Rooftop patios

Downtown Montreal is full of buildings with flat roofs. Not only are the roofs flat, but they're built strong enough to stand up under snow. Despite the wealth of strong, flat roofs, there's a serious shortage of rooftop patios. I find that there's something wrong with that situation. To me, it seems simple that strong, flat roofs should mean lots of wonderful rooftop patios. No one would even need to do very much. They'd just have to put up some barriers to minimize their liability. I think that the denizens of the office buildings would seriously benefit from a little fresh air and a fantastic view.