As the latest installation underscores, green roofs are booming in the region.
In Philadelphia alone, 52 green roofs have sprung up, totaling 10.6 acres by the count of the Water Department, which is tracking them as part of its effort to amass data on storm water.
Storm water is a historic problem for Philadelphia because most of the city's underground pipes combine rainwater and sewage. During storms, the system becomes overwhelmed and water polluted with raw sewage and road dirt overflows into streams and basements.
"Every acre manages or eliminates roughly a million gallons of storm-water runoff a year in the city," said Chris Crockett, acting deputy commissioner of environmental services for the Water Department.
The roofs installed so far manage enough storm water every year to fill a swimming pool the size of City Hall's block with 5.5 feet of water, Crockett said.
The roofs also are seen as a way to improve the efficiency of buildings by keeping them cooler in summer and warmer in winter.
Green roofs top sites including the main Philadelphia Free Library, the Peco building, the Friends Center, Drexel University dorms, the Curtis Institute of Music, the Penn Charter Performing Arts Center, the new Kensington High School for the Creative and Performing Arts, the Urban Outfitters corporate headquarters, and numerous private homes.
'It depends'
At St. Joseph's University, researchers have planted the top of the science building with four types of green roofs, including more than 20 plant species. They also have installed sensors so they can collect data on water absorption, temperature differentials, and more.
An additional 34 green roofs, totaling about 11 acres, have been proposed.
