Building of the week: Chicago City Hall

Chicago City Hall is a big rectangular building that occupies an entire block. It was completed in 1911 and with its neo-classical columns and imposing bulk, it’s a building that projects power. City mayors have no doubt enjoyed the ego boost of working out of such an intimidating structure, but the interesting bit is on the roof.

In 2001 one of the world’s most ambitious green roofs was installed. Over 150 different species of plants thrive on it, 11 storeys up. It’s closed to the public, but beekeepers have hives on it. The other people who come and go are scientists and architects, because the roof was designed as a demonstration project.

Developed with William McDonough, one of the brains behind the Cradle to Cradle concept, it serves as a test bed for living roofs. Pollinators are observed. The wide variety of plants allows them to see which ones do best, especially during dry spells. Temperatures are monitored to see how they affect the building below, and how they reduce the heat island effect. That’s why the garden covers half the building, so that there’s an experimental side and a control side. In summer, the side of the building with the garden is several degrees cooler, saving thousands of dollars in air conditioning costs.

That learning can then be taken elsewhere in Chicago, where green roofs have been actively encouraged. Over 500 buildings have them, a greater area than any other US city. Notable other sites include the largest green roof in the world at Millennium Park, and the country’s largest rooftop vegetable garden at McCormick Place West. The city also hosts the country’s first organic certified rooftop farm, and features a variety of meadows, gardens and farms on top of schools, car parks and offices.

There is a reason why this emerged in Chicago. In 1995 the city experienced a heatwave that overwhelmed ambulance and healthcare facilities. Over 700 heat related deaths were recorded, and the urban heat island effect was identified as a contributing factor. The green roofs programme was put in place as one of many responses to the disaster. It offered a high profile and visible example of City Hall making Chicago more resilient to extreme weather.

With climate change making heatwaves more likely, more cities might want to consider how to reduce urban heat, and whether green roofs might have a role to play.


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