Pittsburgh Post-Gazette 

By Bob Podurgiel

Even though most roofs are still covered with ice and snow, people in Carnegie are thinking green -- as in green roofs.

Dr. Rob Berghage, associate professor of horticulture at Penn State University, gave an audience Friday of about 50 local political leaders, business owners and conservation advocates an introduction into how green roofs are constructed and how they can benefit building owners and the environment at a green roof and green landscaping conference in the Carnegie Borough Building.

Green roof technology can reduce storm water runoff from roofs by as much as 80 percent in the summer and by up to 30 percent in the winter, Dr. Berghage said.

Reducing storm water runoff is a goal shared by the sponsoring groups for the conference including Carnegie Borough, Allegheny County, the Pennsylvania Environmental Council, the Penn State Center -- Engaging Pittsburgh and Highmark. Allegheny Green and the Foundation for Pennsylvania Watersheds also helped to sponsor the conference.

Carnegie Councilman Rick D'Loss said a reason for holding the conference there was the easy access from Interstates 376 and 79, but he also said, "all storm water seems to lead to Carnegie too."

Mr. D'Loss cited the damage to the borough caused by the Hurricane Ivan flood in 2004 as a compelling reason for Carnegie's interest in reducing storm water runoff and for sponsoring the conference.

Mike Finnerty, Allegheny County councilman for District 4, which covers most of the communities in the Chartiers Creek watershed, said the event was an opportunity to discuss ways to prevent flash floods.

Dr. Berghage said a 6-inch shallow root system can absorb rainfall up to an inch and a half, and can get rid of 80 percent of the all storm water in the summer.

"The typical rain event in Pennsylvania is a half-inch," Dr. Berghage said.

The roofs are constructed, he said, by adding waterproofing material to the roof deck, then covering that with a moisture mat and root barrier usually made of heavy plastic sheeting.

A filter fabric is then layered on top along with a drainage layer of lightweight aggregate usually made of pumice. A lightweight soil mix and the plants are then placed on top.

The plants often resemble a meadow landscape and make heavy use of sedums that absorb and retain large amounts of water, making them ideal for storm water retention and surviving drought conditions.

A green roof can add 7.5 to 30 pounds per square inch of weight to a structure, depending on the depth of the vegetation and the root zone, which can vary from 6 to 12 inches in depth.

A roof designed for storm water management usually needs only a thin, shallow system of 6 inches, Dr. Berghage said.

He pegged the costs of a green roof from $10 to $30 per square foot and said one can cut air-conditioning costs by up to 30 percent.

Most of the green roofs installed to date have been on the flat roofs of commercial buildings, though Dr. Berg-hage said they can be installed on pitched roofs by using a tray system to hold the plants -- but it is more costly.

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