Full
Text Reference Document
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Introduction
The concept of sustainable design has emerged in response to the growing problems
attributable to conventional development. Concerns over
fossil fuel emissions, the increasing rate of resource depletion and habitat
destruction, the growing global demand for cheap energy sources, global climate
change and the widening gap between the quality of life for the first and
third world demand a unified response. This response has evolved into
one of design. Not only the design of our built infrastructures, but also
a redesigning of the way human societies view and interact with the natural
world. Officially coined in Our Common Future, a 1987 report by the World
Commission on Environment and Development, sustainable development is defined
as "development that meets the needs of the present without compromising
the ability of future generations to meet their own needs."2(pg. 5)
The sustainable development movement has grown out of this sense of regional
and global responsibility.
The fundamental concepts of sustainable design stress the need for symbiosis
between human development and the natural world, a responsibility to repair
design flaws by looking for new and existing ecologically sound alternatives,
and shifting concentration from short-term quarterly profit statements to
long-term goals of sustainability.3 The realm of built structures arguably
has the most to gain from sustainable design; from the way we design our homes
and communities, to the way we design our major cities.
Working toward key factors such as renewable energy sources, capturing/retaining
storm water to reduce runoff, developing passive heating and cooling systems,
and increasing overall building productivity while reducing the amount of
resources used can all be found as examples of sustainable development. Greenroofs
are one element of sustainable architecture, which provides benefits in resource
conservation, reduced building operating costs, water retention and purification,
runoff water reduction, easing urban heat island effect, and supporting local
wildlife.
What is a greenroof?
The title "greenroof" describes the roofs
of a building that is covered with a growing medium, substances other than
soil alone, which support root development/growth and living vegetation.5
The term "greenroof" is used here in the
same vein of logic as used by greenroofs.com; that is the term greenroof
more accurately describes a roof covered with vegetation than the term green
roof. The logic is that a green roof may simply describe the color of a roof,
and have nothing at all to do with the fact that a roof does or does not have
a covering of growing medium supporting a living, breathing organisms.5
Although the title greenroof also misleads a bit,
as greenroofs are not always green, rather the variety of plant
life often exhibits many different colors throughout the changing seasons.
Greenroofs are not the only type of roof to have
vegetation, for example there are rooftop gardens used to grow produce, and
ornamental hangings of plant life to beautify the building. Although these
other types of rooftop vegetation house there plants in containers or planters
rather than in a spreading growing medium on the roof itself. The primary
difference between other types of rooftop vegetation and a greenroof
is the continuity of the growing material used, and therefore the amount of
area that is covered by plant life. This fundamental difference serves to
provide benefits such as improved insulation, water conservation, and prolonged
roof life for the area of the roof covered by plants.
What greenroof designs
are there and how do they differ?
Generally there are two types of greenroof designs,
extensive and intensive.5 Due to the numerous unique features of every
project, slight variations in these two general designs are common.
Extensive greenroofs are not particularly focused on
aesthetic appeal because they are not intended for human traffic, aside from
occasional maintenance upkeep. This design, sometimes referred to as a
low-profile greenroof, incorporates a thinner layer
of growing medium that can range from 2-6 inches in depth and helps restrict
the height of plant growth.4 The growing material used is composed of
>70% inorganic or mineral material and <30% organic matter.5 This
design can be installed and maintained on slopes as great as 30 degrees due to
its lighter weight and low growing plant life.5 A low-profile design is
often found on smaller roofs due to space and weight restraints as well as on
very high rooftops as it would be dangerous to encourage human presence on
them. Extensive greenroof designs typically weigh
15-50 pounds per sqft.7
Intensive greenroofs on the other hand are designed
with human interaction in mind and incorporate a more varied array of plant
life including but not limited to bushes, trees, and native grasses. This
design, sometimes referred to as high-profile greenroof,
requires a deeper growing medium of 8-24 inches to be effective due to the
greater diversity of plant species as well as more intensive watering and
fertilizing during the first few months after installation.4 The growing
medium found on intensive greenroofs employs near equal
levels of organic and inorganic material, with a slightly heavier composition
of inorganic material.5 Benches, gazebos, and other sorts of
recreational equipment can be found on intensive greenroof
tops. Intensive greenroof designs typically weigh
80-150 pounds per sq-ft, making the load bearing capacity of the structure a
much more important factor than with extensive greenroofs.7
Rooftop vegetation of the past:
From the ancient hanging gardens of Babylon to Icelandic homes and barns,
vegetation has been utilized ornamentally and functionally on rooftops for
thousands of years.12 Due to a lack of heating resources and a harsh
climate in more Northern cases, layers of sod were laid to insulate the
building while strips of birch bark functioned as a water proof layer
protecting the underlying wooden beams.14 In more temperate locations,
layers of reeds and tar were used to form a primitive roof membrane to protect
from leaks and overgrowth.13 The same motivation drives green building
initiatives today: a scarcity of resources combined with pressing ecological
concerns and an enduring passion for the natural world.
Contemporary Examples:
Visionaries and design leaders have undertaken contemporary applications of greenroofing technology in many parts of the globe.
The United States currently has 365 listed greenroofs
covering a combined area of 5,836,683 sq-ft.5 Chicago, Illinois is
home to 23 greenroofs covering a combined area of
1,697,373 sq-ft and has recently made a name for itself as a modern green
city through the implementation of green design and technology.5 The
City Hall building in downtown Chicago was arguably most important rooftop
to be greened in the area because it sent the message that mayor Richard Daley
as well as others in places of political power, are backing the green building
movement that is growing in the United States.6 Installed in 2001,
the 20,300 sq-ft semi-intensive greenroof atop City
Hall welcomes human interaction and recreational use.5 This particular
greenroof saves $5,000 annually on utility costs.5
Cities all over the US are beginning to see greenroofing
as a worthwhile, long-term investment opportunity illustrated by 89 different
greenroofing projects throughout the lower 48 states (in highest
density on the East and West coasts).5 The density is no doubt somewhat
attributable to the large amount of rainfall the coastal regions experience,
keeping maintenance costs down and diagnostic benefits up.
Installation/Maintenance:
The first question anyone interested in installing a greenroof
should ask him or herself is what needs must the greenroof
meet while addressing the limiting factors of construction. Are they purely
aesthetic, purely functional or as is most often the case, some combination of
both? The primary limiting factors for a greenroof
are slope of the roof, weight limitations of the building, and the budget
allocated for the project.5 Greenroofs located
on high-rise buildings often focus only on functional benefits, as human
traffic on the rooftop is virtually impossible and/or restricted. Retrofitting
introduces unique hurdles as well. An existing structure may have load barring
problems as a result of age and wear; also the roof decking itself may need to
be replaced due to wear and tear. In any case but especially when retrofitting,
one would be greatly advised to hire a statistical consultant to provide
specific information about weight constraints and overall structural integrity
in an effort to avoid problems both during and after installation.
The upfront costs of installing a greenroof are
without argument more expensive than a traditional rooftop. However, many
see the higher upfront cost as an investment rather than money out the door.
The cost savings are found in the long-term benefits a greenroof
provides, as to be discussed later in the Benefits section. Extensive greenroofs may range in installation cost from $15 to $20
per sqft.8 Intensive greenroof designs often
range from $25 and higher per sqft.5 Of all
the greenroof components installed, the roof membrane
is the single most expensive.5
A greenroof is comprised of six layers on top of
the roof decking itself. The layers consist of a water proof membrane, a layer
of insulation, the water drainage and filtration system, a root impenetrable
layer to prevent clogs in the drainage/filtration system as well as damage
to the membrane beneath, the growing medium, and the vegetation.18
First, a waterproof cover is laid down on the roof decking called a roof
membrane; this membrane prevents water from damaging the building underneath.8
An insulating layer of material may be placed on the roof membrane if so
desired, atop of which a root impenetrable barrier is installed to protect the
roof membrane and insulation.8 Next comes the drainage and aeration
layer, this layer allows the growing medium to expel excess water to prevent
stagnation as well as reduce the weight burden of the roof. On very shallow
extensive greenroofs, this layer is sometimes
combined with the filtration layer due to weight constraints.8 This
layer sometimes is fitted with a mechanism that prevents drainage, creating a
water retention bank to be used either by the above plant life or elsewhere,
for example in the case of gray-water. An additional root barrier is installed
above the drainage system to prevent clogs and root damage.8 Finally, the growing medium is spread and the plants sowed,
or the individual modules equipped with living plants are installed.
A modular or "block" system can be used as an alternative to the more
traditional approach of laying a continuous spread of growing media. A modular
system consists of individual "blocks" filled with a pre-selected
growth media and rooted plants.11 This design is best suited for
extensive greenroofs as there are soil depth
restrictions tied to modular systems. The attractive aspects of installing a
modular system include the ability to inspect or remove any individual module
for maintenance or for seasonal rotation.11 There is also a higher
likelihood of the plants surviving the installation period if they are
established and allowed to mature prior to installation at the project site,
resulting in less need to irrigate/fertilize.7
Both intensive and extensive greenroof designs
require additional watering during the first year or so after installation to
allow the plants to become fully established and self-sufficient. A permanent
irrigation system is sometime installed based on the design of the greenroof, the variety of vegetation to be raised on its
surface, and the local climate conditions.7 An irrigation system is not
usually necessary, as is the case of a modular greenroof
system, however; all greenroof designs require some
form of additional watering for the first few months after installation to
allow the plants to develop root systems capable of surviving on their own.
Drip irrigation or sprinkler systems are typically the variety installed on
greenroofs.8 Fertilizers may be applied to plants if desired although
heavy inputs are not necessary, as excessive growth of plant life is not
encouraged. Slow release fertilizers are the variety most often chosen for greenroof applications.5 Fertilizers with soluble
nitrogen should be avoided as it can contaminate runoff water; such fertilizers
include those derived from animal products, such as blood or manure, which may
also contain unwanted/harmful bacteria.5 Seasonal weeding by maintenance
personnel must be done to prevent unwanted species of vegetation from
colonizing the rooftop, typically the result of wind or bird droppings. Leaves
and other rubbish should be routinely cleared from the roof to prevent clogs in
the drainage system, and the drainage system itself should be monitored
routinely as its improper function can result in monumental consequences for
the vegetation as well as the building itself.
Growing Medium:
The growing medium is a lightweight man-made material capable of retaining
a large amount of nutrients, maintain thermal stability to promote a healthy
subterranean environment for root colonies, and drain quickly. The drainage
capabilities of the growing medium and the drainage system installed are very
important because substances that drain slowly have lower daytime temperatures
and warm up/thaw more slowly than well drained growing mediums, contain far
less oxygen than well draining mediums which inhibit soil aerobic organism
activity, have a far greater likely hood of supporting plant diseases, parasites,
and or fungal growth, and limit uniform plant and root growth.1(pg. 173)
Weight limitations are one of the most crucial factors to consider when
selecting a rooftop-growing medium. If the medium is too heavy, it will damage
the structural integrity of the entire building rendering all other possible
benefits of the greenroof void.
Such examples of man-made growing materials include Diatomaceous Earth, a
man-made substance formed by firing hydrated silica mineral at high temperatures
in order to form a light and porous substance.1(pg. 158)
Natural materials such as expanded clay and slate pebbles may
also be used as the growing medium. The smaller the particles are, in the
case of materials like expanded clay or slate, the better they will retain
water.19 This being said, materials like expanded slate or clay drain
much more quickly than other organic media such as peat or humus.
Soil is not chosen as a growing-medium because it is very heavy (especially
when wet), and does not have adequate drainage to meet the demands of a rooftop
setting. Soil only drains as well as the least permeable horizon allows.1(pg.
18) Although soil is not used alone, it may
be mixed with different materials that encourage an environment suitable to
rooftop conditions.
Vegetation:
There is no one specific plant species suitable to the needs of a greenroof, although the vegetation must meet certain requirements
for it to be used in a rooftop setting. So long as the species has good regenerative
capabilities, a shallowly spreading root system and the capability of withstanding
periods of strong direct sunlight, drought, wind, and cold there is likely
to be rooftop application.29 The key issue
when selecting the vegetation for a greenroof is
the local climate conditions.
Speaking again in generalities, the species of vegetation most associated with greenroofs are the alpine succulents, or sedums. The name
alpine succulent derives from the fact that these particular plants are typically
found in arid habitats ranging from 3,000 to 8,000 feet above sea level, and
are capable of storing water for extended periods of time.9 Alpine
succulents also prefer rocky growing conditions very close to the conditions of
greenroofs.9 Sedums can sprout roots from an individual clipping without
any chemical stimulants; this vigorous regenerative quality makes them a
beneficial and easy to care for plant species.
Native grasses and flowers may also be chosen depending on the specific area,
but alpine succulents are often selected over native vegetation due to their
wide applicability for rooftop settings. Larger plants such as bushes and trees
are sometimes found on intensive greenroofs, though
they are planted in growing containers because of their tendency to have
strong, spreading roots capable of damaging drainage systems, and their
increased need for space in order to survive.1
Benefits:
The benefits derived from greenroofs are numerous.
They include both economic as well as ecological implications, and extend
beyond the individual structure home to the greenroof.
A greenroof improves the thermal stability of the
building itself, resulting in lowered operating costs by reducing heating/cooling
burdens. The improved insulation of a greenroof
not only helps cut down on excess entropy occurring within the building, but
also extends the life of roofing materials as well as reduces maintenance
expenditures.22 In the hot summer months, the building will be able
to retain a greater amount of cool air inside and in the winter the structure
can more efficiently maintain comfortable temperatures with less use of heating
supplements. In cities where a large amount of rooftops have been greened,
there has been improvement in the thermal stability of the overall area via
reducing urban heat island effects. Cities are particularly susceptible to
heat island effects, which occur all year round but most noticeable and destructive
during the summer months due to a large amount of radiant heat being absorbed
by bare concrete, gravel, and/or asphalt rooftops. Urban areas experiencing
urban heat island effects are typically 2 to 10°F hotter than nearby rural
areas, which results in increased energy demands, air pollution, and heat-related
sickness/mortality.16 Greater amounts of energy must be used to combat
urban heat island effects in cities during the hottest months, for example
air-conditioning is utilized much more heavily causing more fossil fuels to
be burned to supply the necessary energy, which creates more smog.23
Smog and other air pollutants are highly sensitive to temperature conditions,
for example as the temperature raises so does the accumulation and concentration
smog.23 Greenroofs provide improved evapotranspiration
by using absorbed solar heat energy rather than just reflecting it back into
the atmosphere where it becomes trapped by pollutants. When vegetation draw
moisture in through their leaves, it is evaporated and dispersed by utilizing
heat and wind as its vehicle, thereby rendering radiant heat diffuse and reducing
the urban heat island effect.24 Vegetated roofs only reflect about
20% of sun's energy back into the atmosphere.25 In addition to easing
urban heat island effects, an overall reduction in air pollution levels as
can be seen in cities with a large amount of greenroofs,
like Tokyo for example.17
A secondary although no less important benefit of reflecting solar radiation
can be seen in the dramatically extended life of the roof decking and membrane.
Extending the life of roof materials cuts down on the constant
demand for new materials, thus helping to stretch our scarce resources further
without impacting the integrity of their benefits. The roof membrane
and decking protection provided by a greenroof extends
roof life up to twice as long as conventional roofs.4
Storm water runoff stabilization and purification holds the strongest suit
in many areas of the country where the primary ecological concern is not heat
island effects or building operating costs, but rather floods and water quality.
Greenroofs reduce the amount of contaminated runoff
water entering local watersheds as well as the amount needing to be treated
by the municipality. Vegetated roof systems have been shown to retain 60%
- 100% of the storm water that comes into contact with the rooftop.10
In addition to this benefit the vegetation will drastically reduce the amount
of contaminants in the runoff water. Nitrogen for example, is common in nature
but is also produced in mass by human society and excreted into the environment
by automobile exhaust, agricultural products, and industrial byproducts/wastes.26
Nitrogen eutrophication is a major problem for aquatic
ecosystems, however rooftop vegetation absorbs and metabolizes nitrogen before
it can harm terrestrial or aquatic communities.26
There are many community and social benefits associated with greenroofs as well, including greater demand for recycled
products such as compost, mulch, and local nursery plants that help support
the local economy.4 In addition to benefits for local business, many
hospitals and therapeutic recreational facilities have begun to utilize the
beautifying effect of rooftop vegetation to enhance their patient's/customer's
enjoyment and benefit for the duration of their stay.15 This is partially
due to aesthetics and partially due to the physical benefits greenroofs provide such as improved air quality and noise
reduction. Rooftop vegetation additionally supports local insect and bird
populations, which help to keep the numbers of invasive and nuisance insects
down without the use of harmful and expensive pesticides.
A greenroof will provide more benefits for its building
occupants, owners, and surrounding communities the more care and consideration
is given to the installation process and specific bioregional conditions.
Methods:
Pennsylvania is home to 31 greenroofs, covering a combined area of 282,616
sqft.5 Allegheny College has planned to construct a new Theater and
Communication Arts building that will have a greenroof. Great care must be
taken in the installation and upkeep of the new greenroof on campus in order
to ensure the longevity of the greenroof's benefits as well as to promote
sustainable/green building design.
The question I proposed to answer was, how can I develop a website to better disseminate information pertaining to the new greenroof on the Allegheny College campus to current/perspective students? In addition to answering this question, I have toured three greenroofs in the surrounding area to further my knowledge of how greenroofs operate and how they are maintained. The three greenroofs which I toured in the fall semester of 2006 were located at: The Asbury Woods Nature Center in Erie, Pennsylvania; The Pittsburgh Children's Museum in Pittsburgh, Pennsylvania; and Hamerschlag Hall of Carnegie Mellon University in Pittsburgh, Pennsylvania.
By utilizing textual, web based, and firsthand sources, I have constructed this website and the present Full-Text document in order to further the understanding of interested persons.