If your bathroom exhaust fan runs but the ceiling still turns mouldy, the fan is almost certainly moving far less air than it should. In most Sydney bathrooms the cause is one of four things: a dust-clogged fan, a blocked or disconnected duct, a fan switched off too soon, or a fan that is too weak for the room.
The tricky part is that the sound tells you nothing. A fan can hum along happily while barely pulling any air out. So before you scrub the ceiling again or repaint, it’s worth finding out what the fan is really doing.
Start here: the toilet paper test
This takes two minutes and needs no tools.
- Close the bathroom door and window, then switch the fan on.
- Hold a single sheet of toilet paper flat against the fan grille.
- Watch what happens.
If the paper clings to the grille, the fan is pulling air at a reasonable rate. If it flutters, slides off or drops, the fan is weak or blocked, and that is very likely why your mould keeps returning.
A second check is the mirror. Take a normal hot shower with the fan on. If the mirror is still heavily fogged ten minutes after you finish and the walls feel damp, the moisture is not leaving.
The four usual culprits
1. The fan is clogged with dust
Bathroom fans collect fine dust, hair and lint, and the damp air turns that dust into a sticky layer on the grille, blades and housing. A coating on the blades makes the fan less efficient, and a blocked grille cuts the airflow further.
Signs: the fan sounds louder or rattles, the grille looks grey and furry, or the toilet paper test fails but the fan still spins freely.
2. The duct is blocked, crushed or not connected
Behind the ceiling, the fan connects to a duct that carries moist air outside. That duct can fail in several ways:
- It has been squashed or kinked by insulation, building work or someone walking in the roof space.
- It has come loose at the fan or at the outlet, so the air spills into the ceiling.
- It sags in the middle, so condensation collects in the low point and runs back towards the fan.
- The outside flap or cap is clogged with lint, dust or nesting material.
The most serious version is a fan that discharges into the roof space instead of outdoors. All it does then is move the moisture upstairs, where it can settle on roof timbers and insulation and cause mould you can’t see. A fan should vent outside the building.
If you can safely and legally access the roof space, you can sometimes spot a loose duct. If you can’t, a technician with a camera or proper access can.
3. The fan is switched off too soon
Most mould problems start in the minutes after the shower, not during it. Steam keeps lingering and condensing on the coolest surface in the room, which is usually the ceiling.
Many fan manufacturers and building advisers suggest leaving the fan on for around 15 to 20 minutes after a shower. If your fan is wired to the light and you switch both off when you leave, it gets only a fraction of that time.
A fan with a run-on timer or a humidity sensor solves this automatically. If you rent, ask the owner or agent whether one can be fitted.
4. The fan is too weak, or in the wrong place
Some fans are simply under-sized for the room, especially in bathrooms that have been enlarged or have a large shower. A fan positioned far from the shower can also miss most of the steam.
Fans also lose performance as they age. A fan that coped fine ten years ago may now be well below what the bathroom needs.
This is the one cause that cleaning won’t fix. If the toilet paper test still fails after a thorough clean and a clear duct, the fan likely needs replacing with a stronger or better-positioned unit, which is a job for a licensed electrician.
When the fan is not the problem
Sometimes the fan is fine and the moisture is coming from somewhere else. Consider these if cleaning and fixing the fan doesn’t help:
- A roof leak above the bathroom, often shown by a stain that grows after rain.
- A plumbing leak in the wall or ceiling cavity above or beside the shower.
- Failed waterproofing around the shower, where moisture travels behind the walls.
- A closed-off or undersized window in a bathroom that depends on natural ventilation as well.
Mould that appears in a pattern, keeps returning in exactly the same spot, or comes with a damp smell or bubbling paint is worth getting checked rather than repainting over.
Why Sydney bathrooms struggle
A few local factors make this problem more common here than you might expect.
Humid summers. When the outside air is already humid, there is less room for steam to escape and walls dry more slowly. Ventilation has to work harder.
Windowless bathrooms. Many apartments and townhouses have internal bathrooms where the fan is the only ventilation. If it’s weak, there’s no backup.
Shared or long duct runs. In apartment buildings, fans often connect to long ducts or shared risers. A blockage anywhere along the way affects every bathroom on that line, and you can’t see inside it.
Older homes. Some older bathrooms have no fan at all, or an original fan that has been running for decades.
What you can safely do yourself
Some basic upkeep is simple and makes a real difference.
Clean the grille. Switch the fan off at the wall first. Most covers pull down or unclip. Wash the cover in warm, soapy water, rinse it and let it dry completely.
Vacuum the visible parts. Use a soft brush attachment to remove dust from the blades and the inside of the housing you can reach. Wipe gently with a damp cloth, taking care not to push water into the motor.
Check the outside outlet. If you can see the vent cap from the ground, check that the flap opens and isn’t clogged.
Extend the run time. Leave the fan on after showering rather than switching it off as you leave.
Clean it regularly. A grille clean every few months and a fuller clean about once a year is a sensible routine, with more frequent attention if the bathroom is used heavily.
What to leave to the professionals
Some jobs shouldn’t be a weekend project:
- Anything involving wiring. Replacing a fan, or working on the electrical connection, needs a licensed electrician. If you open the unit, turn off the power at the switchboard as well as the wall switch.
- Ducts in the roof space or ceiling cavity. Working in roof spaces carries risks from heat, falls and electrical hazards.
- Older ceilings. In homes built before 1990, some ceiling and wall materials may contain asbestos. Don’t cut, drill or sand them. If you’re unsure, have it assessed before any work.
- Blockages you can’t reach. Cleaning the full length of the duct, not just the grille, takes proper tools. Our exhaust fan cleaning page explains what a complete clean covers, and if your home has ducted heating or cooling too, the related duct work is covered under air duct cleaning.
Removing the mould you already have
Fixing ventilation stops new mould, but the old mould needs removing too.
- Ventilate the room and wear gloves and a mask.
- For light surface mould, use a cleaner made for mould and follow the label, or a mild solution recommended for your surface.
- Wipe, then let the area dry fully.
- Repaint with a bathroom or mould-resistant paint once the surface is dry and the cause is fixed.
If the mould covers a large area, has soaked into the plaster, or returns within weeks after you’ve fixed the fan, the moisture source is still active and needs professional attention. Painting over it only hides the problem for a short time.
A simple order of attack
If you want a clear plan, work through it in this order:
- Do the toilet paper test and note the result.
- Clean the grille and vacuum the housing, then test again.
- Check the outside vent cap.
- Leave the fan running for 15 to 20 minutes after every shower for a week or two.
- If the test still fails, have the duct and fan inspected.
- If the fan is fine but the mould persists, look for a leak or waterproofing problem.
The takeaway
Recurring bathroom ceiling mould is usually a ventilation problem, not a cleaning problem. A noisy fan isn’t necessarily an effective fan, so test it, clean it, give it enough run time, and check that the duct actually takes the moist air outside. Fix the moisture, and the mould has far less reason to come back.