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In my enthusiasm for the potential benefits of greenroofs, I
would certainly be remiss if the potential problems of greenroofs were not also
explored. A discussion of some of the possible physical, environmental,
and aesthetic issues follow:
PHYSICAL ISSUES:
Waterproofing Integrity
Every reputable roofing company will guarantee and provide a
warranty for the waterproofing integrity of their membrane(s), including
greenroofing providers. Water leakage from drainage backups or possible root puncture
could lead to interior damage if the correct waterproofing membrane system, root
barrier, and drainage layer are not selected. Of course, when choosing a
greenroof system and/or contractor, it is advisable to check references on completed projects
for waterproofing success. Vulnerable areas where leakage is possible
include abutting vertical walls, roof vent pipes, outlets, air conditioning
units, perimeter areas, etc. A thorough water flood test needs to be
conducted for leaks after installation of the waterproofing membrane to ensure
quality control, certainly before the other layers are applied.
Pesticide Leakage from Roof Materials
Recently, an environmental science magazine began testing
drainage from greenroofs in an effort to measure any pesticide runoff. The
potential does exist for certain elements, such as iron and aluminum, to seep
out and infiltrate our ground water. Care must be exercised in selecting
thick membranes to ensure no release of pollutants, and the materials used in
foundations and pathways on a greenroof should not leach carbonates. This
would also be a good opportunity to choose greenroofing companies who use
environmentally friendly roofing components.
Additional Support Considerations
For extensive and intensive greenroofs with
projected live loads of higher than 17 pounds per square foot,
consultation with a structural engineer is a requirement. Additional
growing media depths, large plants
such as trees, walkways, seating areas, parking areas, etc. will command greater
structural support, and a greater layer build-up of the greenroof system. For
example, deeper planting beds can be constructed over internal columns and walls
to provide a higher loading capacity. Considerable costs can be associated
with these high interactive projects, especially with renovation or retrofitting
projects.
Unwelcome Wildlife Problems
I have had people ask be about the possibility of attracting
rats, raccoons, squirrels, spiders and the like with greenroofs. Because
of their watertight quality, I would suspect it would be extremely difficult (or
impossible) for these larger animals to enter a home through the roof.
But because a natural habitat is created, perhaps some undesirable critters
would be invited, and then their proximity might put off some folks. I
actually haven't read or heard anything regarding this issue; please share any
thoughts or experiences.
ENVIRONMENTAL ISSUES:
Certain aspects of greenroof technology could be challenged in
terms of their positive environmental characteristics and sustainability
factors. While examining these issues, perhaps the ultimate answer lies in the
varying degrees to which a concept is considered environmentally sound and sustainable. In other words, there may not be black and white, or cut and dry responses to these
important concerns.
Native vs. Cultivated Plants
For example, is it environmentally correct to use plants other
than natives on greenroofs? Purists would probably say that only
natives should be used in most landscaping projects, including
greenroofs. A true representation of the land would, in fact, aim to replicate its
original natural habitat, and therefore native plants would be considered
essential to the design.
However, several factors come into play here. First of
all, these are very planned landscapes, and we are trying to replicate the
physical conditions of plants living in the ground, but many feet above.
Soil substrates definitely aren't native soils (too heavy, easily compacted,
etc.) - so certainly this isn't a natural design to begin with. We are already limited somewhat with the greenroof plant palette due to
their exposure to harsher climatic conditions. (For the most part, the
reference here is to extensive greenroofs, as plant selection is much greater
for intensive greenroofs because of their higher soil depths, etc.)
Secondly, we are importing an established European technology with proven plant
species, and many of these are readily cultivated and available here.
Also, if we self-limit our plant selection, the greenroof environment will be
lower in bio-diversity.
Greenroofs will
only function if the vegetation is successful, so initially it may be necessary
to use proven adaptive species, and then introduce and promote our own native
plants as they are proven successful. Unless a specific design program calls for
an exotic look, I believe that ultimately, it will be in our own best environmental interest to establish natives as the primary plant material for
greenroofs.
Irrigation Requirements
Upon reviewing my first draft, UGA Professor Darrell Morrison asked me to consider a couple of possible criticisms, the first being "In a
hot, dry climate, is it environmentally sound to have a green roof if it
requires irrigation to establish/maintain it as a green roof?"
First of all, a hot and dry climate would dictate a distinct
list of plants that are extremely drought and heat tolerant, especially
succulents. Many species that grow in full sun have developed mechanisms
to reduce leaf evaporation, e.g., wax coated and spiked leaves.
Of course, any landscape requiring regular irrigation would be
high maintenance and expensive in terms of supplemental water usage and cost. If
you design a golf course on a greenroof, then it would not be very
environmentally sound because of these factors, in addition to the negative
aspects of the monoculture of turfgrass. However, as seen from the Optima and ZinCo examples in the
Greenroof Components section, techniques have been developed to automatically
capture and release stored rainwater within the greenroof system. Although
these details would increase the initial capital investment cost of the
greenroof, they would quickly pay for themselves in terms of irrigation savings
as well as providing an argument for sustainability.
If you compare all the natural
processes that are mimicked by a greenroof environment to its potential irrigation
requirements, I believe the greenroof would still be considered environmentally
sound, although not necessarily self-sustainable.
AESTHETIC VALUES:
Professor Morrison also asked, "Are the aesthetics of
green roofs always positive? Can the good points about green roofs be sufficient
to 'lose' the aesthetic quality?" The examples offered throughout
were selected to reflect the wide variety of greenroof styles chosen by the
owners and designers. Some are more natural, some very designed, but
certainly all are organic in that there are plants on the roof! If not
planned correctly, an unsightly, overgrown appearance could result. Of
course, if you are trying to recreate a meadow habitat designed to attract
wildlife, this may actually be your goal.
Also,
the organic nature of plants needs to be acknowledged in that seasonal
fluctuations and periods of severe drought and heat will be reflected in the
look of the plants themselves. Therefore, colors, heights, and plant density
will most likely change with the seasons.
Some people may feel organic architecture is inappropriate or
"unnatural" for any building, and that is their own aesthetic
value. If no or low maintenance is desired, then the design must dictate the
correct choice and placement of plant material. A hands-off program may
result in a wild and overgrown look. Beauty is in the eye of the beholder, but surely the many arguments
for the ecological and economic benefits of greenroofs can outweigh
aesthetically weak designs. Below are some examples of less structured organic
design
which
some may find either objectionable or lovely because of its wildness:
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