MAY 2, 2005

ASLA Green Roof Rounding Out
Designers discuss three preliminary elements to the roof.

In a conference call late last month with the ASLA Green Roof Task Force, Michael Van Valkenburgh, FASLA, discussed some of the preliminary elements Michael Van Valkenburgh Associates Inc. (MVVA) and Conservation Design Forum (CDF) are considering as possibilities for the ASLA green roof. During the conference call, Van Valkenburgh described three tentative elements of the roof with the task force: two half cylinders, or “waves” of vegetation that would dominate the roof, an observation platform that would consist of a grated steel walkway placed over plantings, and a masonry structure that would house the stairwell and elevator shafts. Here are some details for each element:

Detail of vegetation "waves" as seen from above. courtesy of MVVA Inc.

Waves of vegetation
Each side of the roof could be covered by two half cylinders planted with vegetation. These cylinders would be made of a lightweight foam that has been used in other projects, including Van Valkenburgh’s Kraus Campo project at Carnegie Mellon. The material has the triple-threat advantages of being lightweight, durable, and cost-effective. The idea behind the cylinders is that they will envelop visitors to the roof as they came up the stairs in a pasture of green and only reveal the surrounding roofscape when a visitor is actually standing on the observation platform. To regain the feeling of being enveloped by the roof garden, a visitor will need only sit down and the roofscape—and in essence, the city—will disappear.

The Task Force had several questions regarding the concept of the cylinders, including whether passersby would able to see the cylinders from street level. Because educating the public about green roofs is a major component of the project, ASLA wants to ensure that as many people are exposed to the roof as possible. Van Valkenburgh assured the Task Force that the cylinders will be viewable from the street, and will, in fact, “look like the back of a whale rising out of the water.” He added the firm would run computer models to show how the cylinders would look from street level and would create string models of the cylinders on the roof to demonstrate more concretely that they could be seen from the street.

Another intriguing idea discussed was creating two types of plantings on the otherwise matching cylinders. This could create a kind of green roof science experiment, with vegetation planted on one cylinder in a traditional way, while laying only soil on the other cylinder and allowing nature to take its course. On this cylinder, birds and the wind would be responsible for “planting” by depositing native seeds in the soil. Van Valkenburgh noted that this technique has worked on German green roofs, but he acknowledged that the design solution would have to include a way to secure the loose soil so it won’t blow away.

Detail of the observation platform walking survace. courtesy of MVVA Inc.

The observation platform
The innovative idea for the observation deck was first proposed by CDF, Van Valkenburgh said, and received praise from the jury. Under the design, the platform would be raised from the surface of the roof, allowing enough room to lay down soil and a thin layer of plants, before putting down an industrial steel grate. Plants would literally grow up through the floor. This solution would ensure that the roof is as green as possible, with almost every square foot covered in vegetation.

Detail of stairwell structure with glass enclosure. courtesy of MVVA Inc.

The stairwell structure
Due to fire codes, a “solid masonry structure” must be built over the roof access. Although this will create some additional expenses for the project, it may help to solve the practical issue of where to put heating and cooling equipment that now sits on the roof. The firm suggested that some of these units could be moved off the main roof and onto the stairwell structure and be surrounded by shrubs and other plantings. This would create more room on the roof, and the plantings on the structure could be partially irrigated by the runoff from the cooling units.

In one version of the plans, a glass structure would be built off the stairwell structure, creating an observation greenhouse, rather than just an observation platform. This would have the dual advantages of allowing four-season access to the roof, as well as shelter during inclement weather. Both the Task Force and Van Valkenburgh agreed that while this would be nice to have, budget permitting, it is not a necessity. Other options for shade and shelter on the viewing deck were discussed, including a metal industrial structure supporting an arbor of vines and a retractable awning.

Discussing trees
Van Valkenburgh also told the Task Force what had been discussed with CDF regarding the inclusion of trees on the roof. Since significant structural support would have to be built around the building, at a great cost to ASLA, in order to support trees, the Task Force concurred that this was an option the Society was not likely to explore. Second, it was a case of “just because we can, doesn’t mean we should.” Again members of the Task Force emphasized that this was to be a teaching roof and should present solutions that can be emulated across the city and within a reasonable budget.

Ultimately, Van Valkenburgh agreed, saying all three elements—the dual cylinders, the stairwell structure, and the viewing platform—were “working within the bounds of the project,” and he added that a green roof does “run the risk of being overdone.”

The two firms will continue to refine their initial designs for the ASLA green roof in close consultation with ASLA. They are scheduled to return to Washington, D.C., for an on-site meeting in the near future to discuss these preliminary designs.

Detail of the green roof's major elements. courtesy of MVVA Inc.

Click here for boards and sketches presented during the conference call.

Email this Article to a Friend

Back to LAND Online

.