The entire roof system was manufactured by Seele in Germany with representatives from its New York office brought in to oversee installation.

Because the roofs would span multiple structures and be exposed to the winds blowing off the Potomac River, structural analysis was a critical element of the complex design. To minimize loads on the concrete-structure buildings, designers opted for a mix of sliding and spring connectors that would allow for up to 2 in. of expansion.

Portions of the roof also cascade off the edges. Along the south elevation facing the National Mall, one “wing” of the roof extends 40 ft away from the building. Three 35-ft-long steel props fan out from a horizontal structural beam along the fa�ade and meet the roof near the middle of the cantilever, leaving the tip of the roof untouched.

“Those moving connections were the most challenging part of the design,” Goodwin says. “Once you allow the fa�ade to move a few inches, then you are dealing with the tie-ins to other materials. To the north there’s a metal roof it needs to tie into. Along the roof, there’s also a PVC membrane it needs to tie into, and above the roof terrace there are fixed metal flashings. You need to create expansion joint details to allow this structure to move significantly compared to the building.”

Since the roof was built over vast atria spaces, a complex scaffold system had to be designed that would not only fill the void between the buildings but would also be precisely tiered to match up with the curvatures of the roofs. The five-story high, 20,000-sq-ft horizontal scaffold is fully loaded down to slab on grade at the parking level below the main floor.

Reshoring was added in the garages to support the scaffold above. After several months of design, the scaffold took nearly six weeks to assemble.

Construction of the roof began in June 2009. Sections of the shell were preassembled on the ground using mostly bolted connections to minimize welding in the field. Each piece was then flown in with a tower crane.

Once the entire shell was set, the scaffold was designed to allow the crew members to stand in each frame and receive each 200-lb glass panel as they were individually picked and placed by the tower crane. Placement of the first glass panels began in October and work was completed in February.

Substantial completion for the entire project is scheduled for late this year with occupancy in early 2011.

With the unique roof system now in place, the signature white dove has already revealed itself to D.C. residents and throngs of tourists. John Stranix, owner’s representative for USIP, said the building at first looked like “your typical Washington building,” but with the roof in place, people have taken notice.

“This is going to be an important addition to the Mall,” he says.