Chicago’s eleven-story city hall is an historic architectural landmark. The structure was designed in 1911 by Holabird & Rochek. The roof, which covers 33,000 square feet, was carefully analyzed by an extensive design team and deemed capable of carrying the additional weight of a greenroof.

Under the supervision of Mayor Richard M. Daley, The City of Chicago’s Department of Environment began construction of a pilot greenroof project as a tool to demonstrate the effectiveness of various vegetated and non-vegetated surfaces in combating energy loss and the heat island effect. This project was developed to establish information for the Urban Heat Island Initiative with the United States Environmental Protection Agency. This information will then be used to establish a system for large-scale implementation of greenroofs. The effects are monitored with the use of infrared technology and weather stations posted upon both vegetated and tarred sides of the building. Other specific areas monitored are rainwater runoff and native versus non-native plant growth success rates.

As part of the overall design, an undulating topography was created by the installation of 12”-24” mounds composed of insulation boards covered with the growing medium. Several layers are incorporated into the greenroof. These include layers for waterproofing, drainage, filtration, and growing medium. Lightweight soils composed of nutrient and mineral rich elements contain layers of mineral and organic soil mediums such as compost and mulch, humus soil mixture, and a low organic matter bulk mineral blend. These layers provide anchorage for plants, as well as superior drainage and aeration. On average, these components weigh 65 pounds per square foot, as opposed to standard soil, which weighs 110 pounds per square foot.

Chicago’s city hall rooftop was designed for maximum water storage during and following precipitation. This slows down water runoff via steady infiltration through soil layers and vegetated mass. A portion of this water is stored in tanks and used within the garden to deliver water to a drip irrigation system. The drip irrigation system is used during droughts or for initial plant establishment.

Plants cover 20,300 square feet of the rooftop, and were chosen for their ability to withstand this harsh growing environment. Plants are divided between test zones with intensive, semi-intensive, and extensive areas, and are organized by bloom color. Most of the 20,000 plants of 150 varieties are native to the region. When tested for surface temperature differences in August of 2001, temperatures in the vegetated areas of the roof were 91-119 Fahrenheit degrees, in comparison to the area covered with paving that reached 126-130 degrees. The tarred area of the same building climbed to 169 Fahrenheit degrees.

Source: City of Chicago
http://www.ci.chi.il.us/Environment/rooftopgarden/temperature.pdf
Source: City of Chicago http://www.ci.chi.il.us/Environment/rooftopgarden/medium.pdf
Plan © Conservation Design Forum, Inc.
http://www.mcdonoughpartners.com/projects/p_chicago.html
Source: ASLA Online
http://www.asla.org/meetings/awards/awds02/chicagocityhall.html
Source: William McDonough +Partners
http://www.mcdonoughpartners.com/projects/p_chicago.html