At The Wharf, Vegetative Roofs Play a Key Role in Storm Water Management

The Wharf is a riverfront community spanning nearly a mile of the Potomac River. The neighborhood features high-end hotels, luxury condominiums, retail shops, commercial offices and a music hall. Photos: The District Wharf

According to the Washington D.C. Department of Energy and the Environment, the District is home to more than 3 million square feet of vegetative roof assemblies (VRAs). A commitment to responsible storm water management has helped the nation’s capital rank first among U.S. cities when it comes to green roofs installed. From the massive 500,000-square-foot VRA atop the Douglas Munro U.S. Coast Guard headquarters (featured in the September/October 2018 issue of Roofing) to elegant boutique cocktail bars, VRAs are helping Washington, D.C., manage storm water runoff and providing unique vantage points for taking in national landmarks. The Wharf, in the District’s Southwest quadrant, represents a “neighborhood” approach to green roofs.

A riverfront community spanning nearly a mile of the Potomac River, The Wharf is a mixed use, public-private development, including high-end hotels, luxury condominiums, retail shops, commercial offices and even a 6,000-seat music hall. While the design and aesthetics of different buildings at The Wharf project a sense of diversity, VRAs are a unifying element. In addition to providing visual interest and elevated spaces for public gatherings, VRAs help The Wharf achieve its sustainability and water management goals.

Managing Storm Water, Supporting Sustainability

Water has long been a defining element of life in the District, whose neighboring waterways include not only the Potomac, but also the Anacostia River, Rock Creek and Chesapeake Bay. To help manage storm water runoff, the District makes use of a massive cistern system, permeable pavements, and extensive use of vegetative bio-retention supported by VRAs. The District of Columbia’s Department of Energy & Environment mandates measures that retain runoff from a 1.2-inch storm event through green infrastructure and capture reuse systems. Specific storm water retention rates are specified relative to a building’s footprint.

The vegetative roofs help tie together the diverse types of buildings in the area. Photos: The District Wharf

In addition to helping manage storm water runoff, protected roof membrane assembly (PRMA) systems at The Wharf also help to support the development’s sustainability performance goals, which started at the master planning stage. Sustainability objectives supported by The Wharf’s VRAs include reducing storm water discharge and improving thermal performance through the cooling and shading properties of soils and plantings — an important consideration in a region renowned for its humid summers. VRAs are also desirable in urban areas for the role vegetation can play in helping filter pollution and providing habitats for birds and other wildlife. Finally, VRAs offer an aesthetically pleasing environment for employees, occupants and visitors.

The Wharf development was designed to achieve LEED Gold, while individual buildings targeted LEED Gold or Silver. Constructed in phases, Phase One opened in 2017 and took nearly 15 years to complete. While the Great Recession slowed construction, an “upside” of the delay was that a proliferation of PRMA assemblies across the District helped inform the assembly of green roofs at The Wharf. Several high-profile buildings that employ similar roofing systems include the MGM Casino, the National Museum of African American History and the National Archives.

A PRMA Approach to Support Performance

More than half of the roofs in The Wharf make use of an Owens Corning PRMA that includes Owens Corning FOAMULAR 404 and 604 extruded polystyrene insulation (XPS) supplied by PPSI Maryland. XPS delivers unique water resistance and strength properties that differentiate it from other insulation products and make it ideal for VRA applications. In fact, the demanding conditions on rooftops helped prompt the National Roofing Contractors Association (NRCA) recommendation to use only XPS insulation for VRA applications. The innate water resistance of XPS helps a roofing system retain its R-value and energy-saving power while retaining its compressive strength to bear a significant amount of overburden. Strength is critical in a PRMA application, as the insulation must be able to withstand the weight of vegetation, rainfall, growing media, pavers and surrounding surface materials.

Sustainability objectives supported by The Wharf’s VRAs include reducing storm water runoff, improving thermal performance, and providing habitats for wildlife. Photos: The District Wharf

At the highest levels, the rooftop plants and vegetation help serve as a giant sponge to absorb the rainwater. In a PRMA roof, insulation under the plant layer, growing media and filter/drainage layer is placed above a waterproofing layer which directs water horizontally to a series of overflow vaults. A network of massive 700,000-gallon cisterns throughout the District collect and control the release of water.

In addition to water, wind was another consideration when planning VRAs at The Wharf. Rock curbs from Hanover Architectural Products help mitigate against winds coming in off the riverfront. Other roofing materials that help The Wharf PRMAs achieve performance include waterproofing membrane 790-11 Hot Rubberized Asphalt by the Henry Company, EMSEAL expansion joints to tie together air barriers, and Hanover Architectural Products pavers and sedum mats supplied by Sempergreen.

Contractor Coordination and Communications Are Key

As a community-focused development, The Wharf celebrated its grand opening with several public events. Hometown rock band the Foo Fighters performed for a sellout crowd at the Anthem music center on October 12, 2017. The enormous task of completing multiple buildings against a very tight timeline was a challenge felt by all of the contractor trades on site, according to Brian Davis, general superintendent at James Myers, the roofing contractor charged with installing green roofs at The Wharf. The immensity of the project required careful scheduling and logistics among roofers and other trades. Teams worked throughout the night and seven days a week as Phase One approached completion.

Photos: The District Wharf

As the countdown to the October 12 grand opening approached, construction teams followed tight schedules outlining exactly what team members would be working in what area at a particular time. The logistical demands of building nine buildings over six blocks made traffic bottlenecks an ongoing challenge throughout the project. Scheduling details had to consider not only the District’s notorious rush hour traffic but also events and attractions in the area. For example, the Washington Nationals baseball team played a number of weeknight baseball games at home. Trades working on The Wharf were required to clear the streets three hours before the first pitch of each Nationals home game. While navigating event schedules and a tight construction schedule, contractors also had to maintain high levels of safety and quality.

The public event commemorating the completion of Phase One celebrated a one-of-a-kind public space winding along one of the nation’s most historic riverfronts with an epic rock concert. More quietly, the completion of Phase One celebrated Washington’s role as a leader in the installation of VRAs while helping the District achieve sustainability goals and comply with storm water management mandates.

MATERIALS

Insulation: FOAMULAR 404 and 604 extruded polystyrene, Owens Corning, www.owenscorning.com

Waterproofing Membrane: 790-11 Hot Rubberized Asphalt, Henry Company, www.henry.com

Expansion Joints: EMSEAL, www.emseal.com

Sedum Mats: Sempergreen, www.sempergreen.com

Pavers: Hanover Architectural Products, www.hanoverpavers.com

Douglas A. Munro Coast Guard Headquarters Boasts One of World’s Largest Green Roofs

The Douglas A. Munro Coast Guard Headquarters Building in Washington, D.C., has more than 550,000 square feet of green roof space. Photos: Owens Corning

Like a 550,000-square-foot sponge, the vegetative roof assembly (VRA) atop the Douglas A. Munro Coast Guard Headquarters Building in Washington, D.C., absorbs rainfall while managing its release into the city’s sewer system. Green roofs are a storm water management “best practice” in the nation’s capital, which boasts more than 3.2 million square feet of green roof space.

Gordon Contractors installed the green roof on the 11-level, 1.2-million-square-foot headquarters, which is also home to several independent field commands including the National Pollution Fund Center and Marine Safety Center. Key performance objectives included complying with D.C.’s stringent storm water regulations, as well as federal Environmental Protection Agency (EPA) rules requiring 95 percent of storm water to be collected onsite.

Beyond complying with local and federal storm water mandates, the project team creating the vegetative roof sought to create a rooftop habitat that supports biodiversity and helps birds and other wildlife thrive. The result of a careful and collaborative approach to moisture management is a high-performing roof that ultimately received LEED Gold certification from the U.S. Green Building Council. Working as a system, the roof’s components help reduce rainwater runoff while helping to reduce pollutant loads and protect natural resources such as the nearby Chesapeake Bay.

Numerous Challenges

As every roofer knows, no two projects are alike. The location of the Douglas A. Munro Coast Guard Headquarters on hilly terrain provided plenty of natural obstacles, including nine of the eleven levels being built into a hillside. The site

Gordon Contractors installed the green roof on the 11-level headquarters. Performance objectives included complying with D.C.’s stringent storm water regulations, as well as federal EPA rules requiring 95 percent of storm water to be collected onsite. Photos: Owens Corning

features a series of stair-stepped green roof terraces that help gradually move water from higher to lower levels and eventually into a pond through elevation changes of 120 feet. According to Steve Gordon, president of Preservation & Protection Systems Inc. (PPSI), the company providing design and installation support with the Owens Corning FOAMULAR extruded polystyrene insulation and Henry 790-11 Hot Rubberized Asphalt used in the VRA, the stair-step design required a complex approach to the roof’s varied elevations. To meet this challenge a complex network of expansion joints by EMSEAL tied together multiple air barriers within the system. Another challenge was the magnitude of the project, spanning more than a half-million feet across multiple levels and 176 acres. Timing was critical, as plants and vegetation required quick delivery and transplanting into their new environment, particularly during D.C.’s sweltering summers. These living materials couldn’t simply be covered with a tarp until better conditions prevailed, but had to be quickly transported, installed and irrigated.

The Douglas A. Munro Coast Guard Headquarters serves 4,000 occupants, so comfort and energy efficiency were important concerns. From the outset of the project, the integrated roofing team was tasked with designing a vegetative roof assembly that would help decrease the building’s heating and cooling energy usage, lower long-term maintenance costs and extend the life of the roof.

A Solution to Achieve Performance Goals

The project team selected a Protected Roof Membrane Assembly (PRMA) to deliver the water management required to meet storm water mandates, as well as deliver energy efficiency through R-value performance and strength to support vegetation requirements. The PRMA places the insulation layer above the waterproofing membrane, a reversal of traditional roof systems. PPSI recommended Owens Corning FOAMULAR 404 and 604 extruded polystyrene (XPS) insulations for use in the PRMA. The water resistance and compressive strength of the XPS insulation provided the integrity needed for long-term roof performance and helped the Douglas A. Munro Coast Guard Headquarters Building ultimately achieve LEED Gold certification.

The Douglas A. Munro Coast Guard Headquarters was built on hilly terrain, and nine of the eleven levels were built into a hillside. The stair-stepped green roof terraces help gradually move water from higher to lower levels and eventually into a pond. Photos: Owens Corning

Given the sheer size of the project, it was inevitable that the roofing team would encounter several scenarios requiring strategic problem-solving throughout the installation. For example, the team adjustedthe engineered soil specification to reduce the loads within the structural tolerances for the roof structure and carefully addressed a range of pH, moisture, organic matter and nutrient levels to support the variety of plants. Not only did such careful attention deliver strength performance, it’s also achieved the objective of attracting wildlife. The terraced landscape has welcomed not only birds and butterflies, but the occasional deer wandering onto a rooftop.

Evaluating Results

Since its completion, the VRA at the Douglas A. Munro Coast Guard Headquarters has attracted widespread attention from landscaping groups, engineering firms and organizations interested in sustainability. These groups are interested not just in the building’s unique design but in its resilience and performance. Steve Gordon of PPSI says the resilience of the headquarters’ roof is reflected in its record of no leaks. “We’ve had no leaks,” Gordon says. “The reason we use hot fluid applied waterproofing on green roofs is because we want to avoid any leaks in the building. At the end of the day, the biggest liability in a roof is water.”

After successfully navigating a range of challenges and opportunities, the U.S. Coast Guard Headquarters’ VRA epitomizes the convergence of aesthetics and performance and is recognizedas one of the largest green roofs in the world.According to the Landscape Performance Foundation, the headquarters’ vegetative roof retains up to 424,000 gallons of rainwater.

In a small way, the performance reflects the integrity of a hero an Act of Congress honored when naming the headquarters, according to Captain Will Smith, Commanding Officer U.S. Coast Guard Base NCR. Captain Smith noted, “The Coast Guard’s only Medal of Honor recipient, Douglas Munro earned the award for his selfless sacrifice as a landing craft pilot at Guadalcanal while evacuating marines from a beachhead under heavy fire from enemy forces.”

TEAM

Architect: WDG, Washington, D.C., www.wdgarch.com
General Contractor: Clark Construction Group, Bethesda, Maryland, www.clarkconstruction.com
Roofing Contractor: Gordon Contractors, Capitol Heights, Maryland, www.gordoncontractors.com
Independent Rep Agency: Preservation & Protection Systems Inc. (PPSI), Laurel, Maryland, www.ppsimd.com

MATERIALS

Insulation: FOAMULAR 404 and 604 extruded polystyrene, Owens Corning, www.owenscorning.com
Waterproofing Membrane: 790-11 Hot Rubberized Asphalt, Henry Company, https://us.henry.com
Expansion Joints: EMSEAL, www.emseal.com
Pavers: Hanover Architectural Products, www.hanoverpavers.com
Sedum Mats: Sempergreen, www.sempergreen.com