Exactly how hot does it get up there in the summer. I would be pretty concerned, if the temperature usually gets above 95 F or so. In principle, if electricity and water are available you could rig up a swamp cooler (misting fan) for them; in practice I dunno about Milwaukee but I know that much of the upper Midwest is fairly humid in the summer and that would really limit how much cooling value you'd get. Another possibility is to put frozen jugs of water (basically, ice blocks) in the shaded coop with them, that they can cuddle up to to stay cool, but I really dunno how much good that would do them for, like, an entire summer (as opposed to just the occasional heatwave). It might be a problem.
You might look into something like Naked Neck ("Turken") chickens; they are not as, ah, conventionally decorative <g> as typical chicken breeds but are supposed to be pretty heat tolerant *and* not as cold-intolerant as most of the
other heat-tolerant breeds.
In winter, on a rooftop, you would need a LARGE coop with well-enclosed run, all of it very well draftproofed and tied onto the building so it doesn't blow away
; and it is possible that you *might* need heat. Put a max-min thermometer up there, not directly on the asphalt but raised up a couple feet on blocks if possible, and see how bad it gets. I wonder whether it might be possible to winter them somewhere else, like in somebody's backyard? That would be FAR easier and more pleasant...
Free ranging sounds like an exceedingly totally bad idea, unless you want chickens fatally plummeting onto pedestrians below at random intervals and making a horrible mess. And your run will probably need a floor of plywood or something like that, to prevent the chickens from scratching through the roof asphalt and causing leaks. Chickens have sharp strong toes and they aren't afraid to use them
Good luck, interesting project,
Pat