For people living in cities plagued by the heat island effect, air pollution, stormwater, and the psychological and physical effects of living in an unnatural environment, turning the rooftops of big buildings into living gardens and parks seems like an ingenious way to mitigate some of those problems, and make urban life more pleasant as well.
Green roofs began to catch on Europe in the 1960s, but it wasn't until the 2000s that the movement began to take root in the U.S. and other countries. In 2017 alone, there were more than 1,000 green roof projects completed in 39 U.S. states and five Canadian provinces, covering nearly 5.4 million square feet (502,000 square meters) of rooftop space with soil and plants, according to an annual survey conducted by Green Roofs for Healthy Cities, (GRHC) an industry organization. GRHC founder and president Steven Peck says in an interview that at least 25 North American cities, including San Francisco, Washington, D.C. and Chicago, have enacted legislation that either requires green roofs on buildings or provides incentives to create them.
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 reduce 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 2015 in the Journal of Environmental Psychology found that simply spending 40 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 counterparts who only got to look at a bare concrete roof.
At this point, you might be wondering: If green roofs have 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 Denver Green Roof Story
The city of Denver, Colorado, learned that lesson after its voters decided in 2017 by a 54.3 to 45.7 percent margin to pass the nation's most aggressive green roof ordinance, which mandated that all new buildings over 25,000 square feet (2,322.6 square meters) in area 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. (Here's a summary of the ordinance's requirements.) Businesses, real estate developers and Denver mayor Michael Hancock opposed the measure, but pro-green roof activists utilized social media "and a lot of community meetings" to build popular support, according to Brandon Rietheimer, the initiative's lead organizer.
After the election, Rietheimer joined a city task force set up to figure out how to implement the new requirements, and the group soon discovered problems. Three separate engineering evaluations of Denver's building stock revealed that 85 to 90 percent 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 8 pounds per square foot (0.09 square meters), Rietheimer says.
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.
"We felt like we were losing a lot of the benefits, so we really needed to make changes, " Rietheimer says.
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 is now awaiting approval 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 will be higher for new construction, with provision for multiple paths to meeting the requirement.
While some supporters who voted for green roofs might be disappointed, "when people see the benefits, at the end of the day it's much better," says Rietheimer, who remains confident that many buildings still will opt for green roofs.
"It sounds like it's stepping back, but it really was for the greater good while balancing the realities," says Jennifer Bousselot, an assistant professor in the Department of Horticulture and Landscape Architecture at Colorado State University, who also served on the task force.
Cost Is an Issue
Aside from structural limits, cost is another challenge. Bousselot says that putting a relatively shallow installation of a few inches of soil could cost between $15 to $35 per square foot in the Denver area. A deeper layer capable of growing larger plants and retaining more moisture would be even more costly.
The revamped ordinance will reduce that economic pressure. Katrina Managan, energy efficient buildings lead for the city of Denver, says in an email that compliance costs would be reduced by 20 percent, and that the cost of new construction would rise by just 1 percent or less under the new rules.
But even though Denver may have to loosen its stringent green roof requirements, Bousselot 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.
"We're urbanizing at such a rate that we have no other option," she says. "If we're going to green, we've got to green our roofs."