Even moderate storm events can overload the combined sewer system in the City of Philadelphia, resulting in serious pollution problems in downstream watersheds. As part of a company-wide effort to incorporate environmental practices into its operations, PECO has installed a 45,000 square foot green roof on top of an eight-story section of its corporate headquarters. The roof is the largest urban green roof installation on an existing building in Pennsylvania.
The roof is expected to add nearly one acre of pervious cover, which will absorb 60 to 70 percent of the estimated 1.5 million gallons of rainwater that fall on the structure annually, diverting thousands of gallons of peak runoff away from the Schuylkill River, which runs along the site’s western border. During the green roof’s peak growth period in summer, rainwater absorption is projected to be 85 percent. During heavy storm events (e.g., the ’10-year storm’), the roof is expected to reduce runoff by at least 50 percent. The green roof is also expected to help insulate the building during poor weather and help cool it inside and out during warm weather. In addition to reducing runoff volume, the roof is designed to filter nutrients and other contaminants from the runoff.
The roof’s vegetation is supported by four layers: a base layer of waterproofing material, a layer of drainage media, a barrier layer of geotextile fabric to prevent the soil from clogging the drainage media, and a 2-inch layer of soil. Vegetated mats containing hardy sedum varieties were laid down on the soil layer (this adds an inch of soil ). Designers opted to use a pre-planted layer of sod to help the vegetation become established, as the roof is prone to heavy winds blowing off the river that could strip the soil layer.
A fact sheet about the project is available for download by clicking here.
Watershed: Schuylkill River
Estimated to be approximately $1 million
Ben A. Lepage
Senior Environmental Project Manager
2301 Market Street, S7-2
Philadelphia, PA 19103
Manager of Facility Operations
2301 Market Street
Philadelphia, PA 19101