That quote alone is a striking illustration of why green roofs are a good thing: they reduce the burden on a building's air conditioning system. At the same time, they're a nice place to hang out and relax.
There's a really fantastic infographic, too, that explains how green roofs work without trapping too much (heavy) water on the roof.
Since the plants and other materials that do make up a green roof can weigh 15-25 pounds per square foot, according to the article, soil typically isn't used, because rain compacts it and it holds too much water.
Check out the article and the diagram at Scientific American and then get to planning your own green roof. (Don't forget to consult an engineer or architect!)