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bathroom fan just blows into the roof space is that ok

The question, put plainly: Where does a bathroom exhaust fan have to vent to?

The short answer

It should discharge outside the building. Venting into a roof space dumps moist air into an enclosed cavity, which causes condensation, mould and, over time, damage to timber and insulation.

TradieAnswers editorial · written in-house and sourced to the authorities listed below

Checked against 4 published sources, listed in full at the end of this page. How we source this

A shower puts a substantial volume of water into the air. A fan that discharges into the roof cavity moves that moisture from a room designed to handle it into a space that is not, where it condenses on the underside of the roof and on any cold surface it finds.

What it causes

  • Mould on ceiling linings and in insulation, often first noticed as a stain in a corner.
  • Corrosion of fixings and, in metal roofs, of the sheeting itself.
  • Insulation that becomes damp and loses much of its effectiveness.
  • Conditions that favour timber decay and attract termites.

Doing it properly

  • Duct to an external vent, through the eave, the gable or the roof, not simply into the cavity.
  • Keep the duct as short and straight as practical, since every bend costs airflow.
  • Insulate ducting that runs through a cold space so moisture does not condense inside the duct and run back.
  • Fit a backdraft flap at the outlet so it does not become an entry for weather or vermin.

Sources

Every source below was fetched and its response recorded on the date shown. Where a government site blocks automated clients, that is noted rather than reported as a successful check.

  1. National Construction Code · Australian Building Codes BoardChecked 2026-08-10 · responded 200 OK
  2. NCC Volume Two · Australian Building Codes BoardChecked 2026-08-10 · responded 200 OK
  3. Home Energy Rating (NatHERS) · Australian GovernmentChecked 2026-08-10 · responded 200 OK
  4. Building and Plumbing Commission · Building and Plumbing Commission VictoriaChecked 2026-08-10 · live, but blocks automated clients so the specific page could not be confirmed

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