Most roofs in a city are covered in asphalt or black tar, reflecting heat and adding little to increase the city’s inhabitants’ quality of life. Green roofs, also known as living roofs take all that otherwise underutilized space and turn it into something useful.

A green roof is the roof of a building that is partially or completely covered with vegetation, a growing medium (soil), and a waterproofing membrane. Green roofs can be classified as extensive, semi-intensive, and intensive. Extensive green roofs provide an ecological protection layer, and are not meant to serve as parks or gardens. Extensive green roofs are best suited to structures with lower load bearing capacity, and are the most low-maintenance type of green roof. Intensive green roofs provide some of the ecological benefits, but also acts as gardens which are meant to be enjoyed. Intensive roofs require significantly more upkeep than extensive roofs. A semi-extensive green roof fall between extensive and intensive roofs interns of the requirements for upkeep and intents for use.

Stormwater retention is a major benefit of green roofs. In a typical urban environment, rainwater flows over paved surfaces into the sewer system and out into nearby bodies of water. Stormwater flowing through an urban environment picks up urban pollutants, damaging water quality. Excessive stormwater runoff can also lead to riverbank erosion and flooding. In cities with sewer systems built before 1930, storm drains are typically combined with the sanitary sewers that take wastewater to water treatment plants. During storms, this can lead to combined sewer overflow, during which untreated wastewater gets dumped into rivers and lakes. Green roofs can reduce peak flow of stormwater runoff by 65% and increases the time for water to flow from the roof to a sewer by three hours.

Green roofs are one of a few strategies cities can take to combat heat island effect. The term “heat island” describes a developed, urban area that is hotter than surrounding rural areas. The annual mean temperature of a city with population of a million or more people can be 1.8—5.4°F , (1—3°C) warmer than in surrounding areas. Growing a layer of vegetation on a roof lowers the temperature of the roof’s surface and the surrounding air.

Green roofs also increase the aesthetic value of roofs. An intensive green roof can provide an attractive space for building occupants, as well as a pleasant view for those in nearby buildings. A green roof can also reduce stress and increase worker productivity.

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