Welcome to brain Stuff production of iHeartRadio, Hey brain Stuff Lauren vocal Bam. Here. For people living in cities plagued by the heat island effect, air pollution, stormwater, and all of the psychological and physical effects of living in a crowded concrete environment, turning the rooftops of big buildings into living gardens and parks seems like an ingenious way to mitigate some of those problems, and green roofs aren't exactly
a new idea. According to Graham Hopkins and Christine Goodwins book The Living Architecture Green Roofs and Walls, the earliest green roofs were the hanging gardens of Babylon and other roof gardens on stone temples, created around six b C. But modernly green roofs began to catch on in Europe in the nineteen sixties, and it wasn't until the two thousands that the movement began to take root, so to speak,
in North America. In Seen alone, there were more than one thousand green roof projects completed in thirty nine U S States and five Canadian pro pances, covering nearly five point four million square feet. That's about five thousand square meters of rooftop space with soil and plants. At least twenty five North American cities, including San Francisco, Washington, d C. And Chicago, have an acted legislation that either requires green
roofs on buildings or provides incentives to create them. And there's growing scientific evidence that green roofs are beneficial, with studies showing that they lower street level temperature and reduce fine particles of air pollution, as well as reduced and delay runoff from rainstorms. And then there are the mental health benefits of more exposure to green space and plants.
A study published in in the Journal of Environmental Psychology found that simply spending forty seconds gazing at a rooftop flowering meadow helped to restore experimental subjects attention, and that those who had such a view made significantly fewer errors and performed better on tasks than their counterparts who only
got to look at a bare concrete roof. At this point, you might be wondering, if green roofs so many benefits, then why aren't even more cities sprouting greenery on their rooftops. While green roofs make sense in a lot of ways, requiring their installation isn't as simple as it might seem. The city of Denver, Colorado, learned that lesson after its voters decided in by a fifty four to vote to
pass the nation's most aggressive green roof ordinance. It mandated that all new buildings over two square feet an area that's around two thousand three square meters, devote at least a portion of their roof surface to vegetation, and required some existing buildings to go green whenever they replaced their roofs as well. Businesses, real estate developers, and Denver Mayor Michael Hancock all opposed the measure, but pro green roof activists utilized social media and quote a lot of community
meetings to build popular support. That's according to Brandon Rethheimer, the initiatives lead organizer. After the election, Reetheimer joined a city task force set up to figure out how to implement the new requirements, and the groups soon discovered problems. Three separate engineering evaluations of Denver's building stock revealed that nine of existing large buildings would have to be exempted
from the green roof requirement. That's because their structures weren't capable of handling the additional weight of adding a rooftop membrane at least several inches of soil and vegetation, which worked out to about eight pounds per square foot. Another problem was that the ordinance allowed buildings to combine solar panels with plants to meet the green roof requirement, which might have resulted in less vegetation on rooftops than proponents
had envisioned. That, in turn, would mean that green roofs wouldn't provide as much relief from the heat, island effect, pollution, and stormwater runoff as hoped. Eventually, the task force decided that the best solution was to write a new ordinance to replace the one voters had approved. That measure, which was approved by the city Council, includes more flexible requirements
and additional options. Instead of everyone having to put in a green roof, buildings would have the option of installing a cool roof that wouldn't absorb as much sunlight and combining that with more vegetation on the property at ground level, or other similar measures. As a result, the amount of total green space required is higher for new construction, with provisions for multiple paths to meeting the requirement. While some
supporters who voted for green roofs might be disappointed. A Retheimer said, when people see the benefits at the end of the day, it's much better. He remains confident that many buildings still will opt for green roofs. We also spoke with Jennifer Boslot, whose name I hope I'm pronouncing correctly and who is an assistant professor in the Department of Horticulture and Landscape Architecture at Colorado State University. She
also served on the task force. She said, it sounds like it's stepping back, but it really was for the greater good. While balancing the realities aside from structural limits, cost is another challenge US. A Lot says that putting a relatively shallow installation of a few inches of soil could cost between fifteen to thirty five dollars per square foot in the Denver area. A deeper layer capable of growing drew plants and retaining more moisture would be even
more costly. The revamped ordinance produced that economic pressure could train a man again. Energy efficient Buildings lead for the City of Denver told US via email that compliance costs would be reduced and that the cost of new construction would rise by just one percent or less under the new rules. But even though Denver had to loosen its stringent green roof requirements, Boslot still sees green roofs as the future, both in the Mile High City and elsewhere.
As more green roofs are built, the cost will drop, She says, a quote. We're urbanizing at such a rate that we have no other option. If we're going to green, We've got to green our roofs. Today's episode was written by Patrick J. Kaiger and produced by Tyler Clang. Brain Stuff is production of iHeart Radio's How Stuff Works. For more in this and lots of other growing topics, visit
our home planet, how stuff Works dot com. And for more podcasts from my heart Radio, visit the iHeart Radio app, Apple Podcasts, or wherever you listen to your favorite show. Smoke