A green roof is simply a rooftop with living plants which actually is not a new technology. In fact green roofs have been a standard practice in construction as early as 7th century B.C. in the development of the Hanging Gardens of Babylon. King Nebuchadnezzar II commissioned the construction of terraced roof gardens for his wife, Amyitis, as a remembrance of the lush land of her hometown. They were even used in the Roman Empire when pressures of development limited the amount of land for gardens. Green roofs were not only valued for their aesthetics, but they were utilized for a more practical purpose, as the soil and plants were found to provide effective insulation from inclement weather. Vikings were known to layer the walls and roofs of their homes with turf and sometimes seaweed to insulate the homes against wind and rain. Scandinavian rooftops were traditionally covered with sod from the surrounding meadows to insulate the homes from the harsh winters.
Green roofs continued to be utilized throughout the world for both practical and aesthetic purposes, but it wasn’t until the 1960s that researchers and landscape architects began to develop the most recent technologies utilized for the construction and installation of green roofs. German landscape architect, Professor Hans Luz, envisioned the use of green roofs, not only for their aesthetic and insulative properties, but as a solution to the environmental degradation in urban areas. In the 1970s an extensive amount of technical research was conducted, primarily in Germany, to create a roof that still provided the historic benefits of a green roof, but could meet modern day standards (Green roof structure). Since then, the use of green roofs has boomed in Europe, with an estimated 10% of the rooftops in Germany ‘greened’. Today green roofs can be found throughout the world in various shapes and sizes (Green roofs around the world).
The modern green roof consists of several layers. The roof begins with the structural support of the building which is either existing (or can be retrofitted to accommodate a green roof) or is built as a part of the construction of the building. The structural support can exist of wood beams, steel, concrete, etc. Most importantly the structural support should be able to support the excess weight of the green roof (from the soil and plants). Next there is a roofing membrane and a waterproof membrane that provides protection to the structural support of the building. Insulation can be applied next, but this is optional. One of the distinguishing elements of a green roof is the drainage/storage layer. This layer is designed with 'cups' that allow for water storage while also encouraging drainage and aeration of the soils and providing a root barrier. This layer is topped with a soil media that is engineered to be both light weight and capable of retaining rain water. Finally, various plants are placed within the soil media.
The types of plants that can be supported by the green roof depend on the type of green roof, extensive or intensive. The main distinction between the two is the depth of the soil media. An intensive green roof has a soil layer deeper than 6 inches which can accommodate larger plants. An extensive green roof consists of a soil layer between 3 and 6 inches which supports succulent plants.