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Even roofs are going green: ASU to study benefits of gardens atop buildings

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The CityScape retail and restaurant plaza lies in the heart downtown, and though it could have just been another concrete landscape in the desert, its blueprint included trees and grassy knolls.

Paul Coseo, an Arizona State University assistant professor of design and sustainability scientist, says this spot atop a parking garage can be considered a green roof – and such part of a global trend toward incorporating vegetation on the tops of structures.

Coseo said CityScape is not unlike Chicago’s Millennium Park, once a withering parking lot and now turned into a vibrant space built over one.

One difference between the two parks, though, is the climate they exist in, and Coseo said it’s important to better understand the benefits of green roofs in an arid region. That’s why he’s received grants to lead a 10-month pilot research project focusing on their potential.

“With green roofs, especially in this climate, it might take some adjustment in terms of seeing what survives and what doesn’t,” he said. “So my plan is to work with a local horticulturist to figure out plant material that will do well.”

His green roof will be installed in March atop the Design School in Tempe. He said it will be relatively modest – about 144 square feet and 8 inches deep.

Coseo, along with a few students and hopefully a few volunteers as well, said he plans to experiment with a variety of plants and see how they change over time. He also hopes to track birds, insects and other creatures that encounter the garden.

“I’m going to measure things like soil temperature and moisture, survival rates of the plants,” he said. “Which ones live or die, or maybe if new plants come in. Sometimes you get seeds that fly in from the surrounding area.”

Most of the research will be done over the summer, which will allow Coseo to measure the effects of heat.

“That’s the critical time here,” he said. “If it can survive the summer, then it can definitely survive the winter.”

The project will study the benefits of the roofs, which could include reducing the urban heat island effect, which contributes to air pollution and increased energy consumption; capturing storm runoff, leading to less flooding; and increasing biodiversity in urban environments.

One of Arizona’s few green roofs topping a building is a 5,000-square-foot desert garden on the Tempe Transportation Center, installed after an ASU research project not unlike Coseo’s.

Bonnie Richardson, an architect and principal planner for the city of Tempe, said the key goal in installing the garden was reducing heat in the building. She said that the depth of the soil is also helping prolong the life of the roof itself.

“In a desert climate, the lifetime of a roof is a little shorter, oftentimes you’ll get cracking,” Richardson said. “This helps protect the roof.”

Along with the insulation benefits, the roof also helps catch storm runoff and filters it into collection storage system to reuse for things like irrigation, she said.

“In Arizona we typically get really fast rains, sometimes it can be so quick that we get a lot of flooding,” Richardson said. “By having mechanisms on site to help maintain the water, that’s beneficial for everybody and it keeps the water out of the street.”

Despite the arid climate, the green roof has been doing relatively well since its installation, with exception of a little browning on one of the plants at the end of summer, she said.

“We have a lot of people who do climb the mountain and see the garden, it does make people think about the opportunities,” Richardson said. “We wanted to have people think about the potential that we could do more green roofs here and we could save energy.”

However his research turns out, Coseo said one thing’s for sure: Green roofs add to the aesthetics of buildings.

“Even if it turns out that it’s only beneficial because people enjoy their environment more, maybe that’s enough,” he said.