Sourced from How Stuff Works
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.