"During heavy rains, water that hits impervious surfaces such as buildings, roads and parking lots flows into city drains. The city's sewer system is unable to handle the excess water and ends up overflowing, releasing more than 1 billion gallons of raw sewage into surrounding rivers each year, he said." - http://www.federaltimes.com/index.php?S=4044663
From a Federal Times article that covers the Department of Interiors green roof program. It cost $228,000 to cover a portion of the 100,000 square foot roof.
I know this building well, I park outside of it a lot when I'm in DC for softball games.
The other interesting reference in the article is "integrated photovoltaic roof membranes" which are very similar to shingling material that you can put on your roof and prevents water from getting in while also obviously acting as a photovoltaic (PV) system. Very nice.