I wouldn't bother with the roof at all and go with a vertical rack. I'm a bit brand biased to Yakima:
HangOver - Bike - Activity
Other option is NorthShort rack:
NSR 4-Bike Rack | North Shore Racks
These options are what we see in the PNW all the time. Holds the bikes closer to the car as well. And unlike a tray style carrier it's easier to load/unload.
I never carry that many bikes so I have a 2 tray carrier from Yakima that I could extend if I want to.