If your bathroom exhaust fan is ducted into the roof cavity instead of outside, every shower pushes warm, wet air into the roof space, where it condenses on cold roof sheeting, timbers and insulation and grows mould out of sight. The Australian Government’s YourHome ventilation guide says extractor fans should be vented to the outside and not into roof spaces or other cavities. Fixing the duct stops new moisture; mould already in the roof space needs separate treatment.
Why a fan into the roof is a problem
Shower air is warm and loaded with water vapour. The roof space is colder, especially on winter nights, so when that air reaches the underside of the roof sheeting, the battens, the bottom chords of the trusses and the insulation, it drops below its dew point and turns to water. YourHome’s condensation guide notes condensation can form on the underside of roofing whenever its surface temperature falls below dew point, and its ventilation page warns that exhaust fans and range hoods opening into the roof space raise water vapour levels and fire risk.
Over time this wets:
- Roof timbers, where mould establishes on the battens and truss chords
- Foil sarking, which can hold water on its surface
- Insulation batts, which absorb moisture, lose performance and become a growing surface
- The top of the ceiling lining, which eventually shows as stains in the room below
Coastal air in the Illawarra already carries a lot of moisture, so there is little spare drying capacity in the roof space to absorb the extra load. In apartment buildings a ceiling void can be shared between units, so one badly ducted fan can affect neighbours; mould in strata apartments covers who is responsible for what.
Signs your fan is venting into the ceiling
- Brown staining or bubbling paint on the bathroom ceiling around the fan grille
- Mould on the ceiling that starts at the fan and spreads outward
- Water marks on the ceiling away from any plumbing
- A musty smell in the bathroom or the rooms either side, often worse in cold weather
- Insulation near the fan that feels damp or smells musty, if you can see it from the manhole
Checking where yours goes (safely)
From inside the house and the yard:
- Turn the fan on and hold a tissue to the grille. If it is pulled in firmly, the motor is working, but that says nothing about where the air ends up.
- Go outside and look near the bathroom for a capped outlet on the roof, a vent in the eaves soffit, or a louvre on an external wall.
- If there is no outlet anywhere, the duct, if there is one at all, probably ends in the roof space.
Looking in the roof space itself carries a real fall risk. Only step on joists or trusses, never the ceiling lining, which will not take your weight, and leave anything involving the fan’s wiring to a licensed electrician. If access is tight or you are unsure, leave it to a professional.
What proper venting looks like
| Termination | Notes |
|---|---|
| Through the roof | Weather-capped penetration; needs a roofer to flash it so it does not become a leak |
| Through the eaves soffit | Often the easiest retrofit; needs a louvred or flapped outlet so wind and rain cannot blow back |
| Through an external wall | Common where there is no roof space above, such as some apartments |
A few details make the difference:
- Keep the duct short and well supported. Long, sagging flexible runs reduce airflow and collect water.
- Insulate the duct through a cold roof space, so the moist air does not condense inside it before it gets out.
- Fit a backdraught damper or non-return baffle. YourHome recommends ducting fans outside with non-return baffles and draught stoppers to stop unwanted air exchange when the fan is off.
- Check the fan can push air that far. A weak fan may need replacing when the duct is extended.
- Let it run on. A fan on a run-on timer keeps clearing moisture after the shower ends, when the room is most humid.
Who does the work
Re-ducting a fan to the outside is a small job, but it is not DIY. Any change to the fan’s wiring is electrical work for a licensed electrician, and a penetration through the roof needs a roofer so it stays watertight. A poorly sealed hole in the roof creates a leak that costs more than the original fix.
If the ceiling is already stained or the roof space smells musty, the fan may not be the only source; a roof leak or a leaking shower can look similar. A mould inspection that includes the roof space will separate them. The subfloor and roof void mould service covers treatment of timbers and removal of mouldy insulation, and bathroom ceiling mould removal covers the visible ceiling. For the bigger picture on fans, heat-recovery systems and humidity habits, see ventilation upgrades for mould prevention.
Frequently asked questions
How do I know if my exhaust fan vents outside?
Look outside near the bathroom for a capped roof outlet, an eaves vent or a wall louvre. If there is none, the duct probably ends in the roof space; tracing it from the fan inside the roof confirms it.
Is it allowed to vent a bathroom fan into the roof cavity?
YourHome notes that the National Construction Code includes condensation provisions for new homes, including requirements for exhaust systems, and recommends venting bathroom, laundry and kitchen exhaust directly outdoors. Many older homes were built or altered before this, so a fan into the roof is common, but it is still a mould risk worth fixing.
Can I cut a vent hole myself?
It is better not to. Any wiring change needs a licensed electrician, and a roof penetration needs proper flashing from a roofer to stay watertight.
Will fixing the duct get rid of the mould in the ceiling?
No. It stops new moisture going into the roof space, but mould already on timbers and insulation stays until it is cleaned or removed. An inspection will show how far it has spread.
Should the fan run after I finish showering?
Yes. The room is most humid straight after a shower, so a fan that switches off with the light stops when it is most needed. A run-on timer or humidity-sensing fan fixes that.