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Roof mounted exhausts are sometimes the best way to go. When they are connected to ducting as big as 12" in diameter, they do stick up a bit but that is so the air doesn't have to make to abrupt a turn. The steel backdraft damper is pushed open by the airflow and gravity will drop it back into place when the flow stops. Back-flows, air trying to push into the building, are blocked. Inside the flap is a bird screen that will keep bird from flipping the louvers open and nesting in the ducting.

The metal sleeve accomodates 12" ducting.