Green Roofs On Campus

At the University of North Carolina (UNC) in Chapel Hill, there is an odd little valley that has always defied development. It took a green roof to get things going.

Today, the University is building a $70-million development in and around the valley. Called Rams Head Center, the project will fill the valley with a four-level parking facility, the roof of which rises to the grade of the surrounding land. Around the parking structure, at the level of the roof, builders are putting up a student commons, a dining hall and a student services facility.

To tie the buildings together with a courtyard, the university will cover the parking garage with a green roof. The roof will include a high-quality rubberized asphalt membrane, an 8-in. layer of gravel, and 36 in. of soil capable of sustaining grass, shrubs and trees as tall as 30 ft.

Green roofs have long played an aesthetic role on college and university landscapes. But today's green roofs provide functional, as well as aesthetic, benefits.

The roof of the Rams Head Center parking garage will, for example, control storm water runoff. The greenery will absorb a good portion of storm water. The rest will filter through the gravel layer - a purification measure - and flow into cisterns built into the roof deck. A series of waterfalls at one end of the building will direct water to a nearby stream.

Green roofs also create stable, living ecosystems. Their insulating properties conserve energy and reduce sound transmission. A collection of green roofs can reduce the warming effect of urban heat islands by as much as four degrees.

Roofscapes, Inc., of Philadelphia, a firm that specializes in green roof design, and Cahill Associates of Westchester, Pa., a water-resource engineering firm, collaborated on the design of the complex green roof at Rams Head Center. According to Charles Miller, president of Roofscapes, the contemporary architectural focus on sustainable design has spurred interest in green roofs for all kinds of buildings, including those on college and university campuses.

In addition to Rams Head at UNC, Roofscapes' higher education projects include green roofs for the Papazian building at Swarthmore College in Swarthmore, Pa.; the Science & Student Services Complex at Foothill College in Palo Alto, Calif.; and the Residence Life Resource Center on the Santa Barbara campus of the University of Southern California.

Green Roof Design

According to the Roofscapes Website, at , green roofs fall into two design categories: intensive and extensive.

Intensive green roofs have been around for years. They feature soil covers that are 12 in. to 40 in. deep and accommodate numerous conventional landscaping features, including small trees, fountains and ornamental ponds. The UNC parking garage roof is an intensive green roof.

Extensive green roofs employ thin soil covers - no deeper than six in. These are low-maintenance designs that feature perennial flowering plants with fleshy leaves. Called succulents, these plants usually require little care and no irrigation.

Irrigation and Maintenance

According to Miller, properly designed green roofs generally do not require irrigation unless they are built in arid climates.

When irrigation is required, Miller's design philosophy is to introduce water at the of the soil layer with capillary or active trickle systems. "You can use spray irrigation," Miller says. "But this produces more weeds and greater maintenance requirements."

The green roofs being designed by Roofscapes at Foothill College and USC Santa Barbara will use capillary and trickle irrigation systems respectively. Once the plants on an extensive green roof have taken root, maintenance involves weeding and infill transplanting twice a year. Intensive green roofs require the same level of maintenance as a garden.

Waterproofing

Whether a system is extensive or intensive, all green roof designs require a high-quality water- proofing layer. According to Miller, green roofs can work with different waterproofing systems, but the highest standards must govern their construction. "For example, we'll use a PVC membrane, but we want it to be at least 60 mils," Miller says. "We'll also use modified bituminous, but it has to be at least two-ply. Other practical waterproofing systems include coal tar pitch. Bottom line, you need premium waterproofing. There are no bargain selections in waterproofing for a green roof."

A Fundamental Problem

High initial costs have generally restrained the adoption of green roofing systems. "Cost is the fundamental problem," says Miller. "Prices range from approximately $11 to $30 per sq. ft., depending upon the thickness of the soil layer and the overall area to be covered."

By contrast, roofs made with high-quality waterproofing membranes cost between $5 and $15 per sq. ft.

Then again, green roofs will likely last much longer than conventional roofs. "Done right, these systems can last for 75 years," Miller says. "No one knows for sure how long they will last. Green roofs have only been around for 35 years or so. But the good ones are still holding up well."

Looking to the future, construction, energy and maintenance costs will likely continue to rise. At some point, the high initial costs of sustainable building practices, including green roofs, may come to seem less onerous to institutions searching for long-lived building systems offering environmental and operational cost benefits.

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