Green Roofs may have been in use for thousands of years in Scandanavian countries. Birch bark waterproofing was used over a wooden roof and sod was placed on top of it to hold the birch bark in place. The natural oils in the bark maintained the water proofing.
Today's Green roofs used existing models as an end goal but needed to incorporate more flexible technology to accommodate changing architecture and materials.
This simple concept can be applied to more complex systems.
Many variations exist on this basic model.
Systems using vegetated trays accommodate drainage for heavy rain events in urban environments.
Urban "Tray Based" green roofs can be custom "coiffed" for specific plantings. These flexible systems give us options for all sorts of structures...when we think about it...doesn't it seem like a good idea to turn carbon dioxide into oxygen and manage storm water at the same time?