A roof is considered a green roof when it is covered, either partially or completely, with vegetation and growing media that have been planted atop an installed waterproof membrane. Green roofs have been around for hundreds of years. Their economic and environmental benefits have caused them to gain popularity in recent years. Also called “living roofs,” these unique rooftops absorb rainwater, provide insulation against heat, cold and noise, help lower urban air temperature and even provide a habitat for area wildlife. Green roofs can contribute to LEED points when properly installed.
There are two basic types of green roof systems. Intensive green roofs are thick and can support a wide range of plant types, however they are more expensive to maintain and require more frequent monitoring. Thinner, lighter layers of vegetation constitute extensive green roofs and are an easier and less costly to maintain, although there is some restrictions in terms of the type of growth that can thrive in an extensive green roof system. In some cases a green roof may include a rooftop pond used to treat grey water. While they do offer some environmental and ecological benefits, container gardens situated on roof tops do not technically meet the definition of a green roof system.
Green roofing often requires the installation of added layers underneath the waterproofing membrane to serve as root barriers and to provide for adequate drainage and irrigation. The specific requirements of green roof systems vary widely depending on the slope of the roof, the type of plants being used and the geographic location of the roof system. Although the installation of a green roof can initially require a somewhat large investment, the cost is often offset by increased commercial real estate value and by the preservative properties the green roof provides to the roof structure and supporting building.