May 31, 2005
ASLA Green Roof Design Nearly
MVVA and CDF present detailed designs and planting
Last week representatives from Michael Van Valkenburgh
Associates, Inc., and Conservation Design Forum, Inc., came to ASLA
headquarters to present what may very well turn out to be the final
design concepts for the ASLA green roof project. The major elements
of the roof remain intact (see
“ASLA Green Roof Rounding Out” in the May 2 edition
of LAND Online), but several of the elements have been
tweaked to ensure that the roof provides the Society with the best
and most varied demonstration green roof possible. Michael Van Valkenburgh,
FASLA, Chris Counts, ASLA, project manager and associate in charge
of the project, and John Gidding, project designer, presented the
overall design and technical aspects of the roof; while David Yocca,
ASLA, project principal for CDF, presented on plant selection.
In presenting the tweaked design of the roof, Van Valkenburgh noted
that the north “wave” would now look more like an asymmetrical
sloped ramp rather than a symmetrical roll. Van Valkenburgh noted
jokingly that this change “may offend the strict formalists,”
quickly adding that, on a more serious note, the new design will
increase the visibility of the roof from the street and give those
on the roof a greater sense of “being held in” by the
waves. It will also create new microclimates for the plantings by
more drastically changing how much water is held on the surface
of the wave. Plants higher on the wave will clearly receive less
water than those lower on the slope, meaning the wave must now have
a semi-intensive green roof system. The south wave will not change
drastically from the original design, Van Valkenburgh said, but
will continue to be symmetrical design, covered by an extensive
green roof system.
In another change, the width of both waves will be
drawn in, providing space for visitors to circulate around the forms
so they can be fully viewed and understood. The edges of the forms
would be encased in brushed metal, Van Valkenburgh said, while also
floating the idea of sealing one end in transparent plastic so visitors
could see a cross section of the wave’s construction. “So
we’d have a Museum of Natural History moment there,”
he said, “a kind of ant farm effect.”
Expanding the platform
Another major change to the design is the expansion of the grated
viewing program. Under the original design, a grate with sedums
planted underneath composed a relatively small part of the roof—a
viewing platform set between the two waves. With the narrowing of
the waves, that area has now been expanded to include walkways around
the waves. This will allow the entire roof to be covered by plantings.
The grill will be three inches off the surface of the roof, Van
Valkenburgh said, meaning that the sedums planted underneath have
a good chance of actually growing through the grillwork. “This
is a really nice thing that CDF has brought to the project that
will really change the ambience that’s up there,” he
Few changes to the stairwell
Although the design team and the ASLA Green Roof Task Force debated
several changes to the stairwell, most of the elements of this structure
were left intact. One exception to this is that earlier plans to
have a glass enclosure jutting off the structure—creating
a sort of observation greenhouse—have been all but scrapped.
Also, earlier discussions to place an awning on the structure have
given way to creating natural shading using a vine trellis that
would be perpendicular to the roof.
At this point, the designers are still planning to
move the heating and air conditioning equipment to the top of the
stairwell structure and placing intensive plantings around them.
These plantings would be partially irrigated by the runoff from
the air conditioning units.
courtesy of Michael Van Valkenburgh and Associates, Inc.
The roof structure
In addition to the design elements of the project, MVVA’s
Gidding also presented some further details on the structure of
the roof. Gidding noted that between the waterproofing, and drainage
layers—standard elements of a green roof—the team will
be using a fabric protection layer, which he described as a soft,
fleece-like material. This layer will add an increased level of
protection for the waterproofing layer as the roof is constructed.
Another interesting addition will be the use of trex beams, which
will be added to the mound cladding. The beams will help distribute
the weight placed on the viewing platform more evenly across the
roof. Under the current design, the roof would be able to have an
occupancy rating of up to 20 people. The roof would actually be
able to hold many more people, but fire codes dictate the occupancy
After Van Valkenburgh completed his presentation on the roof design,
David Yocca presented the firm’s proposals on the plantings
that will be used on the green roof. Yocca said that generally the
plant selection would maximize the demonstration aspect of the roof,
liberally mixing intensive and extensive plantings.
The sedums used underneath the grating, Yocca said,
would be pregrown material, which will already be established and
should help sustain it in the environment under the grating. The
moisture gradient of the mats will range from dry to mesic, allowing
the plants to “move” and find suitable moisture locations.
Yocca recommended using pregrown mats from Xero Flor—the same
material that was used on the Ford Motor Company’s Dearborn,
Michigan, truck plant—for these plantings, but he added that
the firm will do additional research on any companies that provide
this type of product and bid out the contract accordingly.
For the south wave, which would also be an extensive
roof, CDF again recommended pregrown mats that would contain plantings
that are somewhat longer and thicker than those used under the grating.
The north wave would contain an eight-inch growing medium and would
be planted with plant plugs and some clippings. Above the stairwell
structure, Yocca recommended planting shrubs and other more robust
material since they will be partially irrigated by the condensation
from the air conditioning units.
“Our overall objective is to create a
living landscape that is, in some ways, a barometer of the surrounding
environment,” Yocca said of the planting selection. “This
fits with our philosophy of helping people reconnect with nature
and understand, even in an urban environment, what is going on around
View of the roof
from street level
image courtesy of Michael Van Valkenburgh
and Associates, Inc.
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