Yes, this is possible and its called the green roof system. In Germany they began to use this method in the 70's mostly in order to prevent roof damage, but by the 80's they noticed that these green roofs have a positive effect on the environment in that they reduce urban heat, filter dust particles and increase bird and butterfly habitats. I don't see why it couldn't be applied in any city and under any conditions. I don't now about man power, but the point is that it has been done before and there are green roof projects going on in Toronto and Detroit already.