Green Roofs on Historic Buildings: City Hall, Chicago, Illinois

Aerial view of Chicago City Hall green roof on the west end of the City Hall-County Building.

Courtesy of the City of Chicago

Building constructed: 1911
Roof type: Semi-intensive, test/research
Size: 20,300 square feet
Year installed: 2001

The green roof on the Chicago City Hall building was installed as part of a Environmental Protection Agency study and initiative to reduce the urban heat island effect in the city and improve air quality. The one–block wide, twelve–story building had a substantial roof area that was ideal for planting and data collection. The green roof only covers the City Hall half of the City Hall-County Building, allowing for comparative testing of the green roof and traditional roof.

As a semi–intensive green roof, a combination of 20,000 herbaceous plants, 112 shrubs, and 2 trees were planted. The growing medium varied from a depth of 3 inches for the extensive plantings to 24 inches for the intensive plantings, and the semi–intensive areas averaged 8 inches in depth.

Plantings are set back from the roof to limit visibility from the street below and allow access for maintenance of the building facades.

Courtesy of the City of Chicago

Sedums and grasses were planted in the extensive areas, shrubs and deeper rooted plants in the semi–intensive areas, and the two trees in the deepest, intensive areas. The intensive areas were installed on cantilevered platforms over structural columns to support the additional weight. Portions of the roof that were not planted are used to collect rainwater for periods of drought which is stored in two 150 gallon cisterns. A supplemental irrigation system was used to establish the plants and can provide further supplemental water during periods of extreme drought.

Initially the city wanted to make the green roof accessible to building users. Despite the height of the building, the very low parapets meant any railing, if not set back from the parapet, would be highly visible as well as pose safety concerns. A railing set further back from the edge of the roof would have also reduced the area available for the green roof. Currently the green roof is only accessible for maintenance.

A variety of plantings were used on the green roof of Chicago City Hall.

Courtesy of the City of Chicago