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Speaker 1: Welcome to brain Stuff production of iHeartRadio, Hey brain Stuff

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Speaker 1: Lauren vocal Bam. Here. For people living in cities plagued

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Speaker 1: by the heat island effect, air pollution, stormwater, and all

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Speaker 1: of the psychological and physical effects of living in a

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Speaker 1: crowded concrete environment, turning the rooftops of big buildings into

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Speaker 1: living gardens and parks seems like an ingenious way to

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Speaker 1: mitigate some of those problems, and green roofs aren't exactly

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Speaker 1: a new idea. According to Graham Hopkins and Christine Goodwins

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Speaker 1: book The Living Architecture Green Roofs and Walls, the earliest

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Speaker 1: green roofs were the hanging gardens of Babylon and other

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Speaker 1: roof gardens on stone temples, created around six b C.

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Speaker 1: But modernly green roofs began to catch on in Europe

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Speaker 1: in the nineteen sixties, and it wasn't until the two

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Speaker 1: thousands that the movement began to take root, so to speak,

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Speaker 1: in North America. In Seen alone, there were more than

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Speaker 1: one thousand green roof projects completed in thirty nine U

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Speaker 1: S States and five Canadian pro pances, covering nearly five

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Speaker 1: point four million square feet. That's about five thousand square

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Speaker 1: meters of rooftop space with soil and plants. At least

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Speaker 1: twenty five North American cities, including San Francisco, Washington, d C.

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Speaker 1: And Chicago, have an acted legislation that either requires green

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Speaker 1: roofs on buildings or provides incentives to create them. And

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Speaker 1: there's growing scientific evidence that green roofs are beneficial, with

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Speaker 1: studies showing that they lower street level temperature and reduce

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Speaker 1: fine particles of air pollution, as well as reduced and

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Speaker 1: delay runoff from rainstorms. And then there are the mental

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Speaker 1: health benefits of more exposure to green space and plants.

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Speaker 1: A study published in in the Journal of Environmental Psychology

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Speaker 1: found that simply spending forty seconds gazing at a rooftop

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Speaker 1: flowering meadow helped to restore experimental subjects attention, and that

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Speaker 1: those who had such a view made significantly fewer errors

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Speaker 1: and performed better on tasks than their counterparts who only

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Speaker 1: got to look at a bare concrete roof. At this point,

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Speaker 1: you might be wondering, if green roofs so many benefits,

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Speaker 1: then why aren't even more cities sprouting greenery on their rooftops.

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Speaker 1: While green roofs make sense in a lot of ways,

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Speaker 1: requiring their installation isn't as simple as it might seem.

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Speaker 1: The city of Denver, Colorado, learned that lesson after its

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Speaker 1: voters decided in by a fifty four to vote to

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Speaker 1: pass the nation's most aggressive green roof ordinance. It mandated

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Speaker 1: that all new buildings over two square feet an area

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Speaker 1: that's around two thousand three square meters, devote at least

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Speaker 1: a portion of their roof surface to vegetation, and required

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Speaker 1: some existing buildings to go green whenever they replaced their

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Speaker 1: roofs as well. Businesses, real estate developers, and Denver Mayor

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Speaker 1: Michael Hancock all opposed the measure, but pro green roof

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Speaker 1: activists utilized social media and quote a lot of community

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Speaker 1: meetings to build popular support. That's according to Brandon Rethheimer,

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Speaker 1: the initiatives lead organizer. After the election, Reetheimer joined a

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Speaker 1: city task force set up to figure out how to

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Speaker 1: implement the new requirements, and the groups soon discovered problems.

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Speaker 1: Three separate engineering evaluations of Denver's building stock revealed that

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Speaker 1: nine of existing large buildings would have to be exempted

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Speaker 1: from the green roof requirement. That's because their structures weren't

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Speaker 1: capable of handling the additional weight of adding a rooftop

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Speaker 1: membrane at least several inches of soil and vegetation, which

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Speaker 1: worked out to about eight pounds per square foot. Another

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Speaker 1: problem was that the ordinance allowed buildings to combine solar

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Speaker 1: panels with plants to meet the green roof requirement, which

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Speaker 1: might have resulted in less vegetation on rooftops than proponents

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Speaker 1: had envisioned. That, in turn, would mean that green roofs

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Speaker 1: wouldn't provide as much relief from the heat, island effect, pollution,

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Speaker 1: and stormwater runoff as hoped. Eventually, the task force decided

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Speaker 1: that the best solution was to write a new ordinance

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Speaker 1: to replace the one voters had approved. That measure, which

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Speaker 1: was approved by the city Council, includes more flexible requirements

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Speaker 1: and additional options. Instead of everyone having to put in

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Speaker 1: a green roof, buildings would have the option of installing

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Speaker 1: a cool roof that wouldn't absorb as much sunlight and

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Speaker 1: combining that with more vegetation on the property at ground level,

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Speaker 1: or other similar measures. As a result, the amount of

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Speaker 1: total green space required is higher for new construction, with

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Speaker 1: provisions for multiple paths to meeting the requirement. While some

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Speaker 1: supporters who voted for green roofs might be disappointed. A

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Speaker 1: Retheimer said, when people see the benefits at the end

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Speaker 1: of the day, it's much better. He remains confident that

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Speaker 1: many buildings still will opt for green roofs. We also

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Speaker 1: spoke with Jennifer Boslot, whose name I hope I'm pronouncing

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Speaker 1: correctly and who is an assistant professor in the Department

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Speaker 1: of Horticulture and Landscape Architecture at Colorado State University. She

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Speaker 1: also served on the task force. She said, it sounds

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Speaker 1: like it's stepping back, but it really was for the

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Speaker 1: greater good. While balancing the realities aside from structural limits,

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Speaker 1: cost is another challenge US. A Lot says that putting

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Speaker 1: a relatively shallow installation of a few inches of soil

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Speaker 1: could cost between fifteen to thirty five dollars per square

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Speaker 1: foot in the Denver area. A deeper layer capable of

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Speaker 1: growing drew plants and retaining more moisture would be even

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Speaker 1: more costly. The revamped ordinance produced that economic pressure could

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Speaker 1: train a man again. Energy efficient Buildings lead for the

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Speaker 1: City of Denver told US via email that compliance costs

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Speaker 1: would be reduced and that the cost of new construction

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Speaker 1: would rise by just one percent or less under the

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Speaker 1: new rules. But even though Denver had to loosen its

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Speaker 1: stringent green roof requirements, Boslot still sees green roofs as

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Speaker 1: the future, both in the Mile High City and elsewhere.

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Speaker 1: As more green roofs are built, the cost will drop,

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Speaker 1: She says, a quote. We're urbanizing at such a rate

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Speaker 1: that we have no other option. If we're going to green,

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Speaker 1: We've got to green our roofs. Today's episode was written

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Speaker 1: by Patrick J. Kaiger and produced by Tyler Clang. Brain

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Speaker 1: Stuff is production of iHeart Radio's How Stuff Works. For

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Speaker 1: more in this and lots of other growing topics, visit

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