A riverfront neighborhood leverages green roofs to manage stormwater and support sustainability.
Stretching for nearly a mile along the shore of the Potomac River in Washington’s Southwest quadrant, The Wharf is a mixed-use development that includes elevated spaces for gathering using a network of green roofs. Each building’s green roof also adds interest to the skyline with a variety of foliage and green spaces.
Roofing contractor James Myers Co., Beltsville, MD, worked with Preservation & Protection Systems (PPSI), Laurel, MD, and Owens Corning, Toledo, OH, to design and install protected roof membrane assembly (PRMA) systems for vegetative roofs on several buildings throughout The Wharf area. While the vegetated green roof assembly for each building featured unique plants and landscaping, the design concept and materials were selected to manage stormwater runoff. PPSI and Owens Corning had previously developed the PRMA assembly for the U.S. Coast Guard headquarters and were pleased with its performance. “Since its installation at the Coast Guard headquarters in 2013, we’ve had no leaks,” said Steve Gordon, president of PPSI.
The PRMA installed on more than half of the roofs in The Wharf used Owens Corning FOAMULAR 404 and 604 extruded polystyrene insulation (XPS). The XPS insulating material was selected based on its compressive strength and ability to support a breadth of overburden materials including plants, pavers, and a unique network of pedestals and basins holding the soil in place. “We wanted an insulation with the water resistance to handle a variety of overburden types including plants and trees and the PSI to support pavers as well,” said Joe Schneider, RRC, senior project manager at James Myers.
The insulating material was also used in building enclosure areas throughout The Wharf to deliver energy performance with its high R-value. Structural basins under the roof pavers held soil at depths to 4 ft., demanding a high PSI insulation. Beyond water resistance and strength, the XPS insulation helps control water flow as seams in the insulation help slow drainage, and helps avoid overwhelming drainage systems.
As rooftop plants and vegetation soak up the rainwater, the insulation helps to control the flow of water from the plant layer, growing media, and filter/drainage layer to the waterproofing layer and horizontally to a series of overflow vaults. The Wharf makes use of 700,000-gal. cisterns that collect and control the release of water.
Wind in the region presents an additional challenge for The Wharf’s vegetative roofs. Rock curbs from Hanover Architectural Products, Hanover, PA, help mitigate against winds coming in off the riverfront. Other materials used in the PRMA assembly included waterproofing membrane 790-11 Hot Rubberized Asphalt the by Henry Co., El Segundo, CA. EMSEAL, Westborough, MA, expansion joints tied together air barriers in the roofing system. Hanover Architectural Products provided the pavers, and sedum mats were supplied by Sempergreen, Odijk, The Netherlands.
Meeting construction deadlines is always a concern, but the high-profile events surrounding the grand opening of The Wharf project meant there was no room for delay. A concert by hometown rock band the Foo Fighters at the Anthem music center officially launched the opening. The concert sold out quickly, filling the Anthem to its 6,000-occupant capacity. As the days toward the grand opening counted down, the District’s sweltering summer climate brought temperatures in the mid 90’s, and construction teams worked around the clock to complete Phase 1.
“The magnitude of completing multiple buildings against a very tight timeline was a challenge felt by all of the contractor trades on site,” said Brian Davis, general superintendent at James Myers. With so much work occurring at once, scheduling and coordinating the logistics required of multiple trades was key to success. “As we installed pavers with elevated blocks, we were working throughout the night, seven days a week, the final 60 days of the project,” Davis said. For contractors taking on such high-profile projects, Davis said it’s critical to be prepared to adapt, expect the unexpected, and maximize communications with all trades working on a project.
During the final three months of construction at The Wharf, weekly coordination meetings evolved into daily meetings with precise schedules for exactly what team members would be working in what area at a particular time. According to Davis, the challenge of building nine buildings over six blocks made traffic bottlenecks the biggest challenge. Careful attention was paid not only to the District’s notorious rush-hour traffic but also to area attractions and events that draw traffic into the District. For example, the Washington Nationals schedule featured a number of weeknight baseball games, and trades were required to clear the streets three hours before the first pitch of each home game. While assuring schedules were maintained, the contractor also had to ensure safety and quality were not compromised.
The opening of Phase One brought together the District to create a one-of-a-kind public space that has reenergized one of America’s most historic riverfronts. The addition of green spaces atop the buildings has provided a new vantage point from which to take in one of America’s most beloved waterways, while helping achieve sustainability and stormwater management objectives.
Codes Call For Vegetative Roofs
Vegetative roofs are prolific across the nation’s capital. The District of Columbia Department of Energy notes there are currently more than 3 million sq. ft. of vegetative roof in the District. Previous roof designs that influenced the vegetative roof assemblies at The Wharf include several high-profile buildings including MGM Casino, the National Museum of African American History and Culture, National Museum of American Indians, and the National Archives. One of the largest vegetative roofs in the nation, spanning approximately 500,000 sq. ft., sits atop the Douglas A. Munro U.S. Coast Guard Headquarters in Washington. The innovative assembly used on the Coast Guard headquarters served as a model for the assembly of several vegetative roofs throughout the D.C. area and in The Wharf.
In addition to the Potomac, D.C. waterways include the Anacostia River, Rock Creek, and the Chesapeake Bay. Commercial buildings throughout the District make use of an innovative cistern system, permeable pavements, and substantial vegetative bio-retention using a network of vegetative roofs. According to Jeffrey Seltzer, PE, acting director, Natural Resources Administration, Department of Energy & Environment, Washington, regulated development projects in the District are required to install practices that retain runoff from a 1.2-in. storm event typically through green infrastructure and capture/reuse systems employing cisterns.
Vegetative roof assemblies are a key tool employed to help control the flow of storm water into cisterns. The Wharf also uses these elevated spaces to provide a unique vantage for viewing several national landmarks and delivering valuable usable space in a densely populated area.
The specification and installation of protected roof membrane assembly (PRMA) systems for vegetative roofs is an integral part of complying with the District’s requirements to manage stormwater runoff and to support developers’ vision of spaces integrating nature into the occupant and visitor experience. The District has mandated green roofs and specified stormwater retention rates relative to a building’s footprint. As the District also specifies the ratio of pervious versus impervious surfaces, contractors constructing buildings in The Wharf were challenged to achieve the mandated area of greenspace, considering the requirements of vegetation and adequate sidewalk/surface areas.
About The Wharf
As a community that engages visitors in experiences along the riverfront, The Wharf delivers the most public space to the District since the redesign of the national mall in 1902. In addition to offering 14 acres of parks and public space it features a variety of water activities including kayaking, paddle boarding, and water taxis.
The vegetative roofs are also an important component of The Wharf’s commitment to sustainability. Measures to support sustainability started at the master-planning stage and development was designed to achieve LEED ND Gold while targeting LEED Gold or Silver for individual buildings. The Wharf exceeds many of the requirements of the District’s Green Building Act and the Anacostia Waterfront Initiative.
Developed in phases, Phase One of The Wharf took nearly 15 years to complete. Phase One opened on October 12, 2017 and consists of 2 million sq. ft. of space. The final phase is scheduled for completion in 2022. Upon completion, The Wharf will include 1,375 residences, 945,000 sq. ft. of office space, and 800 rooms in four hotels.
A visitor unfamiliar with the project would likely be surprised to learn that the individual buildings are part of a planned community. Cobblestones coexist with glass and steel. Canopies are at different heights and of various shapes. The narrow streets and alleyways that project old- world charm posed challenges during the construction process. “It was an incredible spectacle of logistics to see huge cranes hoisting trees above traffic and truckloads of extruded polystyrene insulation snaking their way through narrow corridors during construction,” said Tiffany Coppock, commercial building systems specialist at Owens Corning.
Perhaps The Wharf’s diverse look is the result of having nine different architects to design the first nine buildings. Amid all this diversity, the Promenade is a central walkway tying the community together and giving The Wharf its most striking attribute—a sense of community. The Wharf attracts visitors and residents not only during the summer months, but throughout the year. Events such as ice sculpting, Curling & Cocktails, and Broomball & Brews keep the public engaged and interacting at The Wharf throughout the year. Nearly every holiday has a special event—including bagpipers on St. Patrick’s Day, Petalpalooza for the annual cherry blossom celebrations, and a Chihuahua race for Cinco de Mayo. At its heart, The Wharf is a gathering place and much of that gathering takes place on the highly engineered rooftops.