Question on bathroom exhaust fan, sarking and roof vent.

Hi,

I have this post in the bathroom section but haven't got many responses so far. I thought I may repost this here to seek some advice from roof ventilation perspective.

I am planning to install (post hand-over) those Light/Heater/Fan units in my bathrooms located upstairs in my house-to-be. I have sarking for my roof. For the fan exhaust, because I have sarking, does it mean that it has to be ducted externally? can I just vent the steam to the roof space and get couple of whirlybirds and it will be fine??