A green roof is an addition of a new or existing roof that involves high-quality water proofing, a root repellent system, a drainage system, filter cloth, a lightweight growing medium and plants. Green roof systems may be modular with drainage layers, filter cloth, growing media and plants already prepared in movable, interlocking grids, or each component may be installed separately. Green roof development involves the creation of contained green space on top of a human-made structure. This green space could be below, at or above grade, but in all cases the plants are not planted in the ground.
There is no single type of green roof that works for all buildings, climates and client needs. Green roofs can be categorized as intensive or extensive, depending on the depth of growing medium. Six inches or less growing medium depth is an extensive roof, also characterized by its lower weight, lower plant diversity, cost and maintenance. Intensive green roofs have more than 12 inches of growing media and tend to have higher plant diversity, higher weight, cost and maintenance.
One benefit green roofs provide is increased savings on heating and cooling energy costs. Results vary according to size of the building, climate and type of green roof, but a Michigan State University study finds that a typical one-story building with a grass roof and four inches of growing medium would result in a 25% reduction in summer cooling needs. Field experiments found that a 6-inch extensive green roof reduced heat gains by 95% and heat losses by 26% compared to a conventional reference roof.
Green roofs decrease the cost of meeting greenhouse gas reductions and adapting to climate change by reducing the urban heat island effect -- the phenomenon of metropolitan areas being significantly warmer than surrounding rural areas, due to the heat-reflecting nature of concrete and other man-made materials and the release of heat from AC systems. The reintroduction of vegetation into cities promotes natural cooling by absorbing, instead of reflecting, the sun's rays, and through evapotranspiration. Traditional black roofs can reach temperatures of 158°F. One study concluded that 25% green roof coverage can reduce the urban heat island effect by up to 3 degrees F. Green roofs can also mitigate air pollution levels by trapping particulates and capturing harmful gases.
Milwaukee is a city that has a combined sewer system (the deep tunnel), whose treatment capacity can be overwhelmed by heavy precipitation, resulting in overflows into Lake Michigan with untreated waste water. By capturing and temporarily storing storm water, green roofs can reduce run-off volumes, thereby reducing the occurrence of combined sewer overflow events. By reducing peak flows, green roofs can also reduce the incidence of flooding and damage by erosion.
Green roofs filter storm water, improving the quality of the run-off. Studies have shown that a conventional roof's run-off contains high concentrations of pollutants from rainwater, roofing materials and atmospheric deposition. The plants and growing media used in green roofs help decontaminate run-off, loading fewer pollutants into the municipal storm water system.