Patrick J. Kiger of howstuffworks writes:

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.

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.

Jennifer Bousselot, an assistant professor in the Department of Horticulture and Landscape Architecture at Colorado State University, 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. 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.”

Watch this excellent video from NPR:

Read more: Why Don't More Cities Require Green Roofs?