters overflow.
- A blocked downpipe doing the same thing even though the gutter looks clean — covered here.
- Failed flashing around a vent pipe, flue or skylight. Bathrooms have plumbing vents through the roof, and those penetrations are a classic entry point.
- A cracked or displaced tile, sometimes dislodged by a possum or by someone walking on the roof.
- Failed ridge pointing letting water track in.
- A skylight — the part of the roof most cleaners avoid.
- A plumbing leak from the bathroom above, or from failed shower waterproofing on an upper floor.
Water tracks along battens and sarking before it drops, so the stain on the ceiling is often metres from the actual entry point. That is why chasing it from inside rarely works and why the roof needs inspecting.
Why wiping it off is not enough
Two reasons.
Mould on a painted ceiling has usually penetrated the paint film. Surface wiping removes what you can see and leaves the hyphae in the coating, which is why it reappears in the same pattern.
And disturbing mould releases spores. Scrubbing a significant area without ventilation and protection distributes it through the house. For anything beyond a small patch, that is remediation work rather than cleaning — the distinction and the costs are in what mould removal costs in Sydney.
Why Sydney bathrooms are bad for this
Mild, humid winters that never get cold enough to dry the air out, a lot of housing stock with inadequate or unducted exhaust fans, and a building tradition of uninsulated ceilings that stay cold.
South-facing bathrooms with no window are the worst case, and small internal bathrooms in apartments are close behind. In a strata building, a shared exhaust riser that is blocked affects every unit on the stack — worth raising with the committee rather than treating as your own problem.
The order to work through it
- Work out whether it tracks the weather or the shower
- If the shower: fix the fan, check where it vents, run it longer
- If the weather: get the gutters, downpipes and roof checked before paying for anything inside
- Treat the existing mould properly once the cause is dealt with
- Repaint with a mould-resistant ceiling paint
The exterior causes are frequently a few hundred dollars to fix and are the actual solution. See gutter cleaning and roof cleaning, both of which include a roof condition check.
