Mould on the ceiling: four causes, and what fixes each

A mouldy ceiling is a moisture map. Mould grows where a surface stays damp, and a ceiling only stays damp for a handful of reasons: steam that has nowhere to go, rain that is getting in, a patch of ceiling cold enough for condensation, or water coming from the floor above. Each leaves a different pattern, and each has a different fix. Cleaning without knowing which one it is buys a few weeks.

The four causes, and how each one looks

The Better Health Channel’s list of common causes of indoor mould reads almost exactly like that table: leaking roofs and walls, faulty plumbing including blocked gutters and downpipes, condensation from cooking, showering and clothes drying, and areas with poor air circulation (Better Health Channel (Victorian Department of Health), Mould and your health). It also makes the point that matters for ceilings in particular: contamination may be in cavities or the ceiling space, and not all of it is visible from the room below.

Steam: the exhaust fan rules, and why an old fan may not meet them

For bathroom ceilings, the building rules are specific. The ABCB Housing Provisions require an exhaust system in a bathroom or toilet to move at least 25 litres a second, and in a kitchen or laundry 40 litres a second; and the exhaust must discharge directly or via a shaft or duct to outdoor air (ABCB Housing Provisions Standard 2025, 10.8.2(2)). A venting clothes dryer must do the same. Where a bathroom has no window or other natural ventilation, a fan that is not run continuously must be wired to the light and keep running for ten minutes after the light goes off, and the room needs make-up air — a gap under the door is enough (ABCB Housing Provisions Standard 2025, 10.8.2(4)).

Two things follow for an existing home. First, these are requirements for new building work. The clause appears in the current Housing Provisions and in NCC 2022, which cross-references it to the 2019 edition; a fan installed before then was built to whatever applied at the time. Second, the rule that the duct must reach outdoor air exists for a reason: a fan that blows into the roof space moves the moisture rather than removing it. If a bathroom ceiling keeps growing mould, look in the roof space at where the duct ends, run the fan with a sheet of toilet paper against the grille to see whether it pulls, and check that the door has a gap.

NCC 2025, which carries the same exhaust clause, took effect on 1 May 2026 in Victoria, Western Australia and the ACT; 1 May 2027 in NSW and Queensland (ABCB, NCC 2025 state and territory adoption information). Renovating a bathroom is the moment to bring the fan up to it.

Roof leaks, cold patches and water from above

Roof and gutter leaks leave a stain first and mould second. The stain can show up away from the actual entry point, because water can run along a rafter or the top of the sheet before it soaks through. If the stain grows after rain, it is a roof, flashing or gutter problem and cleaning the ceiling is the last step. Roof leak cover sets out the storm versus wear and tear argument, and ceiling water damage covers drying the cavity.

Cold patches. Where insulation in the roof space is missing, has been pushed aside by a tradesperson or has slumped, that part of the ceiling is colder than the rest, and warm, moist room air condenses on it first. The result is mould in a shape that follows the gap: a strip along the outside wall, a ring around a downlight, a rectangle near the manhole. The ACT has made ceiling insulation a minimum standard for rentals, with a compliance date of 30 November 2026 for older continuous tenancies (Residential Tenancies Regulation 1998 (ACT) pt 3); a look in the roof space with a torch shows whether the insulation is continuous.

Water from above. Under an upstairs bathroom, kitchen or laundry, a ceiling stain is a leak until shown otherwise: a failed shower membrane, a dripping waste pipe or a washing machine hose. In a unit, the floor above may be another lot owner’s, and the pipe or membrane may be common property; strata water damage covers whose policy pays.

Cleaning it, and when the sheet has to go

For routine clean-up, NSW Health gives mild detergent or vinegar diluted four parts vinegar to one part water. Where the mould is not readily removed, it gives a diluted bleach solution, with gloves, safety glasses and a well-ventilated room. Working overhead, eye protection matters more than usual: whatever is on the ceiling is coming down.

The harder question is whether the plasterboard can stay. NSW Health’s rule is simple: Discard material that cannot be cleaned and dried completely. (NSW Health, Mould factsheet). A ceiling sheet that is soft, sagging, stained right through or crumbling at the paper face is past cleaning. WA Health guidelines treat contamination no greater than 1 m² as a simple job and larger areas as complex (WA Department of Health, Guidelines for Managing Mould and Dampness Related Public Health Risks in Buildings); above that size, or where the mould keeps coming back, it is a job for someone who will also find the source.

Paint on its own is not a fix. An anti-mould paint over a ceiling that is still getting wet hides the problem until it comes through again.

Rentals and insurance

In a rental the four causes split cleanly between the two sides. A roof leak, a missing or broken fan or a leak from a pipe is the landlord’s repair. Steam from a household that never runs a working fan is where tenant responsibility comes in: Queensland’s minimum standard requires premises free of damp and mould except mould the tenant caused, and gives failing to use an installed exhaust fan as the example (Residential Tenancies and Rooming Accommodation Regulation 2025 (Qld) sch 6 item 4). Mould in a rental property sets out each state’s rules.

For insurance, mould is a consequence, not an event: the claim turns on what made the ceiling wet. A storm-damaged roof or a burst pipe can be a listed event; steam and condensation are not. Mould cover and limits works through it.

Common questions