FanTastic uses good materials. Even the covers, lids, whatever you call them, rarely crack but you get a free replacement if one does. That flange will probably outlast everything around it.
Maybe I'm confused. The brackets I've seen for vent covers have a 90* angle. A short leg mounts to the vertical part of the fan/vent flange. Then the long leg mounts to the cover. Mounting surface of cover is parallel to roof surface. Nothing goes down into the roof structure, through the covering, or through the vent/fan flange. No chance to leak.
