XPS + building science = Three high-performing roofs

The commitment of Washington, D.C., to responsible stormwater management has helped it become a leader in the use of vegetative roof assemblies. Photo courtesy the Wharf, Washington, D.C.
The commitment of Washington, D.C., to responsible stormwater management has helped it become a leader in the use of vegetative roof assemblies.
Photo courtesy the Wharf, Washington, D.C.

While inside the arena, attendees enjoy a range of entertainment options from sports and concerts to, of course, rodeos. On the plaza outside, people gather to take in a concert at the pavilion, grab a glass of wine at the ‘cork yard,’ and even get a preview of rodeo animals and handlers prepping for events inside the arena. Windows in the plaza deck let visitors turn their gaze down to the action below.

With so many possible uses for the plaza, the materials comprising the PMRA must safely support foot traffic, pavers, furnishings, and the vegetative assembly portions. Compressive strength is essential to supporting vehicular traffic, including emergency vehicles like firetrucks and tractor trailers filled with concert gear.

To lend strength to the project, the design team opted for XPS insulation for the roof. XPS is used in PMRAs for its high compressive strength values, available up to 690 kPa (100 psi). When installed above the waterproofing membrane, XPS delivers a high level of moisture resistance.

Grade and slope also factor into the roof’s ‘sandwich’ design. The design team required a material that would raise the height of the roof over the building below by more than a foot, while also supporting 200 mm (8 in.) of concrete, sand, and pavers above. Drains located in the bottom concrete layer of the deck sloping in one direction are often attached to inconspicuous linear drains located a foot above the structural deck in the paver roof, which often slopes in a different direction.

Compressive strength was not the only challenge facing the team working on the Dickies Arena PMRA. The 13,006-m2 (140,000-sf) deck was massive in scale, including:

  • layers of reverse tapered insulation in order to create a flat surface;
  • flat fill insulation to raise the overall height of the roof; and
  • tapered insulation to slope to the drains embedded in the top roof layer.

Layers of strength and performance

The roof’s design resembles a sandwich. At the lowest level of the assembly, a waterproofing membrane was hot-mopped over the concrete slab. Located at deck level are 261 drains. Sitting atop the deck layer is the XPS insulation. The next layer in the plaza is made up of filter fabric and a sand bed. Finally, the roof is finished with pavers and planters filled with trees. At the surface level, slot drains collect water and carry it down the assembly to the 261 drains below.

The Dickies Arena plaza deck gets plenty of traffic as the warm Texas weather welcomes visitors to enjoy the outdoors. With a clever mix of design and functionality, the green spaces of the plaza blend well with the venue’s architecture. Visitors can take in the city’s landmarks, get geared up for rodeo action, and soak up a gorgeous Texas evening, and if the state experiences one of its infamous deluges, the plaza deck is engineered to manage the rainwater.

Mixed-use VRAs manage moisture, maximize space

A complex of nine buildings spanning six blocks and a mile of Potomac riverfront, the Wharf in Washington, D.C., is more than a development—it is a whole new neighborhood that has been two decades in the making. As a public-private initiative, the Wharf epitomizes mixed-use with high-end hotels, condominiums, retail shops, commercial offices, and a 6000-seat music auditorium—a new destination for live, work, play, and tourism in the heart of the nation’s capital.

While the structures and functions making up the Wharf are diverse, VRAs are a common element across the development. The roofs incorporated at the Wharf continue a trend seen all around Washington, D.C., which features more than 278,709 m2 (3 million sf) of green roofing. In the district, VRAs are not just trendy, but part of the city’s sustainability and infrastructure efforts.

Sustainable solutions

The Potomac River may be the most famous waterway in Washington, D.C., but the city also sits close to the Anacostia River, Rock Creek, and Chesapeake Bay. In recent years, the district has taken more steps to protect these waterways and better manage stormwater runoff. VRAs have proved to be a key part of this strategy.

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