Green roofs can provide a wide range of benefits to urban communities interested in enhancement and protection of their environment. Some of the most important merits of green roofs are reduction in energy use, decrease in air pollution, improved water quality, enhanced stormwater management and decreasing heat island effect.
According to a USEPA study, green roofs are capable of removing 50% of the annual rainfall volume from a roof through retention and evapo-transpiration. By reducing the amount of impervious surfaces within a developed zone, green roofs reduce the amount of stormwater runoff. Also, because green roofs absorb water, they delay the time at which runoff occurs, resulting in decreased stress on sewer systems at peak flow periods.
For conventional non-living roofs with a slope of 2%, a 96% runoff rate is observed. On the other hand, intensive green roofs may have as low as a 15% runoff rate. The benefits green roofs have regarding stormwater runoff could be amplified by more green roofs in a close-knit area and using green roofs with a deeper substrate layer.
Urban Heat Island Effect
Because urban areas are densely populated with buildings, they tend to be hotter than the surrounding areas, a phenomenon known as urban heat island effect (or UHI effect). Urban heat islands have many negative impacts such as an in increase energy demand for cooling, an increase in air pollutants and greenhouse gas emissions, and impaired water quality.
The vegetative surfaces of green roofs utilize a relatively large proportion of the absorbed radiation in the evapo-transpiration process and then release water vapor into the air which helps to cool air temperatures. Additionally, the shade provided by trees and other shrubbery greatly helps to reduce the rooftop temperatures and the overall heat island effect.
Rooftop vegetation moderates the factors that accelerate a rooftops breakdown such as extreme temperatures, UV radiation, and cold winds, thus dramatically expanding the life of a roof. According to a study in Germany, a vegetated roof on average can be expected to prolong the service life of a conventional roof by at least 20 years. The result of this is not only cost savings to the building’s owner but also a reduction of landfill wastes.
Green roofs can provide habitat (food, shelter, water and breeding grounds) for many different species. Because of their high density, cities severely restrict green space and threaten or destroy habitats so the creation of such green space assumes particular importance in these areas.
Various recent studies in Europe have indicated that green roofs in large cities have high potential as habitat for species negatively impacted by land-use changes. For example, in Basel, Switzerland, surveys of birds, spiders and beetles on green roofs found high diversity levels for all groups, including many species considered rare or threatened.
Green roofs have the ability to significantly improve the beauty of buildings, the visual and environmental diversity which can have positive impacts psychological well-being. Studies across several countries have all shown the correlation between daily contact with nature and human well-being.
In fact, the results of a large survey in the Netherlands showed that the amount of green space in the built environment was positively related to the health condition people said they experienced in their daily life.
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