All vents will have the same problem as roofs are not flat and have dips. This goes for the 14" by 14" fans as well. My plan is to locate the fan on the center of a rib (~4" across). I am pretty good with forming metal and I am thinking of forming a vertical lip (up to 0.1") around the opening. This won't form a seal but will help limit water from leaking inside. From there, I would be relying on butyl putty sheets to create a seal (trimmed to to the mount diameter) and exterior sealed with silicone rubber. I am also thinking of shaping a wooden ring that matches the interior rib profile. That way when I tighten the Ventaline screws it will be against a firm backing and not relying on the sheet metal. That is my thinking at the moment, I'll practice the idea on some test parts before I commit to cutting the hole in the roof.
