July 6, 2006
Photo courtesy ASLA
The American Society of Landscape Architects opened a green roof on its Washington, D.C., headquarters in conjunction with Earth Day this year. “We wanted to show how you can maximize both the environmental benefits and the aesthetic and amenity benefits of a green roof through the landscape architect’s design,” said Nancy Somerville, executive vice president of the ASLA.
The 3,300-square-foot roof, whose design was led by New York–based Michael Van Valkenburgh Associates, replaces a standard black tar surface on the 10-year-old brick building. Visitors to the roof climb a staircase to reach an aluminum-grate landing suspended above various succulents. Sweet fern and fragrant sumac grow among other plants in deeper soil on the roof over the staircase platform. Two formerly imposing air return units are also hidden within slopes of semi-intensive plantings that include African ice plants, evergreen moss flox, and various leafy stonecrops, anchored by a system of steel cables. “Having blooming plants around you, at and above eye level is a very different experience than having them just around your feet,” Somerville said, adding that the roof feels cooler than neighboring roofs, giving visitors an “oasis feeling.”
Roof accessibility was key but costly, with a staircase and entryway taking up two-thirds of the $950,000 budget: The narrow building envelope limited room for construction staging and storage, thereby increasing labor costs; ASLA officials also cite high materials costs in the wake of Hurricane Katrina.
The ASLA has ongoing plans to monitor the roof and share findings to help further green roof development: flow-meters are being fine-tuned to gauge the roof’s stormwater retention, and temperature readings will be compared with those from an adjacent non-green roof.