Vegetated or green roof systems are relatively new to the US - experiencing incredible growth in the past 10 years. Installation of vegetation on roof surfaces was primarily a German civil engineering practice implemented in the 70s to advance environmental sustainability.
Information within the US on green roofs 20 years ago could only be found in German. Today over 75 institutions from NASA to Penn State currently conduct research about green roofs. Legislation has passed on both federal and local levels (learn more from Plant Connection Inc about US legislation). In January of 2017 San Francisco will be the first US city to require the implementation of vegetated roof systems and/or solar panels.
Learn more about how "Green Roofs Take Root Around the World" by Jackie Snow for NationalGeographic.com