pete, I'm not sure what your weather and climate is like out there, but my experience with urban living reminds me that rooftops can get awfully hot, especially in the middle of summer. In SoCal and VA anyway, it wasn't uncommon for temps to reach 120 and even higher on a rooftop, especially with no breeze. In a greenhouse (which already traps light energy and converts it to heat), I would imagine the temp up on a rooftop would have reached a level that no amount of cooling could reasonably have been provided that would have reduced the heat down to the (typical) 95 above which you really don't want to keep orchids.
Again, I have no idea what your particular rooftop conditions are like, so that caution may not apply. But do you have a Hi-Lo thermometer that reads and holds the hottest and coldest temps in an area? You might set one up there under some shadecloth or screen or something so you'd have a general idea of conditions before you dove into the project. Other than that, if you can provide a water and electrical source, I think you'd be able to create a really nice growing space.