May 31, 2005
CDF’s David Yocca on the
ASLA Green Roof
LAND sat down with David Yocca, ASLA, to discuss
the green roof project, the design process, and the objectives of
David Yocca, ASLA, discusses
the ASLA green roof during a meeting with the Green Roof Task
After his presentation to the ASLA Green
Roof Task Force, LAND Online sat down with CDF project
principal David Yocca, ASLA, to discuss his firm’s involvement
in the ASLA green roof. CDF, which designed and implemented the
green roof for the Chicago City Hall, is consulting on the ASLA
project for the lead firm Michael Van Valkenburgh and Associates.
What do you see as the philosophy of the
ASLA green roof? Is it more about educating the public on the concept
of green roofs, is it a design-driven project, or is it most important
to demonstrate the environmental impact of a green roof?
This project is about how all those things relate
to each other and how thinking about them all at the same time—rather
than taking a linear approach—creates a better environment.
So it’s very much about promoting the idea of green roofs
in general and very much about how landscape architects can influence
the design of green roofs in a way that the whole is much greater
than the sum of its parts by thinking about how spaces can be shaped
and how they can be more inhabitable. It’s also an opportunity
to monitor and promote the ecological advantages of green roofs.
Obviously CDF has a lot of experience with
green roof design. What unique challenges are you running into with
the ASLA project? How is this project similar to others you’ve
It’s similar to most of the green roof
projects we’ve done in that it’s a unique set of circumstances.
We have the client, the project team, and the physical realities
of this particular place all coming together to form a solution
that is unique because that set of condition is unique. We’re
certainly able to capitalize on our experiences and those of others
to bring ideas to this set of issues—but the issues are unique,
and so the solutions are unique.
Mainly the challenges stem from the fact that this
building was built with ignorance toward the roof space, other than
as a repository for some of the mechanical elements of the building
and a way to keep water out. So it’s a very narrow definition
of what rooftop space could be. This project underscores the importance
of landscape architects being at the table during the design process,
parallel to the other design professions, because that’s really
the way to get the best result. But now we’re left with a
legacy of things that are hard to move and work around, whereas
if they were integrated into the design of that space from the beginning,
that wouldn’t have to be done. But I think that in reality
there are millions of square feet of rooftop space that haven’t
been designed and built with those thoughts in mind but certainly
Another challenge is that going into this, we wanted
to address the most lessons possible for the widest audience possible,
and I think that we’ve really gotten to that point. There
are ways of looking at this that have applications to any rooftop
There’s been a lot of debate about
whether green roofs prevent leaks or cause leaks, and clearly you
come down in the “prevent–leak camp.” Can you
explain a bit what your thinking on that is?
What has been demonstrated with this technology
for the past 30 to 40-years is that green roof materials, if installed
properly, can act as a protective layer over the standard waterproofing.
The nature of green roof materials is such that they keep the elements—extreme
changes in temperature, exposure to ultraviolet light, puddles of
water—that deteriorate a waterproofing membrane from doing
so. The mentality for roof surfaces has traditionally been, “You
need to see it, so if it starts leaking, you can find the leak and
address it.” Except, by virtue of keeping it exposed, you’re
not protecting it, and you’re making it deteriorate faster.
We deal with situations a lot where we’re proposing
green roofs for a variety of reasons, and the clients say, “Well,
it’s going to cost more than a standard roof.” And the
answer is “Yes, of course it is.” But if you take a
long-range view, the roof will likely last well beyond 30 or 40
What attracted CDF to apply for the ASLA
green roof project? Why was this a project you wanted to be a part
The goals that ASLA stated for this project
are very closely aligned with ours at CDF. And that is to promote
the application of sustainable principals to the built environment
in a way that enhances the built environment. Also, promoting and
educating people on the potential benefits of taking these approaches
is very much what we’re about at CDF. Also, I am a landscape
architect, and many of my colleagues are as well, so we support
ASLA and want to take an active role in being part of the organization.
You’re working with MVVA on this
project. What type of perspective does CDF have that helps inform
the project? And vice versa, how does MVVA look at the project in
a way that informs CDF?
First of all, we’re really, really happy
that ASLA decided to choose the two firms to work on this project
together. Because (A) I think it makes the product much, much better
than it would have been by hiring just one of us, and (B) I think
each of our firms is gaining from the experience of working together
because we do have very different, but compatible, perspectives.
I think that the strengths that we bring—and there’s
certainly a lot of overlap here because we’ve each done green
roofs before—are our company philosophy of making places for
people that are wholesome and healthy by virtue of relating to natural
systems and applying our knowledge of natural systems to the built
environment. I think Michael and his office have the strength of
really crafting the space in a simple and elegant way that makes
it innately appealing to people, even if they don’t know why.
This will really feel like a space that is exciting and interesting
to be in and will be appreciated from a variety of vantage points—both
looking out and looking in.
Ultimately, the blending of this aspect of human-scale
design and ecological systems—the use of water and plants—is
what’s really making this a great solution.
ASLA recently sponsored the Green Roofs
for Healthy Cities Conference here in Washington, D.C., and there
was a lot of talk about intensive and extensive green roofs. These
terms are occasionally substituted for “no design” and
“design.” How would you react to that, and how will
this project build a bridge between those labels—“intensive”
and “extensive”—and incorporate design?
Those terms are meant to inform and enlighten,
but too often they confuse and bewilder. We try to stay away from
using either of those descriptions because they are limiting, and
when you start off thinking, “Well, is it going to be this
kind of roof, or that kind of roof,” it really is only about
changing the roof surface form an impermeable surface to a more
porous surface. Really, where our profession comes in is in applying
the technologies to shape spaces in ways that are meant to serve
people. And there are plenty of examples, large buildings, warehouses,
and other places, where simply putting a different coating over
the roof is all you need to do and all you want to do.
So, when we use the terms “extensive”
and “intensive”—because they’ve become common
terminology—it’s because we’ve gotten past the
more fundamental questions of “What’s this roof space
about, and what are we doing here?” This project is an excellent
example of how these technologies can be applied in ways that aren’t
anticipated by the material manufacturers—it’s really
about using the materials to enhance the space.
What’s the most important thing you’d
like ASLA members to know about the green roof and your involvement
in the project?
One of the most important messages is that thinking
about the often-wasted fifth side of the building is a whole new
frontier for landscape architects and the design profession. The
technology that has emerged over the past 20 or 30 years has provided
an opportunity to be an integral part of the design of urban and
suburban spaces in new ways. It really has the potential to change
the way cities look and feel. Again, it goes back to the core of
what we’re about at CDF, making places better from an ecological
standpoint, because it’s ultimately better from a human standpoint.
Green roofs and rooftop garden spaces can be a really big part of
that overall idea.
I’d like to add that I’m grateful
for us to be a part of this and grateful that the committee that
has formed to help with this project is made up of the members that
it has. It’s really a great cross-section of our membership
and different perspectives, and that’s helped to make it a
very positive experience. Of all the green roof projects I’ve
worked on so far, it’s really been by far the best from my
perspective because we’re all landscape architects, and we’re
all coming from the same place, but with a different perspective.
So it’s been really fun.
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