Once you’ve decided where to mount your inline fan unit and have connected all ducting, it should look something like the above photo. Strapping the flex ducting up to the trusses will make it easier to move around the attic in the future if needed. Important! Never fasten anything to the roof underlayment itself, doing so could cause a leak! The inline fan unit receives power from the main light switch in the bathroom, this way it will come on when the main light is activated. Note: Most people go #2 with the light on.