Un tetto verde o tetto vivente è un tetto di un edificio che è parzialmente o completamente coperto di vegetazione e terreno di coltura, piantati su una membrana impermeabilizzante . Essa può anche includere strati aggiuntivi come ad esempio una barriera della radice e di drenaggio e di irrigazione sistemi. La piantumazione sui tetti, dove le piante sono mantenute in vaso, non sono generalmente considerati come veri e propri tetti verdi. Tetti con stagni sono un’altra forma di tetti verdi che vengono utilizzati per il trattamento di acque grigie .
È una finitura tecnologica della copertura che fornisce diversi benefici all’edificio come la protezione dell’impermeabilizzazione, la regolazione del microclima grazie all’abbassamento della temperatura dell’aria in ambiente urbano e la lotta contro l’effetto isola di calore, l’isolamento termico e quindi risparmio energetico, la riduzione della presenza di polveri sottili, la creazione di nuovi habitat per la fauna selvatica, la regimazione delle acque piovane, oltre al minore impatto ambientale ed estetico.
Esistono due tipi di tetti verdi, i tetti intensivi, che sono più spessi, con una profondità minima di 12,8 cm e può supportare una più ampia varietà di piante, ma sono più pesanti e richiedono più manutenzione, ed una più combinazione leggera, che sono poco profondi, che vanno in profondità da 2 cm a 12,7 cm, più leggero dei tetti verdi intensivi e richiedono una manutenzione minima.
Il termine tetto verde può essere utilizzato anche per indicare tetti che utilizzano una qualche forma di tecnologia verde, come per esempio un tetto freddo , un tetto con collettori solari o pannelli fotovoltaici . I tetti verdi sono anche indicati come eco-tetti , oikosteges , tetti verdi , tetti viventi , greenroofs e VCW H.
È generalmente composto da un “pacchetto” di più strati che comprende:
– Membrana (o manto) impermeabile antiradice
– Strato di separazione e protezione del manto impermeabile
– Strato di drenaggio e accumulo idrico
– Tessuto di filtro
– Substrato colturale
Le caratteristiche più importanti sono la qualità del substrato, la quantità di accumulo d’acqua, la superficie di appoggio dell’elemento di accumulo e la apertura a pori del tessuto di filtro. È solitamente un sistema che presenta spessore e peso ridotti per permettere di essere utilizzato sulle coperture e richiede scarsa manutenzione, poiché viene utilizzata una vegetazione composta di essenze di sedum che devono essere in grado di sopravvivere in situazioni di estrema siccità, con alte capacità di rigenerazione e auto propagazione.
A modern green roof (California Academy of Sciences). Constructed for low maintenance by intentionally neglecting many native plant species, with only the hardiest surviving varieties selected for installation on the roof.
Green roofs are used to:
Reduce heating (by adding mass and thermal resistance value)
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.
Reduce cooling (by evaporative cooling) loads on a building by fifty to ninety percent, especially if it is glassed in so as to act as a terrarium and passive solar heat reservoir – a concentration of green roofs in an urban area can even reduce the city’s average temperatures during the summer
Reduce stormwater run off— see water-wise gardening
Natural Habitat Creation— see urban wilderness
Filter pollutants and carbon dioxide out of the air which helps lower disease rates such as asthma— see living wall
Filter pollutants and heavy metals out of rainwater
Help to insulate a building for sound; the soil helps to block lower frequencies and the plants block higher frequencies
If installed correctly many living roofs can contribute to LEED points
Increase agricultural space
With green roofs, water is stored by the substrate and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation.
Green roofs not only retain rainwater, but also moderate the temperature of the water and act as natural filters for any of the water that happens to run off.
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.
Often, phosphorus and nitrogen are in this category of environmentally harmful substances even though they are stimulating to the growth of plant life and agriculture. When these substances are added to a system, it can create mass biological activity since they are considered limiting factors of plant growth and by adding more of them to a system, it allows for more plant growth.
Elevation 314, a new development in Washington, D.C. uses green roofs to filter and store some of its storm water on site, avoiding the need for expensive underground sand filters to meet D.C. Department of Health storm-water 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 1.4–4.4 degrees Celsius (2.5–8.0 °F) cooler than they are on traditionally roofed buildings nearby. Green roofs are becoming common in Chicago, as well as in Atlanta, Portland, and other United States cities, where their use is encouraged by regulations to combat the urban heat-island effect. Green roofs are a type of low impact development. 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 also provide habitats 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.
Costs and Financial benefits
Roof gardens provide residents of an apartment complex in Tongyang Town, Tongshan County, Hubei with fresh produce
A properly designed and installed extensive green-roof system can cost $108–248/m2 ($10–23/ft2) while an intensive green roof costs $355–2368/m2 ($33–220/ft2) however, since most of the materials used to build the green roof can be salvaged it is estimated that the cost of replacing a green roof is generally one third of the initial installation costs.
With the initial cost of installing a green roof in mind, there are many financial benefits that accompany green roofing.
Green roofing can extend the lifespan of a roof by over 200% by covering the waterproofing membrane with growing medium and vegetation, this shields the membrane from ultra-violet radiation and physical damage. Further, Penn State University’s Green Roof Research Center expects the lifespan of a roof to increase by as much as three times after greening the roof.
It is estimated that the installation of a green roof could increase the real estate value of an average house by about 7%.
Reduction in energy use is an important property of green roofing. By improving the thermal performance of a roof, green roofing allows buildings to better retain their heat during the cooler winter months while reflecting and absorbing solar radiation during the hotter summer months, allowing buildings to remain cooler. 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. With respect to hotter summer weather, green roofing is able to reduce the solar heating of a building by reflecting 27% of solar radiation, absorbing 60% by the vegetation through photosynthesis and evapotranspiration, and absorbing the remaining 13% into the growing medium. Such mitigation of solar radiation has been found to reduce building temperatures by up to 20 degrees celsius and reduce energy needs for air-conditioning by 25% to 80%. This reduction in energy required to cool a building in the summer is accompanied by a reduction in energy required to heat a building in the winter, thus reducing the energy requirements of the building year-round which allows the building temperature to be controlled at a lower cost.
Depending on the region in which a green roof is installed, incentives may be available in the form of stormwater tax reduction, grants, or rebates. The regions where these incentives will most likely be found are areas where failing storm water management infrastructure is in place, urban heat island effect has significantly increased the local air temperature, or areas where environmental contaminants in the storm water runoff is of great concern.
An example of such an incentive is a one-year property tax credit is available in New York City, since 2009, for property owners who green at least 50% of their roof area.
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The main disadvantage of green roofs is that the initial cost of installing a green roof can be double that of a normal roof.
The additional mass of the soil substrate and retained water places a large strain on the structural support of a build. This makes it unlikely for intensive green roofs to become widely implemented due to a lack of buildings that are able to support such a large amount of added weight as well as the added cost of reinforcing buildings to be able to support such weight.
Some types of green roofs do have more demanding structural standards especially in seismic regions of the world. Some existing buildings cannot be retrofitted with certain kinds of green roof because of the weight load of the substrate and vegetation exceeds permitted static loading. Depending on what kind of green roof it is, the maintenance costs could be higher, but some types of green roof have little or no ongoing cost. Some kinds of green roofs also place higher demands on the waterproofing system of the structure, both because water is retained on the roof and due to the possibility of roots penetrating the waterproof membrane. Another detractor is that the wildlife they attract may include pest insects which could easily infiltrate a residential building through open windows.
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. Extensive green roofs traditionally support 10-25 pounds of vegetation per square foot (50–120 kg/m2) while intensive roofs support 80-150 pounds of vegetation per square foot (390–730 kg/m2). 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. Intensive roofs are more park-like with easy access and may include anything from kitchen herbs to shrubs and small trees.
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. Extensive roofs are usually only accessed for maintenance. 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. Some green roof designs incorporate both intensive and extensive elements. To protect the roof, a waterproofing membrane is often used, which is manufactured to remain watertight in extreme conditions including constant dampness, ponding water, high and low alkaline conditions and exposure to plant roots, fungi and bacterial organisms.
Advances in green roof technology have led to the development of new systems that do not fit into the traditional classification of green roof types. Comprehensive green roofs bring the most advantageous qualities of extensive and intensive green roofs together. Comprehensive green roofs support plant varieties typically seen in intensive green roofs at the depth and weight of an extensive green roof system.
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.
Re-creation of Viking houses in Newfoundland
Sod roofs on 18th century farm buildings in Heidal, Norway.
On the green roof of the Mountain Equipment Co-op store in Toronto, Canada.
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 has 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, Norway, Italy, Austria, Hungary, Sweden, the UK, and Greece.
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 uptake has been slow, but a number of cities have developed policies to encourage their use, notably London and Sheffield.
Rooftop water purification is also being implemented in green roofs. These forms of green roofs are actually treatment ponds built into 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 and Waterzuiveren.be Plants used include calamus, Menyanthes trifoliata, Mentha aquatica, etc.)
Several studies have been carried out in Germany since the 1970s. Berlin is one of the most important centers of green roof research in Germany. Particularly in the last 10 years, much more research has begun. About ten green roof research centers exist in the US and activities exist in about 40 countries. In a recent study on the impacts of green infrastructure, in particular green roofs in the Greater Manchester area, researchers found that adding green roofs can 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 centers.”
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”, also known as “biodiverse roofs”, can partly mitigate this loss of habitat by covering the flat roofs of new developments with a layer of locally sourced material. Construction techniques for brown roofs are typically similar to those used to create flat green roofs, the main difference being the choice of growing medium (usually locally sourced rubble, gravel, soil, etc…) to meet a specific biodiversity objective.
In Switzerland, it is common to use alluvial gravels from the foundations; in London, a mix of brick rubble and some concrete has been used. The original idea was to allow the roofs to self-colonise with plants, but they are sometimes seeded to increase their biodiversity potential in the short term. Such practices are derided by purists.
The roofs are colonised by spiders and insects (many of which are becoming extremely rare in the UK as such sites are developed) 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 nationally rare black redstart.
A green roof, 160m above ground level, and claimed to be the highest in the UK and Europe “and probably in the world” to act as nature reserve, is on the Barclays Bank HQ in Canary Wharf.
Designed combining the principles of green and brown roofs, it is already home to a range of rare invertebrates.
Examples by country
Green roofs have been increasing in popularity in Australia over the past 10 years. Some of the early examples include the Freshwater Place residential tower in Melbourne (2002) with its Level 10 rooftop Half Acre Garden, CH2 building housing the Melbourne City Council (2006) – Australia’s first 6-star Green Star Design commercial office building as certified by the Green Building Council of Australia, and Condor Tower (2005) with a 75-square-metre lawn on the 4th floor.
In 2010, the largest Australian green roof project was announced. The Victorian Desalination Project  will have a “living tapestry” of 98,000 Australian indigenous plants over a roof area spanning more than 26,000 square metres. The roof will form part of the desalination plant’s sophisticated roof system, designed to blend the building into the landscape, and provide acoustic protection, corrosion resistance, thermal control, and reduced maintenance. The green roof was designed by ASPECT Studios, ARM / pecvkvonhartel architecture, and will be installed by Fytogreen Australia 
Since 2008, City Councils and influential business groups in Australia have become active promoting the benefits of green roofs. “The Blueprint to Green Roof Melbourne” is one program being run by the Committee for Melbourne.
The green roof on top of the Canadian War Museum in Ottawa looks like a wheatfield, with the towers of Canada’s Parliament visible in the distance
The city of Toronto approved a by-law in May 2009, mandating green roofs on residential and industrial buildings. There is criticism from Green Roofs for Healthy Cities that the new laws are not stringent enough, since they will only apply to residential building that are a minimum of six storeys high. By 31 January 2011, industrial buildings were required to render 10% or 2,000m² of their roofs green. Toronto City Hall’s Podium roof was renovated to include a 32,000 square foot rooftop garden, the largest publicly accessible roof in the city. The green roof was opened to the public in June 2010.
In 2008, the Vancouver Convention Centre installed a six-acre living roof of indigenous plants and grasses on its West building, making it the largest green roof in Canada. The new Canadian War Museum in Ottawa, opened in 2005, also features a grass-covered roof.
During the renovation of the Hamilton City Hall in Hamilton, Ontario that spanned from 2007 to 2010, many efforts were taken to enhance the environmentally-friendly nature of the structure, which included the addition of a grass covered roof.
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 pesticides.
A more advanced method, (aquaponics), being used experimentally in Egypt, is farming fish next to 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.
Green roof planted with native species at L’Historial de la Vendée, a new museum in western France
In France, an 8,000 square metres (86,000 sq ft) extensive, cable-supported green roof has been created on the International School in Lyon. Another huge green roof of roughly 8,000 square metres (86,000 sq ft) has been incorporated into the new museum L’Historial de la Vendée which opened in June 2006 at Les Lucs-sur-Boulogne.
Long-held green roof traditions started in the early industrialization period more 100 years ago exist in Germany. In the 70s, green roof technology was elevated to the next level. Serious storm-water issues made cities think about innovative solutions, preferably with living plants. Modern green roof technology with high performance, lightweight materials were utilized to grow hardy vegetation even on roofs that can hardly support any additional load. In the 80s modern Green Roof Technology was common knowledge in Germany while it was practically unknown in any other country in the world. In Stuttgart, with one of the most innovative Department of Parks and Recreation and with the worlds oldest horticultural Universities, modern green roof technology was perfected and implemented on a large scale.
With the first green roof industry boom in Germany there were quality issues recorded. The FLL formed a committee that is focused on modern green roof technology. FLL stands for Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (FLL)or in English: The German Landscape Research, Development and Construction Society. The FLL is an independent non-profit organization. It was founded in 1975 by eight professional organizations for “the improvement of environmental conditions through the advancement and dissemination of plant research and its planned applications”. The FLL green roof working group is only one of 40 committees which have published a long list of guidelines and labor instructions. Some of these guidelines also available in English including the German FLL-Guideline for the Planning, Execution and Upkeep of Green-Roof Sites. The results of the research and synthesis done by FLL members are constantly updated and promulgated utilizing the same principles which govern the compilation of DIN standards and are published as either guiding principles or labor instructions.
The current Green Roof Guideline was published in 2008. There is also an introduction to FLL to download at a FLL member and promoter. Today most elements of the German FLL are part of standards and guidelines around the world (FM Global, ASTM, NRCA, SPRI etc..
Fachvereinigung Bauwerksbegrünung (FBB) was founded in 1990 as the first green roof association in the world. FBB was founded as an open forum for manufacturers and planners, merchants and operators in 1990. The organization was born from the then-visionary idea of understanding the relationship between nature and constructions not as oppositional, but as an opportunity. Both the green roofing and conventional roofing industries are equally represented. The FBB has developed to become an innovative lobbying group with a strong market presence, internationally known through its cooperation with other European associations. Today, approximately 100 member companies use the multifaceted services offered by FBB, which offers a greater degree of market expertise and competitiveness. “Kompetenz im Markt”.
Today, about 10,000,000m² (or 100,000,000 square feet) of new green roofs are being constructed each year. According latest studies about 3/4 of these are extensive; the last 1/4 are roof gardens. The cities with the most green roofs in Germany are Berlin and Stuttgart. Surveys about the status of regulation are done by the FBB. Nearly one third of all German cities have regulations to support green-roof and rain-water technology. Green-roof research institutions are located in several cities as including Hannover, Berlin, Geisenheim and Neubrandenburg.
Germany is the country with the most green roofs in the world and it is the country with the most advanced knowledge in modern green roof technology. Green Roofs in Germany are part of the 2 –3 years apprentice educations system of landscaping professionals. Since green roof are common knowledge and common sense it is interesting to take a look at the German Wikipedia page for green roofs – there is no need to describe projects, case studies or related research. The green roof technology was implemented before the internet age.
The oikostegi, a green roof on the Treasury building in Athens
The Greek Ministry of Finance has now installed a green roof on the Treasury in Constitution Square in Athens. The so-called “oikostegi” (Greek – oiko, pronounced eeko, meaning building-ecological, and stegi, pronounced staygee, meaning roof-abode-shelter) was inaugurated in September, 2008. Studies of the thermodynamics of the roof in September 2008 concluded that the thermal performance of the building was significantly affected by the installation. In further studies, in August 2009, energy savings of 50% were observed for air conditioning in the floor directly below the installation. The ten-floor building has a total floor space of 1.4 hectares. The oikostegi covers 650m², equalling 52% of the roof space and 8% of the total floor space. Despite this, energy savings totalling €5,630 per annum were recorded, which translates to a 9% saving in air conditioning and a 4% saving in heating bills for the whole building. An additional observation and conclusion of the study was that the thermodynamic performance of the oikostegi had improved as biomass was added over the 12 months between the first and second study. This suggests that further improvements will be observed as the biomass increases still further. The study also stated that while measurements were being made by thermal cameras, a plethora of beneficial insects were observed on the roof, such as butterflies, honey bees and ladybirds. Obviously this was not the case before installation. Finally, the study suggested that both the micro-climate and biodiversity of Constitution Square, in Athens, Greece had been improved by the oikostegi.
Sod roof Church at Hof, Iceland
Sod roofs are frequently found on traditional farmhouses and farm buildings in Iceland.
Switzerland has one of Europe’s oldest green roofs, created in 1914 at the Moos lake water-treatment plant, Wollishofen, Zürich. Its filter tanks have 30,000 square metres (320,000 sq ft) 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 claimed to be the world’s first green roof botanical garden was set up in Augustenborg, 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 SGRI botanical garden incorporates green roofs and extensive landscaping of streams, ponds, and soak-ways 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 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.
In 2012, the shopping mall Emporia with its 27,000 square metre roof garden, was opened. The size of the roof garden is approximately equivalent to 4 soccer fields, which makes it one of the biggest green roof parks in Europe that is accessible to the public.
In 2003 English Nature concluded that ‘in the UK policy makers have largely ignored green roofs’. However, British examples can be found with increasing frequency. A notable early roof garden was built above the Derry & Toms Department Store in Kensington, London in 1938. More recent examples can be found at the University of Nottingham Jubilee Campus, and in London at Sainsbury’s Millennium Store in Greenwich, the Horniman Museum and at 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 the United Kingdom, intensive green roofs are sometimes used in built-up city areas where residents and workers often do not have access to gardens or local parks. Extensive green roofs are sometimes used to blend buildings into rural surroundings, for example by Rolls-Royce Motor Cars, who has one of the biggest green roofs in Europe (covering more than 32,000m² on their factory at Goodwood, West Sussex.
The University of Sheffield has created a Green Roof Centre of Excellence and conducted research, particularly in a UK context, into green roofs. Dr Nigel Dunnett of Sheffield University published a UK-centric book about green roofing in 2004 (updated 2008).
Fort Dunlop has the largest green roof in the UK since its redevelopment between 2004 and 2006.
The undulating green roof of the California Academy of Sciences, under construction in San Francisco in 2007.
One of the largest expanses of extensive green roof is to be found in the US, at Ford Motor Company’s River Rouge Plant, Dearborn, Michigan, where 42,000 square metres (450,000 sq ft) of assembly plant roofs are covered with sedum and other plants, designed by William McDonough; the $18 million assembly avoids the need of what otherwise would be $50 million worth of mechanical treatment facilities on site. Built over Millennium Park Garage, Chicago’s 24.5-acre (99,000 m2) Millennium Park is considered one of the largest intensive green roofs. Other well-known American examples include Chicago’s City Hall and the former Gap headquarters, now the headquarters of YouTube, 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 used on the roof include Achillea tomentosa (woolly yarrow), Armeria maritima (sea pink, sea thrift), Carex inops pensylvanica (long-stoloned sedge), Eriophyllum lanatum (Oregon sunshine), Festuca rubra (red creeping fescue), Festuca idahoensis (Idaho fescue), Phlox subulata (creeping phlox), Saxifraga caespitosa (tufted saxifrage), Sedum oreganum (Oregon stonecrop), Sedum album (white stonecrop), Sedum spurium (two-row stonecrop), Sisyrinchium idahoense (blue-eyed grass), Thymus serpyllum (wild thyme), Triteleia hyacinthina (fool’s onion).
The new California Academy of Sciences building in San Francisco’s Golden Gate Park has a green roof that provides 2.5 acres (10,000 m2) 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 building consumes 30–35% less energy than required by code.
An early green-roofed building (completed in 1971) is the 358,000 sq ft (33,300 m2) Weyerhaeuser Corporate Headquarters building in Federal Way, Washington. Its 5-story office roof system comprises a series of stepped terraces covered in greenery. From the air, the building blends into the landscape.
The largest green roof in New York City was installed in midtown Manhattan atop the United States Postal Service’s Morgan Processing and Distribution Center. Construction on the 109,000 sq ft (10,100 m2) project began in September 2008, and was finished and dedicated in July 2009. Covered in native vegetation and having an expected lifetime of fifty years, this green roof will not only save the USPS approximately $30,000 a year in heating and cooling costs, but will also significantly reduce the amount of storm water contaminants entering the municipal water system.
The 14,000 square feet of outdoor space on the seventh floor of Zeckendorf Towers, formerly an undistinguished rooftop filled with potted plants, make up the largest residential green roof in New York. The roof was transformed in 2010 as part of Mayor Michael Bloomberg’s NYC Green Infrastructure campaign, and supposedly serves to capture some of the rain that falls on it rather than letting it run off and contribute to flooding in the adjacent Union Square subway station.
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 6 seconds/m²/year. Maintenance of green roofs often includes fertilization to increase flowering and succulent plant cover. If aesthetics are not an issue, fertilization and maintenance are generally not needed. Extensive green roofs should only be fertilized with controlled-release fertilizers in order to avoid pollution of the storm water. Conventional fertilizers 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 many available nutrients. The FLL guidelines specify maximum-allowable nutrient content of substrates.