Stormwater Management...on the Roof

(Conservation Currents, Northern Virginia Soil and Water Conservation District, May-June 2003)

A modest condominium community in Fairfax County is greening up. Yes, that’s up... on the roof.

Yorktowne Square has installed a vegetated roofing system, demonstrating an innovative way of managing stormwater runoff. Instead of increasing impervious surface like a conventional roof, a “green” roof can absorb up to 70% of stormwater, which is similar to the absorption rate of a healthy, aerated lawn.

The ultimate priority of any roof, conventional or otherwise, is to keep the building watertight. With green roof technology, the roofing material has a waterproof membrane and root barrier to prevent penetration. The underlying roof deck is designed to bear the capacity of a saturated system.

Mike Perry is president of Building Logics Inc., which is providing the design and roofing materials for Yorktowne Square. While the cost of a green roof is about 30% higher than a conventional roof, Perry explains that a green roof will last 40 or more years. “That’s three times longer than the average life span of a conventional roof,” he says.

“Burying the roof membrane makes all the difference,” states Perry. “Ultraviolet light exposure is the greatest cause of degradation. The vegetation blocks the ultraviolet light and keeps the temperature more constant, which reduces expansion and contraction of the roofing materials. Less wear and tear means a longer roof life.”

A green roof provides additional insulating qualities that are not found with a conventional roof, keeping the building warmer in the winter and much cooler in the summer.

Perhaps the greatest ecological function a green roof provides is its stormwater management capacity. The Northern Virginia Soil and Water Conservation District rarely misses a chance to explain the effects of land use on streams. As development increases, so does the amount of impervious (nonporous) cover. Rooftops, driveways, roads, sidewalks, and parking lots replace the trees, shrubs and other vegetation that can absorb stormwater. More impervious surface leads to more surface runoff. The greater the volume of runoff, the more eroded and polluted are the receiving streams.

On a green roof, plants intercept and delay rainfall runoff and the peak flow rate, reducing the erosive potential of stormwater and eventually returning water to the surrounding atmosphere by evaporation and transpiration.

There are two types of green roofs—intensive and extensive. Intensive is heavily landscaped and may include manicured lawns, shrubs, trees and flowers. These spaces are intended for humans to enjoy, but they are expensive and impractical in most circumstances. Extensive green roofs are simpler and need little maintenance. This type of system uses only a very thin layer of soil and a groundcover type of plant.

The 4,200 square foot roof at Yorktowne Square is an extensive roof, planted with three varieties of sedum plugs, all common to the mid-Atlantic region. Sedums are hardy succulents that can thrive for six to eight weeks without additional water.

Perry expects the roof to have full coverage within two growing seasons. The growing medium is two inches of soil, a special blend that includes shale, sand, and less than 20% organic material. “One of the most interesting things about the Yorktowne project,” says Perry “is that it is an existing wood frame structure that is incorporating a green roof as a retrofit. This is a key feature as opposed to a new structure specifically designed to incorporate a vegetated roof system.”

Peter Bolster, Mark Gatlin, and Michael Furbish of the Furbish Company planted the 8,400 sedum plants. Furbish is a former commercial real estate developer who now focuses only on sustainable development. “I want to do environmentally sensitive design, not commodity product real estate,” said Furbish. “Working on this, a small subcontract, is one step toward my goal. If you make what is environmentally friendly also economically viable, people will be interested.”

Jeanette Stewart, president of Yorktowne Square Condominiums Homeowner Association (HOA), is the leading force behind several innovative environmental projects in the community. Yorktowne HOA is paying for the green roof with its own funds and grants from the Virginia Department of Conservation and Recreation and the National Fish and Wildlife Foundation. Grant funds also are paying for other stormwater management features including rain gardens and cisterns.

Stewart will use the cisterns to monitor the quality and quantity of runoff captured from the new green roof and an adjacent conventional roof, also new. Each has eight drains which feed downspouts that empty on the lawns. Stewart will tap one drain from each roof and run the downspout to a cistern on the ground. If the science is correct, Stewart says there should be less runoff and fewer air pollutants from the green roof. After it is tested, the captured water from both rooftops will be available for residents to use for gardening and car washing.

“We have a working class community with residents from all over the world that care about the environment,” said Stewart. “With state and federal funding and mentoring from EcoStewards Alliance, we are able to show how communities can retrofit their buildings and landscapes to improve the environment.”

Michael Perry has several green roofing projects underway around the country and says the demand for his “time-tested products and services” continues to grow. “In five to ten years people are going to have to find a reason not to use a green roof. It’s the environmentally responsible thing to do.”

For more information about the project, contact EcoStewards Alliance at 703-766-1724 or Michael Perry at 757-340-4201. Jeanette Stewart directs education programs for EcoStewards.