Herrity Building Gets Green Roof


Are you interested in a green roof for your home or business, but don't know where or how to see one? A beautiful, new green roof at the Fairfax County Government Center is easy to access and available for anyone to tour during daylight hours!

The recently completed vegetated roof is located on the upper level of the Herrity Building parking garage. To access the site, enter the garage behind the Herrity building. Navigate to the top deck of the parking garage; you can park your car in any of the parking spaces adjacent to the rooftop garden.

Employees tour Herrity building green roof The 5,000 sq. ft Herrity green roof is a demonstration project, designed to showcase the beauty and variety of green roofs as well as the numerous environmental benefits vegetated rooftops provide. The intended audience includes residents, professionals and county employees.

For drop-in visitors, a 3-panel interpretive display installed at the garden describes the components of green roofs and the advantages of greening your rooftop. Already aligned along the margin of the vegetated area are interlocking trays that form a mini-green roof botanical garden. Each labeled tray is planted with a single species of the tiny, drought tolerant plants called sedums that make up most green roofs.

Within the vegetated roof area, three different planting levels illustrate the three types of green roofs: extensive, semi-intensive and intensive. The largest area, planted predominantly with sedums, is of the extensive type. This is the most common, lightest and most low-maintenance type of green roof. Extensive green roofs have shallow soil layers, typically 3-4 inches deep and are generally not accessible as public green space. Semi-intensive green roofs, like the shallow planters on the Herrity garage, have deeper soils (4-8 inches) and support a greater variety of drought-tolerant plants including shallow rooted perennials. Intensive green roofs are true roof-top gardens intended for public enjoyment, and can include water features, vegetable and herb gardens, or even trees and shrubs. The Herrity green roof includes several deep planters filled with native shrubs and perennials.

Two environmentally-minded Fairfax County citizens, Jeannette Stewart, president of the non-profit Lands and Waters, and Harry Glasgow, a Fairfax County Park Authority Board member and an associate director for the Northern Virginia Soil & Water Conservation District, first proposed a green roof on the Herrity parking structure. They brought the idea to the attention of the Fairfax County Board of Supervisors and staff.

As president of the Yorktowne Square Condominiums Homeowner Association, Jeanette had raised matching grant funds to have the first green roof in Fairfax County, a 4,200 sq.ft. vegetated roof, installed in her community. Her efforts at Yorktowne inspired many to believe that the Herrity garage could be retrofitted with a green roof to provide community education and environmental improvement. Jeanette's goal has always been to harness natural ecosystem processes to protect our streams and soil resources. "At both Yorktowne and at the Herrity garage," she asserts, "the natural benefits our plants and soil provide are being used to help us address the environmental problems we are creating."

One of the primary benefits of green roofs is energy savings. According to the U.S. Environmental Protection Agency, green roofs can reduce rooftop temperatures by up to 90 degrees. In fact, green rooftops can be cooler than air temperature because of evapotranspiration, or the evaporation of water by the rooftop plants during photosynthesis.

A green roof on an open air structure like the Herrity garage will not provide the energy savings it would on a similar enclosed building. However, Fairfax County plans to install green roofs on other public facilities, including the main Fairfax County Government Center, as each roof comes up for repair or replacement. Currently, the county spends $8 million annually on energy to heat and cool its 180 buildings. "Reducing energy use doesn't just make good environmental sense," Barry Hickey (Fairfax County, Department of Facilities Management) stresses, "its also makes good financial sense for the county."

The Herrity garage green roof will help Fairfax County meet its air and water quality improvement goals. At lower rooftop temperatures, ground-level ozone, which is a significant component of urban smog, is produced more slowly. Green roofs also retain and filter rainwater that would otherwise run quickly off in local streams causing erosion and carrying pollutants. Data suggests that green roofs can capture 60-100 percent of the stormwater runoff that would otherwise flow from a structure. The water that does flow from green rooftops is cooler, decreasing the thermal pollution in local water bodies.

A monitoring system is measuring the amount of water leaving the roof and collects samples for water quality analysis in an effort to quantify the benefits it provides. Russ Smith (Division of Stormwater Management) is managing the monitoring effort. "We are comparing the runoff from two equal areas of the Herrity garage, one vegetated and the other a conventional parking area," Smith explains, "So we will be able to clearly quantify any decreases in runoff or improvements in water quality resulting from the green roof." The monitoring components, including a rain gauge, soil moisture sensors and an automated water sampler, will help determine how efficiently the Herrity green roof removes pollutants, decreases runoff temperature, and stores stormwater.

"This project is a success beyond our wildest dreams," enthuses Glasgow. "If we want to promote low impact development in Fairfax County, we should have an example for developers and consultants to see. We now have the Herrity green roof as a model and I hope there will be others." A demonstration of permeable pavement will be installed at the main Fairfax Government Center building in the coming year.

Yorktowne Square green roof

(Conservation Currents, Fall 2008)


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