A green roof, also known as a living roof, is the use of the space on top of a building to grow plants, either decorative vegetation or food crops.
Roofs covered in vegetable matter can help reduce and buffer rain water runoff. They serve as a temperature buffer as well, through the effects of shade and evaporative cooling, as well as thermal mass and insulation value. Numerous green roofs in a city can decrease the average temperature in the summer and help prevent heat islands. By increasing plant life and reducing power usage, green roofs help reduce and uptake CO2 emissions. A green roof may be one feature in a green data center or a carbon-negative data center.
Green roofs also help increase biodiversity in challenging areas, providing new areas for avians and bats. These areal insectivores can help reduce pest insects, while also supporting those populations in the face of recent declines. This can help offset the toll that sky scrapers and other reflective structures take in bird collisions.
Green roofs can require additional thought in building design and construction. Due to the weight of the soil, water and vegetation the building needs to be better re-enforced. True green roofs can't just be potted plants on a roof but require a growing medium directly on a roof and typically use water proofing methods such as impermeable membranes to prevent water damage to the buildings. Some green roofs even use ponds as gray water systems to reuse water for purposes other than drinking.
Green roofs use growing mediums with depths of at least 2 cm (or about 8/10 inch) in the case of extensive roofs, which can support only limited types of vegetation. In the case of intensive roofs, which support a wider variety of vegetation, medium depth is approximately 13cm (5 inches). Along with an intensive roof's increase in supportable life comes an increase in maintenance requirements too.
Buildings with solar heat collectors or photovoltaic solar panels are also sometimes spoken of as green roofs. Most commonly, green roofs are used on commercial buildings in cities where habitat for creatures and carbon-fixing vegetation are less prevalent. Legislation in an increasing number of places, including France, mandate that any new commercial building must be constructed with a green roof or one with solar panels.