I love terraces and rooftops, I consider them the most valuable part of a building (from the dweller's point of view). I also love green and gardens, who doesn't? But laying dirt on roofs is a terrible idea, or at least an anachronic one. In one hand it's an issue of just seizing the rooftops for private recreational-relax use, then potted plants and gardens will flow naturally. In the other hand I see a lot of potential in aeroponics, not only for planted rooftops but verticfal surfaces as well.
Pittsburgh Convention Center is LEED certified gold.
It's the largest "green" building in the world - 1.5 million sq.ft.
This is a portion of the roof...
I was thinking that this thread was about greed roofing materials, which immediately made me think of cedar shingles and the less practical, but incredibly durable thatched roof.
I think that green roofs are great, and if I ever had anything to do with a large project, you can bet there will be several thousand sqft of grass or gardens on the roof. On an urban project, I think it's a really compelling feature actually, and is definitely something that should be encouraged with sensible legislation at the local or perhaps state level.
And its the sensible legislation that we're lacking! And since it's so thin, insurance companies are demanding either massive premiums to insure a green roof or won't insure it at all, stopping developments dead in their tracks unless they give up the green roof elements.
Both my high school and the supermarket where I work in Halifax have green roofs. The school was just opened this year, and the supermarket was opened in the late 70s (there is a plaza on top; it's accessible to residents of the adjoining apartment tower).