By definition, green roofs are green spaces on top of human-made structures. Green roofs are more complicated than simply creating a potted plant garden on a roof. Low-maintenance plants are grown in a multi-layered lightweight system that is, in essence, an extension of the roof. The system includes a root-repellent membrane to prevent plants from rooting in the roof, a drainage system and a growing medium that is lighter than the soil used on the ground.
History: The history of green roofs dates back thousands of years. The most famous green roofs were the Hanging Gardens of Babylon, considered one of the Seven Wonders of the World. These terraced structures, constructed around 500 B.C. were built over arched stone beams and waterproofed with layers of reeds and thick tar. Soil, plants and trees were then planted.
The most recognizable green roofs in North America were installed in the 1930s on Rockefeller Center in New York City. These rooftop gardens continue to flourish today after nearly seventy years of service.
Nature: A green roof consists of vegetation and soil, or a growing medium, planted over a waterproofing membrane. Additional layers, such as a root barrier and drainage and irrigation system may also be included.
Green roofs can be used in many applications, including industrial facilities, residences, offices, and other commercial property. In Europe, they are widely used for their storm water management and energy savings potential, as well as their aesthetic benefits.
However it is indeed being debatable whether these roofs actually mitigate the Storm Water Runoff syndrome.
Studies have shown that the green roofs can play an increasingly important role in storm water management. During rainstorms, green roofs act as a sponge, absorbing much of the water that would otherwise run off. Researchers estimate that three to five inches of soil or growing medium absorbs 75% of rain events that are one-half inch or less.
Green roofs also filter pollution from rainwater. This is achieved by the root systems bacteria and fungi, which utilize the natural filtering processes of bioremediation and phytoremediation. As a result, the non-point source pollutants - Nitrogen and phosphorus, are broken down and detoxified. This beneficial process increases over time as rooftop plants and root systems mature.
Also further the green roofs are classified in twin type, namely
However green roofs cost more than traditional roofs because they require more material and labor for installation. Another factor affecting their price is that green roof contractors are limited in number. As the demand for rooftop gardens increases in India, and as additional contractors come into business, up-front costs are likely to decrease.
However, it is widely known that up-front costs do not tell the whole story. Taking into account future summertime energy savings at the time of purchase brings the price of a green roof closer to that of a traditional roof. Depending on local construction codes, it also may be possible to do without storm water infrastructure investments.
Another factor reducing the cost of a green roof is that vegetation can extend the life of a roof. This is because less solar energy reaches the roof substrate, limiting damage from UV radiation as well as daily temperature fluctuations, which cause repeated contraction and expansion.
· There are many benefits to green roofs, which make them attractive to building owners today.
· One of the biggest benefits to green roofs is reducing storm water runoff.
· Green roofs hold rain like a sponge and allow nature to dissipate the moisture naturally rather than rely on aging storm sewer systems
· Green roofs also reduce urban heat islands by reducing roof top temperatures. The surface of a conventional black roof can reach 180°F. Also vegetation protects the roof from extreme temperatures and works to lower the temperature of our urban environments.
· Green roofs also reduce the energy requirements of a building.
· By reducing the ambient temperature of the roofs surface, green roofs slow the transfer of heat into the building. This reduces the cooling requirements of the facility.
· Green roofs also provide insulation to the structure and therefore also reduce the heating requirements of the facility.
· Other benefits of green roofs are that it reduces air and noise pollution, provides urban microclimates, provides usable outdoor patio spaces, and provides protection from heat, ultraviolet radiation, and physical abuse resulting in a longer lasting roof system.