In 2006, the American Society of Landscape Architects (ASLA) completed a green roof installation on its Washington, D.C., headquarters building to replace the building's original, conventional flat roof. According to ASLA executive vice president/CEO Nancy Somerville, the installation was created as a demonstration project for the group's membership, as landscape architects are primarily the professionals responsible for the design of green roofs. In September, the ASLA released its green roof performance report, outlining the energy savings, water retention, water quality, temperature differences, and plant performance realized over the past year.
The green roof was designed with an extensive shallow-depth portion and an intensive deeper portion, with soil depths and plantings based on the building's structural capacity. Flow meters and rain gauges were installed to track the roof's retention of stormwater, and light meters and thermometers were used to track plant-growing conditions and rooftop temperatures, providing helpful, realworld insights into which green roof methods deliver positive results—and which do not.
The ASLA found that its green roof delivered a range of significant economic and environmental benefits. Between July 2006 and May 2007, the roof prevented 27,500 gallons of stormwater—about 75 percent of all precipitation that fell on the roof—from flowing into D.C.'s sewer and stormwater system. The roof created runoff only when rainfall exceeded 1 inch, and that runoff contained fewer pollutants than typical runoff, the report says. However, the ASLA notes that the roof retained more water during the plants' growing season and less water during the dormant season.
Also, the plantings reduced the air temperature over the ASLA headquarters' roof by as much as 32 degrees in the summer, when compared with a neighboring tarred roof. This temperature reduction helped to mitigate the city's urban heat-island effect. Areas of the roof that had thicker plant growth (and therefore better coverage) were cooler, while more-exposed areas of the roof were hotter.
The ASLA also found that some plant species performed better than others, depending on placement, soil depth, and exposure to heat and light. The four hardy species of sedum (a kind of succulent) planted in the extensive portion of the roof—Sedum album, Sedum reflexum, Sedum spurium, and Sedum sexangulare—did very well, while Sedum lanceolatu and Sedum stenopetalatum were poor performers. The intensive portion's plantings of two sumac species, trumpet vine, and pasture rose grew successfully, while the New Jersey tea plantings did not fare well. Grasses also performed well, along with the perennials nodding onion, thread-leaved tickseed, and butterfly milkweed.
The organization also found that its green roof reduced the building's energy costs during the winter; the insulation it created lowered energy usage by 10 percent. Energy costs were not reduced in the summer because the building was being overcooled, but by correcting the building's cooling going forward, ASLA expects future summertime energy savings of 2 percent to 3 percent.