This "green roof" has been growing, or at least persisting, on top of the Atlantic County Utilities Authority for a number of years now. Funny thing is that the portions of it that are green--those blotches of white clover on the right--are actually considered a weed in this context, and should be taken out.
The brown areas consist of several kinds of sedums that despite their appearance are alive.
You'd think that a legume like white clover, which fixes nitrogen from the air and thus makes the soil more fertile, would be a good thing. But in fact there is a reason to keep the soil poor. A more fertile soil will encourage more weedy growth, which in turn will shade out the sedums and leave nothing but weeds. If a drought comes along, and the weeds die out, the roof will have lost its vegetated cover. Low-fertility is actually beneficial in this situation, and likely has a lot to do with this roof being less weedy than others I've seen. Another reason to limit nutrients is that any runoff not absorbed by the roof will head into the local waterways through the storm drain system.
Here are some sedums planted in what is essentially a miniature green roof, on the Mall near the Botanical Gardens in Washington, D.C.