Photo courtesy the Wharf, Washington, D.C.
Green roofscapes let sustainability and flora flourish
The Douglas A. Munro Coast Guard Headquarters, Washington, D.C., served as a model of sorts for the Wharf project, demonstrating that scale and terrain need not hold back an ambitious VRA design. At more than 46,452 m2 (500,000 sf), the headquarters boasts one of the world’s largest vegetative roofs. However, this roof is no simple flat grassland. The Munro headquarters is an 11-level terraced building. Nine of those levels are built into a hillside. Lush with native plant life, the vegetative roof blends so well into the surrounding landscape, local wildlife—including the occasional deer—treat the surfaces as a natural extension of their habitat.
The building’s striking design is in keeping with its namesake. Douglas Munro is the only member of the Coast Guard to receive the Medal of Honor, earned for his selfless sacrifice as a landing craft pilot at Guadalcanal during World War II. The headquarters serves more than 4000 occupants, and in addition to the Coast Guard, houses several independent field commands including the National Pollution Fund Center and Marine Safety Center.
Designed to go with the flow
Like the Wharf, the Munro Coast Guard Headquarters had to meet strict stormwater retention regulations as well as federal EPA rules requiring 95 percent of stormwater to be collected on site. The vegetative roof—51,097 m2 (550,000 sf) in total—acts like an enormous sponge topping the offices. According to the Landscape Performance Foundation, the roof retains up to 1.6 million L (424,000 gal) of rainwater. The building’s stair-stepped terrace design then moves that water gradually through 37 m (120 ft) of elevation changes, and eventually into a pond.
In addition to meeting stormwater runoff regulations, the project’s designers wanted to go above and beyond with the building’s sustainability profile, creating a roof that would help encourage biodiversity and reduce heating and cooling energy requirements. In urban settings, the plantings on VRAs can also help filter air pollution and create habitats for local wildlife. The roof ultimately received LEED Gold certification from USGBC.
The terraced design, natural landscape, and sheer size of the building (a sprawling 111,484-m2 [1.2-million sf] on 71 ha [176 acre]) had created numerous challenges in developing the VRA for this project. The design utilized XPS insulation over hot rubberized asphalt. A complex network of expansion joints was used to accommodate the many levels of the roof, tying together multiple air barriers within the system.
The roof design has proven highly resilient with no leaks to date and is considered to be a best practice for VRAs. The building has become a magnet for engineers, designers, and landscaping groups interested in its design, performance, and sustainability profile.
Conclusion
As designers gain experience with VRAs, they are experimenting with more creative ways to use these spaces to enhance building form and function. Innovations in materials, testing processes, and performance profiles of materials such as waterproof membranes and insulation are allowing VRAs to be placed in an increasing number of projects, bringing the benefits of green roofs to more places where Americans live, work, and play. The regulatory environment as well as the interest of architects, contractors, and building owners in technologies reducing global warming potential (GWP), also continue to inspire innovations in commercial roofing. As this issue of The Construction Specifier goes to press, manufacturers are introducing XPS insulation in compliance with environmental regulations set to become effective in parts of North America, beginning January 2021. Some of the blowing agents used in insulation offer a 90-percent reduction in GWP.
Tiffany Coppock, AIA, NCARB, CDT, LEED AP, is the commercial building systems specialist at Owens Corning where she provides leadership in building science, system development, testing, and documentation. She can be reached via e-mail at tiffany.coppock@owenscorning.com.
