Using vegetation on roofs dates back to the earliest known structures where plants such as moss, grass and bark gave a primitive form of insulation to keep the house cool in the summer months and warm in the winter. A famous historic green roof is the Hanging Gardens of Babylon, one of the Seven Wonders of the Ancient World. These were constructed in 500B.C. and featured a waterproof layer made of tar and reeds, with plants and trees planted above.
As human technology advanced new methods of roofing were developed, yet live vegetation was still used to insulate many roofs, like the traditional sod roofs of Scandinavia. These consisted of a layer of birch bark covered in sod, laid over gently sloping wooden boards. Nowadays technology has developed to the point that vegetation is no longer needed to insulate our homes. As urban sprawl engulfs much of the rural land it is becoming more important to try and maintain spaces of natural vegetation and replace those lost in an ever growing area of concrete and tarmac.
The modern green roof is defined as a roof that is partially or completely covered in vegetation, living in a growing medium over a waterproof membrane. They are created by layering specially made substrate over a drainage system that can absorb water to allow the plants to grow, but can drain away any excess in order to stop the roof from becoming waterlogged. The vegetation is then added on top and grows into the substrate to produce a mat of plants covering the top of the house.
Green roofs were developed in Germany in the 1960s to try and reduce the problems associated with runoff from impermeable roofs. These include water pollution, sewers nearing their capacity and localised flooding. The addition of vegetation absorbs water and slows the run off of any excess water from the roof, reducing the amount stress put on the drainage systems in a city. The vegetation, substrate and filter layers also remove pollutants from the water meaning cleaner water reaches the people below.
As research continues technology has developed and formed two main types of green roof: intensive and extensive. Keep an eye out for our next article on the different types of green roofs available.