Wow, you are onto something but rest assured this concept is already in use and the idea is gaining leverage in the United States. European countries have green roofs down but it is taking time for Americans to accept the idea. North Carolina includes the system as a best management practice in stormwater control. See the link below for the chapter in the manual.Charlotte just welcomed its first green roof at the Ritz-Carlton! I'm seriously thinking about adding vegetation to the roof of my little shed
http://www.ritzcarlton.com/en/Properties/Charlotte/Information/Default.htm#cust2
http://portal.ncdenr.org/web/wq/ws/su/bmp-ch19

http://www.ritzcarlton.com/en/Properties/Charlotte/Information/Default.htm#cust2
http://portal.ncdenr.org/web/wq/ws/su/bmp-ch19