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. This does not refer to roofs which are merely colored green, as with green roof 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 generally considered to be true green roofs, although this is an area of debate. Rooftop ponds are another form of green roofs which are used to treat greywater. Finally, the term "green roof" may also be used to indicate roofs that utilize some form of "green" technology, such as solar panels or a photovoltaic module. Green roofs are also referred to as eco-roofs, vegetated roofs, living roofs, and greenroofs.
A green roof is often a key component of an autonomous building.
A 2005 study by Brad Bass of the University of Toronto showed that green roofs can also reduce heat loss and energy consumption in winter conditions.
In a recent study on the impacts of green infrastructure and in particular green roofs in the Greater Manchester area, researchers found that adding green roofs will help keep temperatures down, particularly in urban areas: “adding green roofs to all buildings can have a dramatic effect on maximum surface temperatures, keeping temperatures below the 1961-1990 current form case for all time periods and emissions scenarios. Roof greening makes the biggest difference…where the building proportion is high and the evaporative fraction is low. Thus, the largest difference was made in the town centres.”
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. However, green roofs or sod roofs in Northern Scandinavia have been around for centuries. The modern "trend" started when green roofs were developed in Germany in the 1960s, and have since spread to many countries. Today, it is estimated that about 10% of all German roofs have been “greened.” Green roofs are also becoming increasingly popular in the United States, although they are not as common as in Europe.
A number of European Countries have very active associations promoting green roofs including Germany, Switzerland, the Netherlands, Italy, Austria, Hungary, Sweden and the UK . Outside of Germany the City of Linz in Austria has been paying developers to install green roofs since 1983 and in Switzerland it has been a Federal law since the late 1990s. In the UK their up-take has been slow but a number of cities have developed policies to encourage their use, notably in London and Sheffield.
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 the rate of runoff from the roof. It has been found that they can retain up to 75% 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.
Combating the urban heat island effect is another reason for creating a green roof. Traditional building materials soak up the sun's radiation and re-emit it as heat, making cities at least 4 degrees Celsius (7 °F) hotter than surrounding areas. On Chicago's City Hall, by contrast, which features a green roof, roof temperatures on a hot day are typically 14–44 degrees Celsius (25–80°F) cooler than they are on traditionally roofed buildings nearby.
Green roofs are becoming common in Chicago, as well as Atlanta, Portland, and other United States cities, where their use is encouraged by regulations to combat the urban heat island effect. 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 this 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 7 degrees Celsius.
Green roofs have also been found to dramatically improve a roof’s insulation value. A study conducted by Environment Canada found a 26% reduction in summer cooling needs and a 26% 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. Rooftop water purification is also being implemented in green roofs. These forms of green roofs are actually treatment ponds built unto the rooftops. They are built either from a simple substrate (as being done in Dongtan) or with plant-based ponds (as being done by WaterWorks UK Grow System Waterzuiveren.be Plants used include calamus, Menyanthes trifoliata, Mentha aquatica, etc.)
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, 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.
Green roofs can be categorized 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 considered "intensive" because they are labour-intensive, requiring irrigation, feeding and other maintenance. "Extensive" green roofs, by contrast, are designed to be virtually self-sustaining and should require only 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 sod roofs, a traditional feature of many Scandinavian buildings, 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 the use of fewer waterproofing and drainage layers.
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 of assembly plant roofs are covered with sedum and other plants, designed by William McDonough. Other well-known American examples include Chicago’s City Hall and the Gap headquarters in San Bruno, CA. Recently, the American Society of Landscape Architects retrofitted their existing headquarters building in Washington, D.C. with a green roof designed by landscape architect Michael Van Valkenburgh.
Another example of a green roof in the United States is the Ballard Library in Seattle. The landscape architect was Swift & Co. and the building architect was Bohlin Cywinski Jackson. This green roof has over 18,000 plants to help with insulation and reduce runoff. The plants which are found on the roof include Achillea tomentosa (Woolly yarrow), Armeria maritima (Sea pink, sea thrift), Carex inops (pensylvanica) (Long-stoloned sedge), Eriphyllum lanatum (Oregon sunshine), Festuca rubra (Red creeping fescue), Festuca idahoensis (Idaho fescue), Phlox subulata (Creeping phlox), Saxifrage cespitosa (Tufted saxifrage), Sedum oreganum (Oregon stonecrop), Sedum album (White stonecrop), Sedum spurium (Two-row stonecrop), Sisyrinchium idahoensis (Blue-eyed grass), Thymus serphyllum (Thyme), Triteleia hyacintha (Fool's onion). Also on the roof there are 17 solar panels as well as solar film in the windows to help generate electricity for the building. Other things that the library has which are environmentally friendly and can save energy include occupancy sensors that turn off lights when people are not in the room, recycled carpet and windows, waterless urinals in bathrooms, and waste reducing furniture.
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 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.
The new California Academy of Sciences building, currently under construction in San Francisco's Golden Gate Park, has a green roof that will provide of native vegetation designed as a habitat for indigenous species, including the threatened Bay checkerspot butterfly. According to the Academy's fact sheet on the building, the new building will consume 30-35% less energy than required by code.
The roof to Banco di Santander's headquarters in Madrid, Spain is currently home to Europe's biggest green roof at just ove 100,000sqm in size. The roof was made using a mix of both extensive and intensive planting systems.
Also, some green roofs are found on Icelandic Farms and buildings. Iceland is plentiful of these grass roofs.
Some cost can also be attributed to maintenance. Extensive green roofs have low maintenance requirements but they are generally not maintenance free. German research has quantified the need to remove unwanted seedlings to approximately 0,1 min/(m²*year). Maintenance of green roofs often includes fertilisation to increase flowering and succulent plant cover. If aesthetics is not an issue, fertilisation and maintenance is generally not needed. Extensive green roofs should only be fertilised with controlled release fertilisers in order to avoid pollution of the stormwater. Conventional fertilisers should never be used on extensive vegetated roofs. German studies have approximated the nutrient requirement of vegetated roofs to 5gN/m². It is also important to use a substrate that does not contain too much available nutrients. The FLL-guidelines specify maximum allowable nutrient content of substrates.
A more advanced method (aquaponics) 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 get some nutrients from the roots of the plants.