By Chris Kann
Effectively managing stormwater runoff can be complex and challenging for urban planners and developers. In Washington, D.C., where the city is susceptible to the negative impacts of stormwater runoff and sewer overflow events, managing stormwater runoff is critical for protecting public health, the environment, and the city’s infrastructure.
It may be no surprise, then, that in the District of Columbia, where the Department of Energy & Environment is the authority for overseeing stormwater management, there are strict construction requirements for on-site stormwater retention, with the overall goal of reducing the burden on the city sewer system to prevent pollution from reaching natural water bodies. The District’s Stormwater Management Regulations stipulates construction sites “achieve retention of the rainfall from a 1.2-inch rainfall event, which is the ninetieth (90th) percentile rainfall event for the District of Columbia, measured for a twenty-four (24)-hour rainfall event with a seventy-two (72)-hour antecedent dry period.”
So, when it came to specifying the new roofs at the former Walter Reed Medical Center site in Northwest Washington, managing stormwater collection and runoff was a high priority.
The redevelopment site
The Parks at Walter Reed is a 26.71-ha (66-acre) land development of the historic Walter Reed Army Medical Center site, which, when fully developed, will contain 287,999-m2 (3.1-million sf) of new construction and adaptive reuse of historic structures, including retail, for-rent and for-sale multifamily units, townhomes, offices, healthcare facilities, a school, and a hotel. The development will contain approximately 20,438 m2 (220,000 sf) of retail, more than 27,871 m2 (300,000 sf) of office space and ambulatory care, a 200-key hotel and conference center, a top-performing language-immersion school, and 2,787 m2 (30,000 sf) of space for creative and arts use.
In addition, the project will include nearly 2,100 housing units to serve a diverse range of incomes, providing both rental and homeownership options. It will feature new roads, infrastructure, and nearly 8 ha (20 acres) of parks and plazas, all employing energy-saving and water-conserving technologies.
Two of the sites’ new buildings—The Hartley and Clover at the Parks—are six-story mixed-use facilities. Both are stick-built over concrete podiums and have wood roof decks. According to Washington’s strict stormwater management system, each had to be designed to treat and manage stormwater and meet a specific green area ratio on-site.
The facilities were designed and specified by the Washington office of the global architectural firm Torti Gallas + Partners and managed by Julian Goldman, associate principal, and Scott Welch, senior principal.
“This is a significant redevelopment project involving mixed-use residential and retail facilities,” says Goldman. “The Hartley has a grocery store, fitness center, and pet grooming business at the street level plus 323 apartments above that range in size from studios to three-bedroom units. The building features two interior courtyards: one with a pool and an amenity space and the other with a Zen Garden. The Clover at the Parks building is a 60-unit co-living community comprised of 248 individually rentable bedrooms in two- to five-unit configurations. The facility also includes 2,229 m2 (24,000 sf) of street-level retail space, a lounge, library, fitness center, conference room, game room, elevated courtyard areas, and a sixth-floor observation deck.”
Elite Sheet Metal Works (Elite) of Beltsville, Md., installed the roofing systems on both LEED Gold mixed-use facilities. The two facilities include 7,822 m2 (84,200 sf) of roofing and approximately 4,775 m2 (51,400 sf) of vegetative green roofing, which accounted for approximately 61 percent of the total roof area.
Green roofing solution
Since this was new construction, the roofing work had to be completed in sections as construction allowed. In addition, it was a very congested and tight construction area, so the metal contractor had to load the roof several times. Its crew of eight to 12 worked across each facility while the roof was installed.
The roofing installation began amid the COVID-19 pandemic and the related supply chain issues. Securing the materials needed for the project was challenging. Still, in partnership with the roofing system manufacturer based in Carlisle, Pennsylvania, Elite was able to complete installation in a timely fashion.
The roofing assembly
Both facilities have the same assembly. From the plywood deck up, the roofing assembly consists of a self-adhered vapor and air barrier, a 40-mil membrane composed of 35 mils of self-adhering rubberized asphalt laminated to a 5-mil woven polypropylene film.
“The vapor barrier also acts as a temporary roof, so once that was fully down, the facility was essentially dried-in,” says Joe Gretchen, an owner and principal with Elite.
With the vapor barrier down, the crew loose-laid two layers of 66-mm (2.6-in.) polyisocyanurate (polyiso) insulation, with staggered joints to reduce thermal bridging. The insulation was topped and protected by a layer of 1.2- x 2.4-m (4- x 8-ft) sheets of 12.7-mm (0.5-in.) gypsum cover board, which provided a flat and firm substrate for the green roof assembly.
Once the insulation and cover board were in place, an 80-mil white thermoplastic polyolefin (TPO) roofing membrane was installed. The TPO membrane is made with advanced polymerization technology that combines the flexibility of ethylene-propylene (EP) rubber with the heat weldability of polypropylene and offers a weathering package for long-term performance.
The crew fully adhered the membrane with a single-component, low-VOC (<250 g/L), spray-applied aerosol contact adhesive and primer.
After installing the membrane, a cover strip was applied to all the seams in areas designated for overburden installation. This added layer of protection ensures enhanced watertight integrity.
Both rooftops include 762-mm (30-in.) tall parapet walls around the perimeter. To terminate the membrane, the crew mechanically fastened the TPO at the base of the parapet and then welded another section of the membrane at the base, which was secured up and over the top using the single component adhesive. To finish the termination, a shop-fabricated coping cap was installed on top of the parapet wall. A 1-m (3-ft) wide walkway around the perimeter and to the mechanicals allows for periodic roof inspections and maintenance access to the mechanical equipment on the roof.
Green roof sections
The first step for installing the green roof was to map out the specific areas on each roof section that would eventually house the growth media. For this, the crew snapped lines and installed an aluminum garden edge, which they secured directly to the TPO membrane using a membrane primer and 76-mm (3-in.)-wide tape.
“Laying out each area and installing the aluminum edge was somewhat time-consuming because we had to be precise with the measurements,” says Gretchen. “Plus, we had to build a separate area at the base of the penthouse for a deeper layer of growth media for a green screen.”
On the Hartley, a penthouse mechanical enclosure stands about 2-m (7-ft) tall in the center of the roof. The architect designed a ‘green screen’ installed in front of the structure to increase the green area ratio on the roof and hold climbing vines that would eventually hide the structure from the surrounding buildings. In addition, some mechanical and vent units were in each green area on 0.3-m (1-ft) tall curbs that had to be carefully flashed before they could install the growth media.
Once the green areas were mapped out and the aluminum edging was in place, Elite loose-laid a non-woven polypropylene fabric in each area. The material is designed to resist soil, chemicals, mildew, acids, and alkalis, and protects the TPO membrane below.
The next step was loose laying a drainage composite consisting of non-woven filter fabric bonded to a molded polypropylene core that offers superior filtration and prevents small particles from clogging the drainage channels in the aluminum edging.
A highly absorbent moisture retention blanket is often essential for projects with stringent stormwater management requirements. In this case, to meet the specific stormwater needs of the project, two layers of 25.4-mm (1-in.) thick mineral wool blankets were installed. These layers were designed to optimize water retention and filtration while promoting root growth and fostering healthy vegetation.
A layer of 101.6-mm (4-in.) of growth media was then installed over the top. For this, the crew used an outside service to blow the growth media onto the roof and into each garden area. Then, the crew carefully raked the media to the correct depth and compacted it with a 113-kg (250-lb) roller.
The last step was to install sedum mats, which are pre-grown with 17 different varieties of sedums for applications in most climate zones. The mats were about 1- x 1.98-m (3- x 6.5-ft)-long.
“We first wet down the growth media and then rolled out the mats to fit within the garden areas. The nice thing with the mats is instant vegetative coverage, so the roof looks great from the start,” explains Gretchen.
To keep the roof garden green and growing, the Hartley has a 90,000-gal (340,687-L) cistern in the basement that collects water runoff from the building. That water is pumped back to the roof to a pop-up sprinkler system installed in the garden areas for watering. The irrigation system can be activated for sedum maintenance as needed. It also automatically runs after a rain event to contribute to the site’s required stormwater treatment capacity.
For the observation deck at Clover at the Parks, a 111.5-m2 (1,200-sf) plaza paver system was installed utilizing nominal 610- x 610- x 50.8-mm (24- x 24- x 2-in.) concrete pavers and adjustable height pedestals. Elite installed the pavers and pedestals over the same non-woven polypropylene fabric installed in the green roof areas, which provides physical protection to the membrane both during and after the paver system installation. In contrast, the adjustable pedestals ensure a perfectly level assembly. Tables and chairs were placed on top of the plaza paver system so that the residents could enjoy the natural space.
After the installation, the roofing manufacturer issued a single-source 20-year overburden warranty. This warranty, an extension of the roof system warranty, covers the green roof and plaza paver systems, including removal and replacement in case of a claim. Single-source overburden warranties, available exclusively through roofing system manufacturers, offer building owners a single point of contact for any warranty claims, providing peace of mind and protection from unexpected and costly liabilities.
The green roofs at the Parks at Walter Reed offer a wide range of benefits for the community and the building. Not only do they help alleviate stormwater runoff, but the green roof can help extend the life of the roofing membrane by protecting it from extreme temperature fluctuations and ultraviolet radiation. In addition, they help reduce the local urban heat island effect since they minimize emissivity and can help lower nearby air temperatures. Lastly, the green roof and plaza paver systems provide a space for residents to gather, enjoy the space, and breathe in the aesthetically pleasing natural beauty of the green roof, which contrasts nicely against the city views of Washington, D.C.
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Author
Chris Kann brings more than a decade of experience to his role as product manager for specialty systems and sustainability at Carlisle Construction Materials. Since joining the company in 2010, he has dedicated himself to innovation and sustainability, leveraging his degree in the science of plastics and polymer engineering technology. Throughout his career, Kann has spent considerable time assisting architects, specifiers, consultants, and representatives in the design and implementation of environmentally friendly overburden and building envelope systems.
