A green roof is a roof of a building that is partially or completely covered with vegetation and soil, or a growing medium, planted over a waterproofing membrane. The term does not include roofs which are merely colored green, as with green shingles. It may also include additional layers such as a root barrier and drainage and irrigation systems. Container gardens on roofs, where plants are maintained in pots, are not included in this discussion, as they are not considered true green roofs (though this is an area of debate). The term Green Roof can also be applied to roofs that have some form of purpose considered "green", such as solar panels or a photovoltaic module. Green roofs are also referred to as eco-roofs, living roofs or greenroofs (a more grammatically correct terminology)
Benefits of green roofs
Green roofs are used to:
- Provide amenity space for building users — in effect replacing a yard or patio
- Grow fruits, vegetables, and flowers
- Reduce heating (by adding mass and thermal resistance value) and cooling (by evaporative cooling) loads on a building — especially if it is glassed in so as to act as a terrarium and passive solar heat reservoir
- Reduce the urban heat island effect
- Increase roof life span
- Reduce stormwater run off — see water-wise gardening
- Filter pollutants and CO2 out of the air — see living wall
- Filter pollutants and heavy metals out of rainwater
- Increase wildlife habitat in built up areas — see urban wilderness
A green roof is often a key component of an autonomous building.
- "In North America, the benefits of green roof technologies are poorly understood and the market remains immature, despite the efforts of several industry leaders. In Europe however, these technologies have become very well established."
History and uses of green roofs
Modern green roofs, which are made of a system of manufactured layers deliberately placed over roofs to support growing medium and vegetation, are a relatively new phenomenon. They were developed in Germany in the 1960s, and have since spread to many countries in Europe. Today, it’s estimated that about 10 percent of all German roofs have been “greened.”  The United States has some green roofs, but they are not as common as in Europe.
Many green roofs are installed to comply with local regulations and government fees, often regarding stormwater runoff management. In areas with combined sewer-stormwater systems, heavy storms can overload the wastewater system and cause it to flood, dumping raw sewage into the local waterways. Green roofs decrease the total amount of runoff and slow down the rate of runoff flowing off the roof. It has been found that they can retain up to 75 percent of rainwater, gradually releasing it back into the atmosphere via condensation and transpiration, while retaining pollutants in their soil.  Elevation 314, a new development in Washington D.C., uses green roofs to filter and store some of its stormwater on site, avoiding the need for expensive underground sand filters to meet D.C. Department of Health stormwater regulations.
Comb ating the urban heat island effect is another reason for creating a green roof. Traditional building materials soak up the sun's radiation and reflect it back as heat, making cities at least 7 degrees hotter than surrounding areas.  On Chicago's City Hall, by contrast, which features a green roof, temperatures on a hot day are typically 25–80 degrees Fahrenheit (14–44 degrees Celsius) cooler than they are on traditionally roofed buildings nearby. 
Green roofs are becoming common in Chicago, as well as Atlanta, Portland, and other cities, where regulations to combat the urban heat island encourage their use. In the case of Chicago, the city has passed codes offering incentives to builders who put green roofs on their buildings. The Chicago City Hall green roof is one of the earliest and most well known examples of green roofs in the United States; it was planted as an experiment to determine the effects a green roof would have on the microclimate of the roof. Following that and other studies, it has now been estimated that if all the roofs in a major city were "greened," urban temperatures could be reduced by as much as 12 degrees . 
Green roofs have also been found to make dramatic improvements in a roof’s insulation value. A study conducted by Environment Canada found a 26 percent reduction in summer cooling needs, and a 26 percent reduction in winter heat losses when a green roof is used.  In addition, greening a roof is expected to lengthen a roof’s lifespan by two or three times, according to Penn State University’s Green Roof Research Center. 
Finally, green roofs provide habitat for plants, insects and animals that otherwise have limited natural space in cities. Even in high-rise urban settings as tall as 19 stories high, it has been found that green roofs can attract beneficial insects, birds, bees and butterflies. Rooftop greenery complements wild areas by providing "stepping stones" for songbirds, migratory birds and other wildlife facing shortages of natural habitat. 
Types of green roof
Green roofs can be categorised as "intensive", "semi-intensive" or "extensive", depending on the depth of planting medium and the amount of maintenance they need. Traditional roof gardens, which require a reasonable depth of soil to grow large plants or conventional lawns, are labour-intensive, requiring irrigation, feeding and other maintenance. "Extensive" green roofs, by contrast, are designed to be virtually self-sustaining and should only require a minimum of maintenance, perhaps a once-yearly weeding or an application of slow-release fertiliser to boost growth. They can be established on a very thin layer of "soil" (most use specially formulated composts): even a thin layer of rockwool laid directly onto a watertight roof can support a planting of Sedum species and mosses.
Another important distinction is between pitched green roofs and flat green roofs. Pitched green roofs are a traditional feature of many Scandinavian buildings and they tend to be of a simpler design than flat green roofs. This is because the pitch of the roof reduces the risk of water penetrating through the roof structure allowing fewer waterproofing and drainage layers to be used.
Industrial brownfield sites can be valuable ecosystems, supporting rare species of plants, animals and invertebrates. Increasingly in demand for redevelopment, these habitats are under threat. "Brown roofs" can partly mitigate this loss of habitat by coveri ng the flat roofs of new developments with a thin layer of crushed rubble and gravel, ideally obtained at minimal cost from the redevelopment site itself. They are intended to be gradually colonised by spiders and insects and provide a feeding site for insectivorous birds. Laban, a centre for contemporary dance in London, has a brown roof specifically designed to encourage the locally rare Black Redstart. (In 2003 Laban won the coveted RIBA Stirling Prize.) There are similar brown roofs on several nearby buildings in Deptford, including the Creekside Education Centre.
Examples of green roofs
One of the largest expanses of extensive green roof is to be found in the USA, at Ford Motor Company's River Rouge Plant, Dearborn, Michigan, where 42,000 square metres (454,000 ft²) of assembly plant roofs are covered with sedum and other plants. Other well-known American examples include Chicago’s City Hall and the Gap headquarters in San Bruno, Calif. The cities of Chicago, Atlanta and Portland, Ore., also boast numerous green roofs.
Switzerland has one of Europe's oldest green roofs, created in 1914 at the Moos lake water-treatment plant, Wallishofen, Zürich. Its filter-tanks have 30,000 square metres (320,000 ft²) of flat concrete roofs. To keep the interior cool and prevent bacterial growth in the filtration beds, a drainage layer of gravel and a 15 cm (6 in) layer of soil was spread over the roofs, which had been waterproofed with asphalt. A meadow developed from seeds already present in the soil; it is now a haven for many plant species, some of which are now otherwise extinct in the district, most notably 6,000 Orchis morio (green-winged orchid). More recent Swiss examples can be found at Klinikum 1 and Klinikum 2, the Cantonal Hospitals of Basel, and the Sihlpost platform at Zürich's main railway station.
What is believed to be the world's first green roof botanic garden was set up in Augustenborg, a suburb of Malmö, in May 1999. The International Green Roof Institute (IGRI) opened to the public in April 2001 as a research station and educational facility. (It has since been renamed the Scandinavian Green Roof Institute (SGRI), in view of the increasing number of similar organisations around the world.) Green roofs are well-established in Malmö: the Augustenborg housing development near the IGRI botanic garden incorporates green roofs and extensive imaginative landscaping of streams, ponds and soakaways between the buildings to deal with storm water run-off. The new Bo01 urban residential development (in the Västra Hamnen (Western Harbour) close to the foot of the iconic Turning Torso office and apartment block, designed by Santiago Calatrava) is built on the site of old shipyards and industrial areas, and incorporates many green roofs.
British examples can be found at the University of Nottingham Library, and in London at the Horniman Museum and Canary Wharf. The Ethelred Estate, close to the River Thames in central London, is the British capital's largest roof-greening project to date. Toxteth in Liverpool is also a candidate for a major roof-greening project.
Disadvantages of green roofs
Some disadvantages that have been found in the use of green roofs include the need to strengthen the structural support of some existing roofs being retrofit to accommodate a green roof, and the fact that green roof vegetation is often not designed to be walked on by humans. Since most green roof designs originate in Europe, it is also sometimes necessary to adapt those designs for the differing climatic conditions of other areas. 
Green roofs have more demanding structural standards. Many existing buildings cannot be retrofitted to have a green roof because of the weight load requirements for the soil and vegetation. A concrete roof deck makes a green roof much more feasible, as opposed to a metal or wood roof deck. Green roofs also have more exacting standards for the roof system to be installed beneath it, as finding and repairing a leak under 4-12 inches of soil and vegetation is an expensive endeavor.
Green roofs in Egypt
In Egypt, soil-less agriculture is used to grow plants on the roofs of buildings. No soil is put directly on the roof itself, thus eliminating the need for an insulating layer. The plants are grown on wooden tables. Vegetables and fruit are the most popular candidates, providing the family with a fresh, healthy source of food that is free from chemicals.
A more advanced method used at some places in Egypt is farming fish next to the plants in a closed cycle. This allows the plants to benefit from the ammonia excreted by the fish, helping the plants to grow better and at the same time eliminating the need for changing the water for the fish, because the plants help to keep it clean by absorbing the ammonia. The fish also get some nutrients from the roots of the plants as well as oxygen.
- ↑ Earth Pledge. Green Roofs : Ecological Design and Construction. Atglen, PA: Schiffer Pub., 2005.
- Scholz-Barth, Katrin. "Harvesting $ from Green Roofs: Green Roofs Present a Unique Business Opportunity with Tangible Benefits for Developers." Urban land 64.6 (2005): 83-7.
- IGRA - The International Green Roof Association, Global Networking for Green Roofs
- "Plant a Green Roof" (Overview article on benefits of green roofs)
- Green Roofs for Healthy Cities (GRHC), North America's Green Roof Industry Association
- SGRI (Scandinavian Green Roof Institute), Malmö, Sweden
- livingroofs, a UK resource for green roof information and research
- Center for Green Roof Research at Penn State University
- Michigan State University Green Roof Research
- "Sweet Sedum" Profiles of five green roofs, from Metropolis magazine
- A timeline of notable green roofs from Metropolis magazine
- A map of U.S. green roofs and facts about the technology from Metropolis magazine
- May your roof be green Article on green roofs in Egypt
- The Creekside Centre in Deptford (Greenwich) - London, UK has a green roof
- The Urban Ecology Centre - Montreal, Canada, has installed a model green roof and offers public workshops and visits
- The German Landscape Research, Development & Construction Society
- Green Roofs in the New York Metropolitan Region Research Report. Columbia University Center for Climate Systems Research and NASA Goddard Institute for Space Studies, 2006
- The Gaia Institute Non-profit engaged in Green Roof technology, primarily in New York City
- Urban Agriculture Notes (Canada)