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Cities mandate green roofs to capture stormwater, cool environment and support biodiversity

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Posted at 5:30 PM, Jun 14, 2022

Green roofs are vegetated spaces on top of buildings. They offer many benefits like capturing stormwater, cooling the environment, and supporting biodiversity.

They also suck carbon out of the air which helps to reduce the impacts of climate change. Horticulturalist Jennifer Bousselot is planting one atop a Colorado State University building called CSU Spur in Denver, Colorado.

“Adding vegetation is a way to filter that air and essentially sequester a lot of those pollutants,” Bousselot said.

Bousselot says there's a green roof requirement for new buildings of a certain size in the city of Denver.

She was instrumental in getting that green roof ordinance passed, and she says many cities across the U.S. are doing the same.

“There's one in San Francisco," Bousselot said. "There's now one in New York. And then there are older ones in both Chicago and Portland that have been around for decades.”

She says there is proof the mandates lead to more green roofs.

“The best case study comes out of Europe," Bousselot said. "So Basel, Switzerland initiated an ordinance in in 1986 and they now have something to the tune of 15% of all their rooftops vegetated.”

Steven Peck is the founder of Green Roofs for Healthy Cities, a nonprofit based in Toronto, Canada.

“The first major city in North America to do that was in Toronto about 12 years ago, which mandated green roofs on new structures and has since that has resulted in more than 10 million square feet of additional green space in the city,” Peck said.

He says the advantages of green roofs are wide-ranging.

“Air quality, for example, is something that everybody benefits from,” Peck said.

There are also energy savings. Peck says black roofs reach 158 degrees in the summer, while green ones stay around 77 degrees. And green roofs typically retain at least the first inch of rainfall.

In Toronto, Peck says that reduced stormwater by hundreds of thousands of gallons.  And cities like New York are taking note.

“The more water they have flowing into their massive stormwater infrastructure, the more costly it is," Peck said. "So they are incentivizing paying almost 100% of the cost of green roofs on existing buildings throughout New York City.”

Before the pandemic, Peck says more than 40 cities in North America were mandating green roofs, and that number is growing as cities realize its effectiveness in the fight against climate change.

“As our population becomes more urbanized, we're already on our way to being two thirds of our world population in cities, we have to start looking at rooftops as spaces to help make our cities more livable,” Bousselot said.

Note: A lot of the green roof footage for this story was filmed at Denver Botanic Gardens