In April 2000 Mayor Richard M. Daley stood at the very (VERY!) top of City Hall, perhaps feeling a bit of vertigo but mostly overjoyed at the sense of accomplishment. He looked about him and gazed upon the dark roofs of the surrounding skyscrapers. He closed his eyes for a moment, imagining instead a sea of green luscious vegetation. Then, he dazzled the crowd with his bright politician’s smile, de-potted the plant, and repositioned it within the garden before him. He had just inaugurated the Chicago’s most famous rooftop garden: the green roof of City Hall.
The green roof consists of more than 20,000 plants of 150 species. It contains shrubs, vines, and even two trees selected for their ability to adapt to the windy and arid conditions of the roof. This roof serves as a large-scale experiment to test the effects of rooftop gardens on the city’s air quality and temperatures.
Overall the Chicago downtown exhibits temperatures 6 to 10 degrees Fahrenheit greater than the Chicagoan suburbs, as a result of the copious amounts of heat absorbed by tar-roofed buildings. Not only do these roofs impact energy efficiency, but they also have a negative effect on civilians’ health. The higher city temperatures cause chemicals in the air to react, creating more smog. Ozone, a component of smog, poses several health hazards in particular to the eyes and lungs.
The plants of the green roof reflect heat and provide shade. Unlike the traditional tar roofs of other skyscrapers, the green roof cools rather than heats the building. In effect the building employs less energy for air conditioning purposes. Comparing the surface temperatures of the City Hall green roof to the nearby, tar-roof Cook County Building has demonstrated a 50 degree difference between the green and the tar roof.
The City of Chicago clamored, not without legitimate concern, “enough tar roofs!” and learned that simple ideas have resounding impacts. Such is the legacy of switching from black to green.
For more information on the City’s efforts regarding climate change, consult the Chicago Climate Action Plan.
For tips on installing a green roof, consult the City of Chicago’s “Guide to Rooftop Gardening.”