Black spots on the ceiling: what causes them and what fixes them
29 June 2026 · 3 min read
Ceiling mould in a Perth winter is almost always condensation, not a leak. How to tell, and the sequence that actually stops it returning.
Small black spots appearing on a ceiling — often in a corner, often over winter, often in a bathroom or a south-facing bedroom. It is the most common interior complaint we get between June and August.
Condensation, not a leak
A leak makes a stain: brown or yellow, with a defined tide line, usually spreading from a point, and worse after rain.
Condensation makes spots: black or dark green, scattered or clustered, worse in corners and along the edges of the ceiling, and worse overnight and in winter.
The mechanism is temperature. Warm air holds more moisture than cold air. When warm, humid indoor air meets a cold ceiling surface, the air at that surface cools below its dew point and moisture condenses. Mould needs sustained surface moisture, and a cold ceiling in a humid room provides exactly that.
Why it clusters in corners
Corners and edges are **thermal bridges** — the junction between ceiling and external wall loses heat faster than the middle of the ceiling, so it runs colder. Colder surface, more condensation, more mould. The pattern of the mould is a temperature map of your ceiling.
Insulation gaps show up the same way. If the batts above the ceiling are missing, compressed or displaced — commonly around downlights and near the eaves — that patch runs cold and grows mould in a shape that matches the gap.
The mould pattern tells you where the insulation is not
A rectangular clean-edged patch of mould often corresponds exactly to a missing or shifted batt above. It is a genuinely useful diagnostic before anyone goes into the roof space.
Why Perth gets it despite a mild climate
Perth winters are not cold, but they are damp and they are the season when houses get closed up. Add unflued gas heating — which releases significant water vapour into the room — poor extraction, and drying washing indoors, and the indoor moisture load in July is high.
Mild temperatures also mean many Perth homes have modest ceiling insulation and no mechanical ventilation strategy beyond opening a window.
The sequence that works
**1. Fix the moisture source.**
- Extraction in bathrooms and the laundry, ducted outside, with a run-on timer
- Rangehood ducted outside, not recirculating
- Do not dry washing indoors without ventilation
- If you use unflued gas heating, ventilate while it runs
**2. Fix the cold surface.**
- Check insulation coverage in the roof space, especially at the eaves and around downlights
- Repair gaps and displaced batts
- Improve airflow to still corners
**3. Kill what is there.**
Use a fungicidal wash and respect the contact time. Do not dry-brush or sand mould — that aerosolises spores.
**4. Stain block.**
Mould staining bleeds through water-based paint. A stain-blocking primer on the affected areas prevents it reappearing through the new finish.
**5. Repaint.**
A ceiling paint with mould inhibitor in wet areas. Flat for the finish, since ceilings show everything.
If it comes back within a year
Then step one was not completed. The paint is not the failure — the moisture is still arriving faster than the room can shed it. At that point it is worth measuring: a cheap hygrometer in the room over a week will tell you whether indoor humidity is genuinely high, and a look in the roof space will tell you whether the extractor is actually ducted outside or just discharging into the cavity.
That last one is extremely common in Perth housing, and it converts a bathroom problem into a roof-space problem and a mouldy ceiling in the room next door.