Project general details
WORLD GREEN INFRASTRUCTURE AWARD
Chicago City Hall Rooftop Garden
Nominator
Investor
Supplier
Architect
Green Infrastructure Foundation
City of Chicago
McDonough + Partners
Roofmeadow (now Studio Sustena)
Location (City)
GPS
Country
Region
Year of nomination
Chicago
data
United States of America
Americas
2024
Project specifications
Chicago’s City Hall is an historic 11-story building home to the most famous rooftop garden in the city, and one of the more recognized green roofs in the world. The first demonstration project for the City’s Urban Heat Island Initiative, which pioneered the notion of the climate resilience benefits of green roofs in urban contexts, demonstrates a four-degree Celsius heat reduction on a hot summer day, compared with a non-green roof, saving thousands of dollars annually on cooling costs. The innovative design was also one of the first biodiverse green roofs in the US, deploying over 20,000 plants of over 150 different species, mostly prairie plants native to Chicago (shrubs, vines, trees, etc.), which thrive in the harsh, varied conditions Chicago experiences. The green roof led to one of the nation’s first green roof incentive programs, which dramatically increased the number of green roofs installed in the City. This project has received numerous recognitions, including the American
Object type
Roof accessibility
Roof slope
Year of completion
Project in operation?
Administrative and civil construction buildings
Private
0-5 degree
2000
YES
Green roof area (m2)
3605
Green roof specification
Method of establishment of vegetation
Initially planted with 120 species of perennials, herbaceous flowering plants, grasses, succulents, woody shrubs and trees, the garden currently has over 160 species thriving (Dvorak and Carroll 2008). The intended goals to establish a green roof pilot project have been achieved, but the progression and level of maintenance required to keep the project successful has changed dramatically from its installation in 2000 to 2008 (Dvorak and Carroll 2008). The initial 2-year establishment period was plagued with difficulties; a variety of issues resulted in the pilot project being in danger of failing. Because of the high-profile nature of the project, local television stations and newspapers for the first time began to question the validity of green roofs in Chicago. The city responded with action the following spring. Over the next five years, the maintenance team established a maintenance plan where the the gardens performance is reviewed annually, and crews are directed accordingly.
Vegetation
The plants were selected for their ability to thrive in conditions on the roof, which is exposed to the sun and can be windy and arid. Most are prairie plants native to the Chicago region. In order to attain an estimate of the total quantity of plants in stalled on the roof and estimate the survival rate of the fall planting, the Department of Environment hired Conservation Design Forum to conduct an estimate of total number of plants installed. The garden produced an 80 percent survival rate during its first growing season. This was good news for the City in that the survival rate was considered to be much higher than expected, given the late planting of the northern half and met contractual requirements for the installer. The planting design pattern and layout was very closely followed and the diversity included in the initial plant list was followed as well, though a number of substitutions were made. An empirical study of the survival rates of individual species planted was not pa
Vegetation layer
Designed by landscape architect/ecological design team members (Conservation Design Forum and Atelier Dreiseitl), the green roof comprises more than 20,000 plants arranged within several rectangular blocks. The blocks are planted according to bloom cycles such that, as the seasons progress, over 150 species of prairie and woodland plants evolve in radial bands of color, form, and texture. Native grasses, herbaceous plants, flowering shrubs, and two trees (a prairie crabapple and a cockspur hawthorn) provide forage for wild pollinators, as well as honeybee colonies maintained on the roof. The undulating surface—comprising layers of insulation and lightweight soils at varying depths—provides temperature control and moisture absorption. The planted side of the roof registers seven degrees cooler than the unplanted side and captures some 75% of rainwater (stored in a cistern in the penthouse below).
Retention layer
The City Hall design opted for a base of Sika Sarnafil’s Loose Laid Waterproofing System, which was easy to install over the existing surface and acted as a leveling layer. This layer then received various layers of extruded polystyrene insulation and separator sheets followed by the roof membrane on top. An additional protection layer was laid on top of the roof membrane, followed by the additional insulation of a filter layer and finally the growing medium with erosion blankets to keep the soil in place.
Filtration layer
The City Hall design opted for a base of Sika Sarnafil’s Loose Laid Waterproofing System, which was easy to install over the existing surface and acted as a leveling layer. This layer then received various layers of extruded polystyrene insulation and separator sheets followed by the roof membrane on top. An additional protection layer was laid on top of the roof membrane, followed by the additional insulation of a filter layer and finally the growing medium with erosion blankets to keep the soil in place.
Drainage layer
The City Hall design opted for a base of Sika Sarnafil’s Loose Laid Waterproofing System, which was easy to install over the existing surface and acted as a leveling layer. This layer then received various layers of extruded polystyrene insulation and separator sheets followed by the roof membrane on top. An additional protection layer was laid on top of the roof membrane, followed by the additional insulation of a filter layer and finally the growing medium with erosion blankets to keep the soil in place.
Separate layer
The City Hall design opted for a base of Sika Sarnafil’s Loose Laid Waterproofing System, which was easy to install over the existing surface and acted as a leveling layer. This layer then received various layers of extruded polystyrene insulation and separator sheets followed by the roof membrane on top. An additional protection layer was laid on top of the roof membrane, followed by the additional insulation of a filter layer and finally the growing medium with erosion blankets to keep the soil in place.
Protective layer
The City Hall design opted for a base of Sika Sarnafil’s Loose Laid Waterproofing System, which was easy to install over the existing surface and acted as a leveling layer. This layer then received various layers of extruded polystyrene insulation and separator sheets followed by the roof membrane on top. An additional protection layer was laid on top of the roof membrane, followed by the additional insulation of a filter layer and finally the growing medium with erosion blankets to keep the soil in place.
Root layer
The City Hall design opted for a base of Sika Sarnafil’s Loose Laid Waterproofing System, which was easy to install over the existing surface and acted as a leveling layer. This layer then received various layers of extruded polystyrene insulation and separator sheets followed by the roof membrane on top. An additional protection layer was laid on top of the roof membrane, followed by the additional insulation of a filter layer and finally the growing medium with erosion blankets to keep the soil in place.
Category:
GREEN ROOFS
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