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In general, there are two types of green roof: intensive and extensive. Occasionally a third, semi-intensive, is added to that list. The depth of the planting medium and the amount of maintenance required determines which category the roof falls under.
Generally intensive roofs are characterized by thick soil (8 inches to 4 feet), heavy weights, and elaborate plantings. The plants usually include shrubs and trees. Rooftop ponds, incorporated into the overall design, would also fall under this category. Intensive green roofs are installed primarily over concrete roof decks to enable them to withstand the weight requirements. The roof needs to be able to support from 80 to 150 pounds per square foot.
Intensive green roofs require significant maintenance, but they are beautiful. These are what generally come to mind when you mention green roofs in the city. They are usually park-like and accessible to the general public.