Rooftop Gardens Can Help Alleviate Heat in Cities, Study Finds

The rooftop garden at Chicago's City Hall.
The rooftop garden at Chicago's City Hall. DJANDYW.COM AKA NOBODY / Flickr / CC BY-SA 2.0

If you’ve even spent a summer in the city and been able to relate to the song of the same name then you know the feeling of wanting to escape the heat by sitting in a patch of cool grass under the shade of a tree.

Due to an effect called the “urban heat island,” temperatures are often ten degrees higher in cities than in surrounding areas due to the heat absorption and retention of materials like asphalt and concrete, a NASA press release said.

Researchers have been encouraging the replacement of tar and other dark-colored materials used in roofing for several decades. Bright rooftops that reflect the sun or “green roofs” filled with plants and greenery can alleviate some of the extreme city heat, new research by climate scientists from New York’s NASA’s Goddard Institute for Space Studies (GISS) said.

The results of the study were published recently in the journal Sustainable Cities and Society.

“As cities grow and develop, they need to make good decisions about their infrastructure, because these decisions often last for 30 or 50 years or longer,” said climate scientist and civil engineer at Columbia University and GISS Christian Braneon, who was one of the study’s co-authors, reported “In the context of more frequent heat waves and more extreme heat, it’s important to understand how these urban design interventions can be effective.”

The GISS research team looked at satellite images of three green rooftop sites that had been installed in Chicago in the early 2000s and compared them to images taken between 1990 and 2011, NASA said in the press release. They looked at how much surface temperatures and vegetation had changed at the sites and in control sites nearby that didn’t make use of green roofs.

Temperatures were reduced in two out of three of the green roofs — in one of them average temperatures were reduced significantly — though the other with lower temperatures than the control site had begun to rise again toward the conclusion of the study. The results suggested that the success of the green roofs in reducing temperatures may be determined by the diversity of the plants used, location and other factors.

The green roof that did not reduce temperatures was installed on a Walmart that had been built on a vacant lot with grass, so the amount of vegetation actually decreased when the store was built.

“In a lot of places, you might be developing an area that didn’t have something there before; it just had overgrown vegetation,” said Braneon, as reported by NASA. “You might think that putting a green roof on your new building would make a significant impact. But what we see is that a lot of impervious material may also be added there — such as a parking lot around the building. As a result, you might reduce the impact of the parking lot, but you certainly haven’t created the cooling effect that the overgrown vegetation had.”

The base of green roofs can include shallow soil for plants that don’t need as much maintenance, or deeper soil to build a home for a more extensive array of plant species and even trees.

According to Garden and Design, there is a difference between a green roof and a rooftop garden. Any roof, including those that are as slanted as the Alps, can accommodate a green roof, while a rooftop garden is generally heavier and uses space in different ways, so it is usually only appropriate for a roof with a mostly flat surface.

Green roofs are generally lighter and easier to put in because they use shallow trays for grasses or flowering plants, while rooftop gardens are usually set up to house a mixture of plants, trees, flowers and grasses, as well as things that make up an “outdoor room” of sorts.

Factors that can influence how much of a benefit a particular green roof has on its surrounding areas can depend on the structure of the roof itself, the diversity of plants selected and the region where the building is located, the researchers said, as NASA reported.

As global heating intensifies, urban heat islands are expected to as well, which means green roofs will become all the more important.

Often the burden of high temperatures in neighborhoods that have less green spaces and trees disproportionately affects some communities of color, as well as older adults and low-income communities.

“My hope would be that the methods we proposed show a low-cost way for folks working in less-resourced cities — who maybe don’t have access to a university or government researcher — to study their own communities,” said doctoral candidate at Yale University’s School of the Environment and lead author of the study Kathryn McConnell, as reported by NASA.

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