| July 11, 2005
ASLA Green Roof Design Finalized
Board of Trustees gives final approval for budget.
In separate conference calls held last week, the
ASLA Green Roof Task Force approved the
final design for the roof, while the board of trustees gave
final approval to the project’s budget.
In the first conference call, representatives from
Michael Van Valkenburgh Associates presented two final versions
of the green roof design to show where heating, ventilation, and
air-conditioning (HVAC) units will be placed on the roof. In the
first option, which the Task Force ultimately approved, the HVAC
units will be placed next to the south mound of the green roof so
they are not visible to those standing in the central viewing platform
but can be seen by walking around the mound. This will be useful
when giving demonstrations of the roof, because the units—which
are a major consideration when doing a commercial green roof installation—can
be pointed out during a demonstration.
Other advantages of this configuration include the
fact that there will now be more planting area on the roof of the
stairwell structure, and the cost of moving the units is somewhat
reduced. However, the move does create a smaller south mound, which
will no longer be symmetrical with the north mounds, and a slightly
smaller viewing platform for visitors.
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Click the image
for a .PDF of the final green roof plans
Rendering the ASLA green roof
(Image courtesy of Michael Van Valkenburgh and Associates,
Inc.) |
Debating passive planting
After it was decided which design would get
the ultimate seal of approval, the Task Force began an extended
debate on whether the roof would experiment with passive planting—that
is, leave one portion of the roof with just soil and allow the elements
to “plant” the roof with vegetation. Michael Van Valkenburgh,
ASLA, noted that the angle of the mounds would not allow for passive
planting, and the only area that was conducive to the experiment
was the newly cleared stairwell structure roof.
However, some members of the Task Force noted that
using passive planting on that structure would eliminate any hope
for more vigorous plantings such as shrubs and taller vegetation.
This, they contended, might make the roof less visible from the
street. Others noted that access to that portion of the roof would
be limited—with only a ladder going up the side of the structure—meaning
the passive planting may be difficult to monitor. Van Valkenburgh
also cautioned that “anything that grows up there is going
to come off to people as weeds,” while MVVA’s John Gidding
noted that those perceived weeds may spread to other areas of the
roof after taking root.
Still, the idea of allowing passive planting to take
place somewhere on the roof captivated the Task Force, and it was
decided that the stairwell structure roof was where this would take
place. But with no guarantee that the passive planting would work,
the Task Force agreed that alternative plans would be made to plant
the stairwell roof in the event that the experiment failed.
The
ASLA Green Roof at a Glance
Here are the elements of the ASLA green roof, along with
the types of plantings that will be used with each element:
Viewing
platform-The 1,250 square-foot viewing platform will
provide contiguous coverage of the portions of the roof
not covered by the north and south mounds or HVAC units.
The planting system will have a three-inch depth, planted
with sedums. The platform will have a flat metal grate suspended
approximately one inch above the plantings.
South
mound-The south mound will have a five-inch planting
system, planted with sedums. The mound will be constructed
of Styrofoam, and the planting soil will be threaded with
ribbons of non-biodegradable plastic to prevent erosion
and sliding.
North
mound-The north mound will have a six-inch planting
system, planted with sedums. The mound will be constructed
of Styrofoam, and the planting soil will be threaded with
ribbons of non-biodegradable plastic to prevent erosion
and sliding.
Stairwell
structure-The stairwell structure will be a simple
building constructed on top of the building’s roof
to cover the access points. The structure will have glass
glazing at the front to provide a view of the roof during
inclement weather. The roof will be covered with soil to
allow passive planting experimentation.
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