Best Practices In Sustainability: Green Roofs

Nov. 9, 2005

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Rooftops in cities across America are seeing green - and in more ways than one.

Case in point: Chicago.

As a city that sees more than its share of climatic extremes - hot, humid days in the summer and frigid, snowy periods in the winter - the idea of saving money on heating and cooling bills is an idea well received by many in this urban center of nearly 3 million people. It all started at City Hall, where the area’s first green roofing project was completed in 2001.

According to the City of Chicago’s website, that rooftop garden consists of 20,000 plants, shrubs, and trees of more than 100 species, the majority of which are designed to thrive in the often harsh conditions present on the building’s roof - extreme temperatures, direct sunlight, and gusty winds. Results have been impressive: When Chicago city officials compared heat readings between the Cook County Building’s tar roof and City Hall’s planted roof on Aug. 9, 2001, the temperature difference between the two was more than 50 degrees F.

Besides greatly reducing the building’s energy costs, Chicago’s first green roofing initiative has helped mitigate the urban heat island effect - created in the summer by black roofs and pavements that absorb sunlight at a higher rate - and created cleaner air in the midst of a city where vehicles and machinery pollute. When it rains 1 inch, the garden is able to retain and use 75 percent of that water before run-off begins entering the sewer system, according to statistics provided by the city. This reduces strain on Chicago’s wastewater infrastructure after major storms.

Even more remarkable about a project like the one atop Chicago City Hall is that a landscaped roof can last more than twice as long as a conventional roof, which may be why the idea is gaining momentum quickly in Chicago and cities across the nation. In fact, Chicago alone had more than 80 public and private green roof projects under way as of June 2004, some of which have been installed at high-profile facilities like Soldier Field and the recently completed Millennium Park.

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