Atop skyscrapers lay plush green rooftops. | The Green Economy

Cool, Efficient, Beautiful: Green Roofs

beforeAtop the skyscrapers shooting into the sky lay plush green rooftops.

While these green rooftops have an ascetic appeal, they offer solutions to environmental and energy issues faced by cities around the world. Since reaching the U.S. market, ‘green’ has become a staple for many city rooftops.

Along with ascetic value — they are lovely to look at — such enhancements combat excess rainfall, reduce heating and cooling costs, and counter the effects of urban ‘heat islands’.

roof5Excess Water Runoff

Green roofs are able to combat excess rainwater runoff. Typically, rainwater that falls from traditional black city roofs mixes with pollutants when it reaches the street. This polluted water makes its way to the city’s water treatment facilities where it is cleaned and stored. Cleaning and storage requires substantial energy usage.

During periods of heavy rainfall, the facilities are easily susceptible to backup. When that happens, the polluted water enters the city’s waterways without going to the cleaning facilities. According to a New York Times article:

“It can take only 20 minutes of rainfall to start water from toilets flowing into Brooklyn’s waterways.”

The storage potential of green roofs could help decrease the flooding of the city’s transit system during large storms. “A four-inch-thick green roof can hold a gallon of water per square foot. If all the rooftops in the City [New York City] were green, they would represent a billion gallons of storage potential.” (Scientific American)

Reduce Heating and Cooling

According to a 2011 study conducted by Columbia University on green roofs, an un-shaded green roof of 1,000 square meters could yield annual heat energy savings of $330 to $350. And an additional $225 can be saved in cooling.

roof1Combat urban ‘Heat Islands’

Green roofs have been proven to help counter the effects of urban heat island. Solar energy absorbed by city streets, buildings and traditional black tar roofs is re-radiated into excess heat-making city temperatures often hotter than temperatures in the surrounding areas.

According to a 2006 report from the New York City Regional Heat Island Initiative, urban heat island “occurrence during the summer months is of particular public policy concern because of the association of higher temperatures with increases in electric demand due to air conditions, elevated air pollution and heat-stress related mortality and illness.”

roof2The report concluded that the most effective solution to combat this issue is planting trees on city streets. However, such space in city environments is limited, restricting the implementation of this solution. Following “street trees,” the study found green roofs to be the second best solution, eliminating the restrictions of available space. The report found that living roofs cool surrounding air temperatures by 16.4 degrees, slightly less than the outcome of “street trees.” Painting roofs white and open space planting followed.

The best solution would be an amalgam of all the available possibilities. Green roofs, street trees, white roofs and open space plantings would not only help with temperature reduction, but increase the beauty of our city environments.

Exploded

Green roofs – also known as living roofs – are traditionally made of insulation, a waterproof membrane, a drainage layer such as gravel or clay, a mat to prevent erosion, soil and plants. The costs associated with green roofs, however, and the problems that arise from difficult accessibility have elicited criticism. According to Scientific American, the installation of a green roof can range in price from $10 to $35 per square foot, depending on the roof size.

By Kelly Velocci, Special to THE GREEN ECONOMY

This appeared in the 2012 Cities and Buildings magazine.
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