I have a slight radius to my roof where my fan will go and I chose to build it up enough to be a flat surface. Since my Maxx Air fan has a rain enclosure and is already pretty tall, I didn't want to make it any taller, but I also didn't want to stress the plastic by trying to force it to the shape of the roof. The raised area, if you can call it that, was easy to fair into the curvature and isn't really very noticeable. On the other hand, if your fan shroud is short and you don't have clearance concerns (like trying to make sure you can get your camper into your garage) adding a short berm all around sounds like it would hedge a bet against future potential leaks. On the third hand, Tony has a lot of hours on the road with his multiple builds and I'm sure gets plenty of feedback from people using his methods, so if no berm is good enough for him, it's probably not necessary.
