ASLA Headquarters Green Roof
Landscape Performance Benefits
- Prevented 27,500 gallons of stormwater — 77% of all precipitation hitting the roof — from flowing into Washington, D.C.’s overburdened sewer system during the initial monitoring period, from July 2006 to May 2007.
- Reduces the amount of nitrogen entering the watershed, according to results from water quality testing.
- Keeps the roof surface temperature as much as 43.5 degrees cooler than on neighboring conventional black roofs on the hottest summer days.
- Reduces building energy use by 10% over the winter months.
- Has received approximately 5,000 visitors to date, ranging from First Lady Laura Bush to educators, grade school, high school, and college students, landscape architect professionals, local and federal government officials, and international visitors from as far as China, the Middle East, and Australia.
- Provides documentation and educational resources via ASLA’s dedicated green roof website, which had 35,000 page views in the last year.
At a Glance
Michael Van Valkenburgh Associates, Inc.
Former Land Use
636 Eye Street, NW
Washington, District Of Columbia 20001
Environmental stewardship is a core value of the American Society of Landscape Architects (ASLA). For that reason, the headquarters green roof was conceived as a demonstration project and includes ongoing monitoring and research components related to stormwater retention, temperature, water quality, plant performance, and building energy savings. The location of the project in an urban area that faces significant issues related to combined sewer outflows and a degraded watershed makes the demonstration value of the project particularly important and effective.
Learning is the point of this project, as it explores design alternatives and educates key audiences about the benefits of green roofs as well as the role landscape architects play in sustainable design. Extensive media coverage reaching millions of people, thousands of in-person site visits, and a dedicated website with full documentation and education resources have increased understanding, engaged the public imagination, and inspired others to adopt green roofs.
- Two 25-ft wide elevated ‘waves’ form a structural skeleton filled with rigid insulation and covered with a green roof system. Because of their innovative design and placement, they effectively protect the usable/amenity roof space from noise from HVAC units on the ASLA building and neighboring buildings.
- An aluminum grating walking surface over a green roof system increases the total greened area of the roof by 30%.
- The design includes intensive plantings above a new stairwell structure and on top of the existing elevator shaft.
- Soil depths and plantings vary to make maximum use of the roof’s structural capacity, with soil depths varying from 3 ft for the flat, extensive sections to 21 ft on the elevator shaft.
- The plant palette intentionally includes ‘experimental’ plants along with those that have been green-roof tested and are in common use on green roof installations in the Northeastern seaboard of the U.S.
- Ipe wood used for the decking was certified as sustainably harvested by the Forest Stewardship Council and does not require sealants or preservatives.
In 2004, ASLA began to investigate installing a green roof on its 12-year-old, 3-story headquarters building in Washington, D.C. The building’s original rubberized membrane roof had already begun to develop pinhole leaks, and a structural engineering study showed that the roof had additional load capacity to accommodate an extensive green roof. ASLA wanted the roof to serve as a demonstration project, but access was limited to a wall-mounted ladder and hatches. The roof also held two 6ft-by-8ft HVAC units, located toward the center of the roof, and three smaller HVAC units. Because maximing the “green” area and environmental benefits were a high priority, the original program did not envision significant usable space.
During the course of the design process, designers developed a concept that maximized both green space and usable amenity space. The project scope and budget were expanded. The roof design includes two elevated “waves,” covered with a green roof system. The waves create an intimate, semi-enclosed space on the roof and completely hide the HVAC units, which were relocated as part of the project. An existing stairway was extended to the roof level to provide access for viewing and maintenance. Aluminum grating over sedum was used for the central roof area and access path to provide more green coverage while accomodating use. Intensive plantings were incorporated above the new stairwell structure and elevator shaft, which had significant additional structural load capacity. The design also includes experimenting with different plantings and soil depths.
The ASLA green roof, as a demonstration project, was more expensive than most such projects, as it involved building new structures and it incorporates a wide variety of plants and features. It has, however, also reduced operations costs for the building and, factoring the expected longer roof life, the green roof will more than pay for itself over time.
- The aluminum grating is greatly benefitting the sedum planted below because throughout the day it offers just enough shade to allow the sedum to retain its moisture longer. The sedum below the grating has spread to 100% coverage.
- The south wave, originally planted with phlox and Silene caroliniana has been transformed to a completely new look. Over time, the less resilient plants were replaced by the sedums, which withstand the higher summer temperatures for longer periods of time.
- As with any garden, there must be watering. The grasses and other taller perennials require watering several times a week to maintain their beauty. On the south wave, where sedums have replaced other plants, watering is rarely required.
Lead Designer: Michael Van Valkenburgh Associates, Inc.
Consulting Landscape Architect: Conservation Design Forum
Architect: DMJM Design
Structural Engineer: Robert Silman Associates
Green roof plants: Emory Knoll Farms
Funding support: Chesapeake Bay Foundation, National Fish and Wildlife Foundation / U.S. Environmental Protection Agency Chesapeake Bay Program.
Role of the Landscape Architect
Managed all aspects of the project, from design to construction; provides ongoing consultation in response to monitored performance.
Case Study Prepared By
Firm Liaisons: Keith Swann and Terence Poltrack, ASLA