Venting bath fan through flat roof?

I am thinking about putting a Panasonic FV-05VF1 50 CFM Low Profile Ceiling Mounted Fan in my bathroom. It is under a flat roof, with no outside wall nearby. Can such a fan be vented through a flat roof without outside air getting back in for the most of the time that it won't be on? With what sort of roof top duct ending?
Thanks, Don <donwiss at>.
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