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Green roof

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Re-creation of Viking houses in Newfoundland
Several grass roofs can be seen in the village of Bøur in the Faroe Islands.

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 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. 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.

Benefits of green roofs

Green roofs are used to:

A green roof is often a key component of an autonomous building.

According to Green Roofs for Healthy Cities founded by Steven W. Peck based in Toronto):

"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."

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.[1]

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. Today, it is estimated that about 10% of all German roofs have been “greened.” [2] Green roofs are also becoming increasingly popular in the United States, although 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.[1] 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. [3] 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 [4] 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 7 degrees Fahrenheit hotter than surrounding areas. [5] On Chicago's City Hall, by contrast, which features a green roof, 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. [6]

Green roofs are becoming common in Chicago, as well as Atlanta, Portland, and other 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 12 degrees Fahrenheit. [7]

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. [8] 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. [9]

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. [10]

Types of green roof

An intensive roof garden in Manhattan

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 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 green 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.

Brown roofs

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 covering 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

File:Aslagreenroof.jpg
Green Roof above the American Society of Landscape Architects' headquarters building, Washington, DC
Green roof planted with native species at L'Historial de la Vendée, a new museum in western France

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, 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.[11] The cities of Chicago, Atlanta and Portland, Ore. also boast numerous green roofs. [12]

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.

In France, a huge green roof of roughly 8,000 m² (70,000 ft²) has been incorporated into the new museum L'Historial de la Vendée which opened in June 2006 at Les Lucs-sur-Boulogne.

The new California Academy of Sciences building, currently under construction in San Francisco's Golden Gate Park, has a green roof that will provide 2.5 acres (1 hectare) 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 [13], the new building will consume 30-35% less energy than required by code.

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 when retrofitted to accommodate the weight of 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. [14]

Green roofs have more demanding structural standards. Many existing buildings cannot be retrofitted with a green roof because of the weight load of 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.

Non-native insects are being attracted into areas where non-native plants are used for Green Roofs in large numbers. For example, the lack of native plants in city centres has resulted in a decrease in dragon-fly population in city centres in Japan. Part of the problem has been a high-usage rate of lightweight Green Roof technologies that do not use plants that support the local insect population but can act as habitat for some unwanted species. The increase in unwanted insect species habitat has begun to result in a crowding of the native insects. To combat this crowding before it gets out of hand, measures are now being taken by the local governments in Japan (which have supported Green Roofs and Walls with extensive incentive programs for years) to encourage the installation of "Bio-Top's". Bio-Top's are Green Roofs that use native species and water basins specifically designed to support local bird and insect populations. These measures will not strictly require that all green roofs use native plants, as this is not logistically practical due to the small-size of the native plant industry world-wide, but it will require that more Bio-Top's are installed with each year's building permit applications to provide more breeding ground and habitat for native species in order to balance the crowding of native species to the area.

Costs of green roofs

A properly designed and installed Green Roof system can cost 15 to 35 dollars per square foot[citation needed]. The cost depends on what kind of roof it is, the structure of the building, and what plants can grow on the material that is on top of the roof. There are some Green Roof systems in the market that advertise at a cost of 8 to 15 dollars per square foot, however, these systems are usually not properly designed systems and use a cheap dimple board drainage material and use typical landscaping soils, two common and costly mistakes with Green Roofs[citation needed]. Insurance companies in British Columbia, Canada have recently lashed out against the installation of these "low-cost" systems due to several failures of waterproofing and plants with these types of systems and have decided not to insure any buildings in British Columbia that use these types of systems. The insurance companies have however, opened a panel discussion to investigate the insuring of buildings that use properly designed Green Roof systems with several years of experience and thousands of installations behind them, such as the Zinco, G-Sky and Optigruen systems. In the Spring 2007 issue of the Green Roof Infrastructure Monitor (Green Roofs for Healthy Cities web site) , Jörg Breuning reflects the wind and fire loads of green roofs and how German insurance companies handle extensive Green Roofs.

One of the hidden costs of a green roof is the cost of re-roofing when the roofing system beneath the overburden eventually breaks down. Repairs to green roofs can also be costly, even when due to manufacturing defect, as roof warranties related to green roofs generally exclude removal of the overburden to address defects.

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)[2]. 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 [3][4]. 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[5].

Green roofs in Egypt

In Egypt, soil-less agriculture is used to grow plants on the roofs of buildings. No soil is placed directly on the roof itself, thus eliminating the need for an insulating layer. Instead, plants are grown on wooden tables. Vegetables and fruit are the most popular candidates, providing 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 get some nutrients from the roots of the plants, as well as oxygen.

See also

References

  1. ^ Earth Pledge. Green Roofs : Ecological Design and Construction. Atglen, PA: Schiffer Pub., 2005.
  2. ^ Kolb, W. and T. Schwarz (2002). Gepflegtes grün auf dem dach Deutscher Gartenbau(7): 32-34.
  3. ^ Emilsson, T., Czemiel Berndtsson, J., Mattsson, J-E and Rolf, K., 2007 Effect of using conventional and controlled release fertiliser on nutrient runoff from various vegetated roof systems, Ecological Engineering, Volume 29, Issue 3, Pages 260-271, http://dx.doi.org/10.1016/j.ecoleng.2006.01.001
  4. ^ Czemiel Berndtsson, J., Emilsson, T. and Bengtsson, L., 2006 The influence of extensive vegetated roofs on runoff water quality, Science of The Total Environment, Volume 355, Issues 1-3, Pages 48-63, http://dx.doi.org/10.1016/j.scitotenv.2005.02.035
  5. ^ Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V., http://www.fll.de
  • 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.
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Columbia University Center for Climate Systems Research and NASA Goddard Institute for Space Studies, 2006

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