Green roofs are not new; they have been used for thousands of years because they helped insulate, thrived in the sun instead of rotting, and other than the increased structure, they were cheap as, well, the dirt that they were planted in. Then flat roofs came in and were covered in tar and asphalt, which needed a lot of maintenance. Engineers and architects didn't worry much about them; nobody could see them. Roofs became parking lots for equipment.
Via Lauren Moss