Teardrops n Tiny Travel Trailers

I created this build with a continued radii over the entire roof. The problem was to install the fan on the curved roof with a factory installed look. I built the opening with 2x3 wood framing and sanded in the radius best I could to match the roof radius by running a level across the entire roof side by side checking until the radius closely matched the side walls. I slightly flattened the opening so as not to pressure the plastic exterior of the fan frame too much when mounting the fan. I put a few extra thickness of butyle tape on the front and back edges to fill the gap because of roof radius. I then started from front center mtg hole and screwed in the fan frame and worked my way towards the rear mounting screws slowly tightening from side to side forward to rear until the fan frame formed the slight curve radius of the roof until completely sealed down tight on the tape. (patience is key, if you crack the frame here it will cost you a new fan.) The final process is to use self leveling caulking over the screws and edges which hasn't been completed just yet in this photo. For the interior I purchased a fan frame interior trim cover used on air stream trailers. This fit nicely for the fantastic fan 14 x 14 opening. And to accommodate the interior curve of the roof running from front to back I turned the air stream frame 90 deg.

It looks good.... Do you still have a good seal with the cover closed all
around the lid. Otherwise rain can get in while driving down the highway.
I found out the hard way.

Your approach looks like it would work but I'm not sure what would happen when you start placing your screws to fix the vent to the roof.

Here are some alternatives:

• Add a shim to the front and back only to provide a solid surface for screwing down the fan unit to the trailer. You could then either caulk the sides or create smaller shims of varying thicknesses to go under each screw hole on the sides and calk in between. Again I would recommend aluminum stock so you don't have to worry about weatherproofing wood and keeping it from decay.
• Use a product like this https://www.amazon.com/General-Tools-Co ... B00004T7RA to determine the contour and build a frame on the outside to create a flat surface. This could be done from wood and sealed or even by creating it from aluminum stock. The front and back would be relatively easy while the sides would need to have a curve on the bottom surface. Designing this and taking it to a shop with a CNC machine would be the best. I acknowledge that this method is more technical than the first one, but also more cool! lol

Thanks for the reply's and thanks for the heads up and suggestions. I purchased an enclosed removable vent cover that I'll be installing for transport and crappy rainy camping nights.
Also I installed it this way to avoided going the shim route in front and back or adding extra parts and weight. It's all screwed in and tightly sealed down now.

My roof radius is very subtle, but I still did it the way crazycyclist suggested; except it is much easier to do than using a special tool or CNC machine.

First I traced the roof profile on to my side spars and cut the bottoms to match the inside ceiling profile, but left the tops flat (I had to start with slightly taller stock than the regular roof spars). This could be done after the fact using a couple of wedged shims on either side.

Then I added shims along the front and rear vent spars; a little planing and sanding using a medium length board and the whole thing was nice and flat. Because my arch was gentle I only had about 1/8 inch at either end to fair in the foam before glassing the outer skin. Now that it is done you can barely see the transition and it all blends in nicely.