ASLA Green Roof Enters Final Phase of Construction
Representatives from Michael Van Valkenburgh Associates Inc. and Forrester Construction provide construction watch tour of the ASLA green roof in conjunction with the National Building Museum.

Last weekend, as ASLA was gearing up for the formal dedication of its green roof landscape, the Society and the National Building Museum hosted a construction watch tour for more than 20 landscape architects, architects, and interested laypeople to demonstrate how the project will lessen the environmental impact of the building, while at the same time creating a valuable and well-designed open space for the building’s occupants. The tour is the first of several high-profile events that will focus on the green roof project this month, including a press tour on April 25, and the formal dedication on April 26. Before the tour of the roof, participants heard background presentations on the project from Chris Counts, ASLA, project manager for Michael Van Valkenburgh Associates, the landscape architect for the project; John Fritz, senior project manager for Forrester Construction, the general contractor on the project, and Frank Pinto, project development for Forrester Construction.

Design
Counts opened the presentation by noting that the project began when ASLA, finding that it needed to replace its building’s existing roof, decided to undertake a green roof demonstration project that would show not only the environmental and structural benefits of green roofs, but also—and perhaps more importantly—demonstrate how landscape architects can impose their unique talents on a green roof to create a new open space for the building’s occupants. Counts noted that this final aspect of the project is what most intrigued Michael Van Valkenburgh, FASLA, whose firm won the competition to design the roof.

“Michael wanted to explore the design possibilities of green roofs, and really make a place for people,” Counts said. “He wanted to make a statement about what landscape architects have to contribute to the green roof phenomenon.”

In creating a green roof landscape for ASLA, Counts noted that MVVA was forced to deal with several challenges to retrofit the existing structure, not the least of which was extending the building’s staircase to the roof, as the only pre-existing roof access points were two trap doors on the third floor. The roof surface also presented many challenges, including several heating and air conditioning (HVAC) units that would have to be moved to create a viable public space, drainage points that would have to be relocated for the same reasons, and varying structural capacity throughout the roof, not to mention ensuring the space complied with OSHA regulations, and had a waterproofing system that would not leak.

Counts noted that Van Valkenburgh’s design calls for extending the staircase from the third floor, and creating a stair tower that would house the entrance to the roof and cover an existing elevator shaft. The tower would be cinderblock construction and remain untreated to maintain the industrial urban landscape created by the neighboring rooftops. The stair tower and elevator shaft cover would also allow for relatively deep planting beds (24 inches of soil and 12 inches of soil respectively) that could hold larger plantings. Some of these plantings will be visible from a skylight aperture in the stair tower.

Once on the green roof landscape, Counts said, Van Valkenburgh wanted visitors to feel as though they were in a “boundless space”—as if they were occupying a large open space, rather than a relatively tight rooftop. To create this sense, he designed two large waves on the north and south ends of the landscape that would be planted and rise up from the rooftop, surrounding occupants in green and holding them on the roof. These waves will be constructed of corrugated metal—again to reinforce the industrial feel of the surrounding roofscape—and filled with rigid foam insulation. The insulation would be held in the waves, and protected from wind damage, by a wire netting. According to Counts, the soil in the waves will be a constructed soil provided by American Hydrotech Inc., held in place by a honeycomb structure to prevent erosion. The HVAC units will be hidden from view behind the waves, and in one case tucked under the north wave. The top of the north wave will also be visible from the street, thereby “advertising” the green roof to passersby.

The third major element of the design is a thin, three-inch planting surface covered by aluminum grating that will ensure maximum coverage of green roof material, while at the same time allowing occupants to have full access to the roof. In introducing this feature, Counts noted that most green roofs “cannot be seen, cannot be accessed, and cannot be occupied,” but that the ASLA green roof landscape will solve all three of these problems The grating placed over the planting surface will be made of non-skid, recycled aluminum supplied by Ohio Gratings. The aluminum grating will not conduct heat as readily as steel grating, and because it’s recycled, it will lessen the environmental impact of the roof construction. The aluminum will be laid so that its vertical lines guide occupants around the roof and visually disappear within the green roof planting materials, Counts noted. He also said that as occupants walk over the grating, the planting materials will “get a haircut” in high-traffic areas, but will be allowed to grow thickly in low-traffic areas, creating high spots on the roof.

The construction process
After Counts completed his presentation, representatives of Forrester Construction presented on the challenges of the construction process. Pinto began by providing an overview of Forrester’s commitment to sustainable design and LEED standards. He noted that the company has a LEED-certified director who provides training for Forrester’s project managers and aids clients in the documentation process when working on LEED-certified projects. In addition, the company has seven LEED-certified senior executives, with more going through the training process. The company has also drafted a field guide for sustainable projects, which is used for all projects whether they are applying for LEED certification or not. Pinto also noted that because one of its principals, Scott Forrester, is a landscape architect, the company had a personal interest in working on the ASLA project.

After Pinto’s overview, Fritz outlined the challenges Forrester faced during the construction process. He noted that one of the primary challenges was creating the structural steel for the waves, and ensuring that they matched the specifications set out by MVVA Inc. This was particularly challenging given that they were a unique design element to the project. He also noted that, because the waves are connected to the roof structurally, below the waterproofing membrane, ensuring that this area of the roof is leak free was critical. It was also difficult for the builders to know exactly where the waves’ support beams could be connected to the building without punching multiple holes through the existing roof.

Fritz also said that it was challenging working on a project of this size while the building was occupied, particularly when it came to building the stair tower. Because the tight quarters around the building did not permit a crane to be brought in, the cinderblocks for the staircase had to be brought up using an external construction elevator, and then taken across the building using dollies. Also, waterproofing around the stair tower was done “on the fly” as each layer went up to ensure leaks were kept to a minimum. As for the stairwell itself, the metal stairs had to be taken across the roof and lowered into the stairwell by Forrester workers. Concrete was then used to fill in the stairs. Again, all this was done while considering the safety and relative comfort of the ASLA staff. In fact, relocating the HVAC units was the only portion of the building that was done while staff was out of the office.

Of course, walking on a roof that has been waterproofed is never a good idea, let alone taking part in a large construction project with industrial steel, so Forrester performed the major portions of the construction prior to installing the final waterproof membrane, only performing spot waterproofing when necessary.

Apart from the actual construction, there was myriad paperwork that Forrester representatives were asked to deal with as well. Due to the high-profile nature of the project, ASLA received several product donations for the green roof landscape along the way. Working with ASLA staff and MVVA Inc., Forrester’s project managers analyzed each product donation to ensure it was suitable for the project, and worked with the company donating the product to provide the proper specifications. Also, Forrester guided the permit process, which was made more difficult by the complexity and uniqueness of the project.