Charting Our Progress

While Chicago and Washington, D.C., lead the way in planting green roofs, broad swaths of the country remain gray. Why the hesitation? In cities without Chicago’s single-minded program of grants, tax breaks, and regulations, the initial expense might seem prohibitive. Although green roofs cost about twice as much as conventional roofs, they also last about twice as long. And because green roofs perform differently depending on climate, building size, and interaction with other environmental components, it’s hard to anticipate the data for any given structure. But green roofs’ documented advantages—saving energy, controlling runoff, offsetting urban heat islands, and providing new habitats for wildlife—profit the public as much as building owners. If the experiences of Chicago and Germany are any indication, government needs to provide incentives that address short-term costs before green roofs can begin to ease our increasingly dense, hot cities.

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A sampling of data on the benefits of green roofs.

Percentage of 2005 population living in cities:
49

Percentage of 2030 population living in cities:
60

Rise in average global temperature between 1900 and 2000:
1–1.5° F

Predicted rise in average global temperature between 2000 and 2100:
3–5° F

Typical heat-island effect, the temperature difference between a city and the outlying rural area:
6–8° F

Number of people who died in Chicago’s 1995 heat wave:
more than 700

Number of people who died in Europe’s 2003 heat wave:
more than 52,000

Expected citywide mean temperature decrease if Chicago’s
vegetation were increased by 25 percent:
1.9–3.1° F

Fraction of U.S. energy that goes toward cooling buildings:
one-sixth

Temperature of a conventional-roof membrane on a 95° F day:
158° F

Temperature of a green-roof membrane on the same day:
77° F

Heat loss of green roof as compared with conventional roof:
18 percent less

Stormwater-retention rate of green roof as compared to conventional roofing material:
up to six times greater

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