4. GREEN ROOFING 4
Green Roofing
The rise of environmentally-focused and sustainable solutions has brought planners back
to an ancient solution to better commune with nature while creating a more pleasant and efficient
urban environment. Green roofing has been firmly entrenched in European cities and is gaining
popularity in businesses, cities and even personal homes here in the United States
(Environmental Protection Agency, 2013). They are more pleasant to look at, reduce water
runoff, and reduce heat accumulation. This paper will discuss how the green roof works, how it
is constructed, what it costs, long-term issues and feasibility.
Description of a Green Roof
A green roof is a roof that is deliberately constructed to support vegetation. A green
roofing is not a new science, dating back to 4000 B.C. when sacred places were constructed with
vegetated roofs, like sod, over elevated surfaces. (Weiler & Scholz-Barth, 2009). The goal of a
green roof is to reduce pollution through mitigating stormwater runoff by up to 80 percent
(Environmental Science in Forestry, n.d.), reduce the urban heat island effect, filter pollutants,
reduce carbon dioxide, and to maximize urban land utilization. Additionally, the added beauty of
a well-kept green roof is much more attractive than a tar or membrane covered one. Carson
reminded us in “Silent Spring”, that we should strive to live along-side of nature rather than to
control it (Carson, 1962).
Construction
There have been several approaches to green roof construction. All of which necessitate
professional design and structural analysis. Initial costs generally start around $10 per square
5. GREEN ROOFING 5
foot for a simple extensive roof, and $25 per square foot with intensive roofs (Environmental
Protection Agency, 2013). This is significantly more than the typical $1.25 per square foot of a
normal built-up roof. This initial cost can turn away many prospective builders.
The basic construction typically includes a layer of waterproofing, drainage mats, root
barriers, engineered planting material, and plants. These plants vary with the location, depth of
the planting material, and overall use intent of the roof (Environmental Science in Forestry, n.d.).
There are two basic types of green roofs, extensive and intensive (Dvorak, 2015). The
lower maintenance extensive roofs are categorized as having a shallow growing medium and
generally are suited for succulent plant types (Dvorak, 2015), these are sometimes called a living
green roof. Intensive roofs also have a biodiverse variation of plants instead of the succulents,
any plants in this type of roof must be hardy and drought-resistant. Both types of roof will have
the following basic components: vegetation, growing medium, drainage layer, root barrier, and a
waterproofing membrane (Environmental Protection Agency, 2013).
1. Extensive Roof. Extensive roofs have about six to eight inches of growing
medium and will assist in stormwater management (Weiler & Scholz-Barth, 2009). The
resulting excess runoff can be stored in cisterns for use to water the roofs in extended
drought periods (Birch & Wachter, S., 2008, p. 177). The thinner profile of a living
green roof generally runs between 12 to 15 pounds per square foot (Environmental
Protection Agency, 2013).
Each extensive roof will have specific design requirements that necessitates a
structural analysis. Because the weight is comparable to a stone ballast roof with the
waterproof protective membrane, there is usually no structural upgrade (Weiler &
Scholz-Barth, 2009). Generally, there is not many additional costs resulting from needed
6. GREEN ROOFING 6
increased structural support for new buildings. This can make a case for the planner to
provide a greater visual amenity as well as improved environmental quality.
The extensive roof can be used instead of a more conventional stormwater
management method as the rain filters through the heat tolerant plants and erosion control
mediums on top of the soil, through the planting media to be taken in by the roots of the
plant. There is a drainage mat that holds in the water under the soil and excess water
funnels onto the waterproofing material under the drainage mat and into overflow pipes
that connect to drain water systems. Under the waterproofing material there is an
additional layer of insulation over the roof deck. (Environmental Science in Forestry,
n.d.). One obvious benefit of this is that water evaporates from the planting medium and
plants which helps to regulate the surface temperature of the roof. Below is a picture
depicting a common extensive green roof and its different layers.
2. Intensive Roof. The intensive type of roof may include rooftop gardens and a
greater variety of plants. These require more maintenance and usually have a deeper
growing medium (Dvorak, 2015). These are also referred to as landscape over structure
and can be used as an accessible green garden or open space (Weiler & Scholz-Barth,
2009). The depth of the growing medium is eight inches up to several feet deep. The
(Dvorak,2015)
Extensive Green Roof
7. GREEN ROOFING 7
weight of this type of roof can run 50lbs or more (Environmental Protection Agency,
2013). This type of system requires more complex planning to ensure that the roof
structure can support the additional weight. An irrigation system may be necessary
during drought times or dryer periods of the year depending on vegetation requirements.
Layers
As mentioned before, the basic layers of construction starting at the top include:
vegetation, drainage, root barrier, waterproofing membrane, and growing medium. There are
also others that are used like a cover board, thermal insulation, vapor barriers, and other
structural supports.
1. Vegetation. Vegetation types will vary depending on the climate, design and use,
available sunlight, irrigation requirements and anticipated rainfall. The EPA
recommends maintenance that consists of weeding every month when the roof is
installed. According to this federal guideline, this weeding may be necessary every
month or at least quarterly for the first two years and every year thereafter
(Weiler & Scholz-Barth, 2009)
Intensive Green Roof
8. GREEN ROOFING 8
(Environmental Protection Agency, 2013). Additional requirements for green roof
maintenance include fertilizing, replanting, and depending on the plants, irrigation.
a. Extensive green roof. Vegetation for extensive green rooftops is generally a
succulent or drought and wind resistant plants (Werthmann, 2007) and tend to
be shallow rooting perennials (Environmental Protection Agency, 2013).
These plants generally need to have a high water content to be more fire
resistant.
b. Intensive green roof. The deeper growing media allows bushes, shrubs, and
trees. These usually require additional irrigation to be added into the design
(Environmental Protection Agency, 2013).
2. Growing Medium. The growing medium may be soil or may be a specifically
engineered medium that consists of up to 80% inorganic material and 20% organic
topsoil. This will normally be designed to last as long as the roof and will be the lightest
weight that can support the intended plant life (Environmental Protection Agency, 2013).
The porosity, or space made up by air, of the medium is important to take into
consideration (Weiler & Scholz-Barth, 2009). The size of particles within the medium
will dictate this porosity. Sand and gravel are larger and have larger spaces, while
colloids like clay have smaller spaces. The larger the space, the quicker the water will
run through it. One note about colloids, is that they are harder to get wet, but once wet,
they will retain water longer than sands or gravel (Weiler & Scholz-Barth, 2009).
There is often a filter membrane installed over the growing medium that consists
of a geotextile to keep the growing material from washing away and cause clogging of
the drainage systems. (Environmental Protection Agency, 2013)
9. GREEN ROOFING 9
3. Drainage Layer. This layer allows excess water drain from the growing
medium and prevents overloading the green roof (Environmental Protection Agency,
2013). This also allows a layer of air to get into the growing medium to create a healthier
soil. These can be egg crate like material that allows for water storage. Intensive and
extensive may both also have a cistern to collect water for future irrigation purposes
(Weiler & Scholz-Barth, 2009).
4. Root Barrier. The root barrier layer provides a separation and protection for the
waterproofing membrane and other lower layers from leaks caused by aggressive root
systems (Environmental Protection Agency, 2013).
a. IRMAs. Under the root barrier, there are often inverted roof membrane
assemblies (IRMAs), which are located above the waterproofing membrane
that are designed to protect the membrane and provide additional insulation.
These can be made of stones or concrete pavers. These are often used when
retrofitting an existing stone ballast roof (Environmental Protection Agency,
2013).
5. Waterproofing Membrane. A layer of impermeable material to prevent water
damage to the structure of the building. The waterproof membranes used in green roofs
are generally more durable and thicker than the ones used in conventional roofing
(Environmental Protection Agency, 2013). Some green roofs may skimp on the root
barrier and use this instead.
6. Cover Board. This is not always used but is a semi-rigid board that protects the
waterproofing membrane and creates separation and additional support (Environmental
Protection Agency, 2013).
10. GREEN ROOFING 10
7. Thermal Insulation. This is another layer that is not always used, but is installed
either above or below the waterproofing membrane and provide additional insulation to
the required insulation of the building. Note: green roofing is not an accepted substitute
for traditional insulation (Weiler & Scholz-Barth, 2009).
8. Vapor Barrier. This layer is the same as the vapor barriers on the walls of a
house or in a basement. It simply is either a foil or plastic sheet to prevent moisture to
pass through. This is not always used, but is a good idea.
9. Structural Supports. Additional structural supports are often necessary to
support the additional weight of the green roof. This is especially the case in intensive
roofing systems or if a roof is retrofitted with a green roof (Weiler & Scholz-Barth,
2009).
Hydrologic Cycle
The hydrologic cycle is a description of how water is constantly exchanged between the
atmosphere and the ground as precipitation and evapotranspiration. This is important to
understand as it connects how the green roof helps with the heat island effect and the stormwater
drainage problems.
(COTF.edu, 2004)
Hydrologic Cycle
11. GREEN ROOFING 11
There are five primary components of the hydrologic cycle. They are:
1. Precipitation. Defined as “Process of water in the atmosphere returning to the
Earth’s surface in liquid or solid form (Cech, 2010, p. 27)” (rain or snow fall). It moves
as dictated by the surface characteristics on which the precipitation falls and on the
duration and intensity of the storm,
2. Runoff. Defined as “Amount of water that flows along the land surface after a
storm event or from melting snow in the spring (Cech, 2010, p. 32)”. Water can run as
either overland flow or interflow toward a lake, river, or stream. Overland flow indicates
that the water travels above ground (surface). This type of flow occurs when there are
intense, short-duration rains or when there is an impervious surface like a rooftop, or
concrete. Interflow is water that infiltrates the soil and moves laterally just below the
surface (subsurface) of the ground in the soil toward its target body of water. This type
of flow occurs when there is a steady light rain and the ground surface is at least partially
pervious to water (Cech, 2010, pp. 77-78).
3. Surface and groundwater storage. Groundwater is defined as “Water contained
in interconnected pores of geologic material below the land surface (Cech, 2010, p.
105)”. In other words, it is water beneath the surface of the earth which saturates the
pores and fractures of sand, and rock formations into aquifers. Surface water is the
rivers, lakes, oceans above the soil.
4. Evaporation/transpiration. Defined as the loss of water to the atmosphere when
liquid water is turned into vapor. Evaporation happens off of the ground and
transpiration happens when plants release water through their leaves during
photosynthesis (COTF.edu, 2004).
(COTF.edu, 2004)
12. GREEN ROOFING 12
5. Condensation. Condensation turns water vapor into liquid. This condenses into
clouds and become precipitation (COTF.edu, 2004).
Stormwater
Stormwater is basically the water that is not intercepted by plants or soil (Weiler &
Scholz-Barth, 2009). Stormwater management is an old concept. As long as there have been
dwellings, humans have run into the issue of stormwater runoff. The current standard of roofing
and street construction that almost every city in a developed country has consists of an
impervious layer that sheds one hundred percent of the water and shunts it into some sort of
gutter or sewer system and it is removed from the immediate area. The result is that planners
have designed an unhealthy system that shunts the hydrological cycle and causes serious impacts
on both volume and water quality in watersheds.
Another problem is that these surfaces are impermeable and designed to shed all of the
water. It goes into the storm drain and stormwater management systems. The increasing amount
of roofs, parking lots and other paved areas causes an ever increasing amount of water that is
overwhelming the structure of most cities sewage or stormwater systems (Werthmann, 2007).
There are cities that report losing enough stormwater annually to provide over 3.6 million
people with enough water to cover their annual home needs (see Table 2) (Weiler & Scholz-
Barth, 2009). To combat this, many planners are striving to find green and sustainable ways to
reintroduce nature into cities to correct these problems.
13. GREEN ROOFING 13
Computing Water Discharge
As mentioned before, stormwater discharge has become a prevalent issue for many cities.
The goal of stormwater management is to maximize water infiltration into the soils and ensure
that surface and subsurface runoff is controlled to minimize erosion damage and flow volume.
By maximizing soil infiltration, it maximizes use for plants.
There are many factors of water movement that need to be taken into consideration. The
movement of the water through the soil medium, the gradient of the slope and the peak runoff
rate are all important factors to consider when designing a green roof to assist in stormwater
retention and mitigation.
The natural movement of water through the soil is an important place to start. The
standard way that hydrologists calculate how water discharge moves through a medium is to use
Darcy’s Law. Darcy’s Law explains, using mathematical equations, how water discharge moves
through a bed of sand. The equation is (Cech, 2010, pp. 123-124):
(Environmental Science in Forestry, n.d.)
Green Roof as Part of
Stormwater Management Plan
14. GREEN ROOFING 14
𝑞 = 𝐾𝑖
Where 𝑞 = specific discharge per unit area
𝐾 = hydraulic conductivity of the medium
𝑖 = hydraulic gradient
It figures out that the specific discharge per unit area equals the hydraulic conductivity of
the medium multiplied by the hydraulic gradient (Cech, 2010, pp123-124). The hydraulic
conductivity can be described as the actual measurement of rate of flow through a porus material
like soil (Cech, 2010, p. 122).
Once this specific discharge is determined, it is then divided by the porosity of the
aquifer. The result is the actual discharge, which will be a higher number than the specific
discharge because the water can only move through porous space and not the entire cross section
of the aquifer (Cech, 2010, pp126). The size of the particles in the aquifer determine the volume
of water: smaller grains equals less water volume; larger grains equals more volume. In the case
of most green roofs, the soil material is a specially engineered material that is light in weight and
porous (Werthmann, 2007).
Determining Flow
Determining what the gradient is important to stormwater management as well as
determining the specific discharge as it feeds into the overall flow calculations. The ground
plane (slope) of the roof or gradient will dictate the pull of gravity on the water. The more
gradual or smaller the slope and more permeable the soil is will increase the stormwater
retention. Conversely, the steeper the slope, the quicker the stormwater will move out of the
roof, decreasing the amount of retained water.
15. GREEN ROOFING 15
The hydraulic gradient formula looks like this (Cech, 2010, p. 122):
𝑖 = 𝑑ℎ/𝑑𝑙
Where 𝑖 = ℎ𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑔𝑟𝑎𝑑𝑖𝑒𝑛𝑡
𝑑ℎ = 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑒𝑙𝑒𝑣𝑎𝑡𝑖𝑜𝑛 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑡𝑤𝑜 𝑝𝑜𝑖𝑛𝑡𝑠 𝑎𝑡 𝑡ℎ𝑒 𝑡𝑜𝑝 𝑜𝑓 𝑡ℎ𝑒 𝑔𝑟𝑎𝑑𝑒
𝑑𝑙 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑡ℎ𝑒 𝑡𝑤𝑜 𝑝𝑜𝑖𝑛𝑡𝑠
This is important as the rate that the water infiltrates growing mediums or becomes runoff
is dictated by the gravity, slope, and permeability of the surface.
Peak Runoff Rate
Once the discharge rate of the soil and the gradient slope are understood, the maximum
amount of water that the soil medium is designed to handle must be understood to determine if it
is the correct blend in the soil mixture. Determining what the peak runoff rate (PRR) is
important to understand as it represents the maximum cubic feet per second that must be
managed (Weiler & Scholz-Barth, 2009). This number is based off of the theory that the PRR of
the area equals the intensity of rainfall multiplied by the coefficient that represents the variables
(see Table 1), characteristics and size of the drainage area (Weiler & Scholz-Barth, 2009). This
measurement can be adjusted to compute the peak runoff rate (in cubic feet per second) with the
rational method, this formula is used (Weiler & Scholz-Barth, 2009):
𝑄 = 𝐶𝐼𝐴
Where 𝑄 = 𝑝𝑒𝑎𝑘 𝑟𝑢𝑛𝑜𝑓𝑓 𝑟𝑎𝑡𝑒 ( 𝑐𝑢𝑏𝑖𝑐 𝑓𝑒𝑒𝑡 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑)
𝐶 = 𝑟𝑢𝑛𝑜𝑓𝑓 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
Where 0 is completely pervious and allows no runoff and 1 is completely impervious. (The
more natural the soil, the smaller the coefficient.)
16. GREEN ROOFING 16
𝐼 = 𝑟𝑎𝑖𝑛𝑓𝑎𝑙𝑙 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 (𝑖𝑛𝑐ℎ𝑒𝑠 𝑝𝑒𝑟 ℎ𝑜𝑢𝑟)
This indicates the intended storm frequency and how long the water will be concentrated in the
drainage medium
𝐴 = 𝐴𝑟𝑒𝑎 𝑖𝑛 𝑎𝑐𝑟𝑒𝑠 𝑜𝑓 𝑑𝑟𝑎𝑖𝑛𝑎𝑔𝑒 𝑎𝑟𝑒𝑎
What is immediately apparent by looking at C, the runoff coefficient, is that when the
other variables are held equal, peak flows are less when the surfaces consist of natural soils that
allow infiltration.
Now that it has been established how to build a green roof, the hydrologic cycle, and the
way that they can be included into the stormwater management plan, the discussion will move to
how it fits into ecology of an urban environment.
Ecology
Ecology from an urban standpoint has issues that can be addressed by the implementation
of green roofs. The impervious surfaces have created an imbalance in stormwater retention and
do not allow the natural processes to cool off solar energy. The result is that large cities have
created an environment that does not retain water, is not biodiverse, and average temperatures
continually increase due to a “heat island effect” caused by large portions of an urban area being
covered with impervious materials.
By installing green roofs on only 5 percent of a city’s rooftops, it can reduce overall air
temperatures between two and four degrees Fahrenheit (Birch & Wachter, S., 2008, p. 81). The
temperature of a green roof depends on the composition, moisture content of the growing
medium, solar exposure. The green roof stays cooler because of the shading and
evapotranspiration (Environmental Protection Agency, 2013). The secondary effects of this will
17. GREEN ROOFING 17
include dropping the need for electricity to air condition buildings and increasing urban water
reserves. There has been a broad acceptance of these benefits and many initiatives have been
adopted by cities across America and in Canada.
Green Roof Initiatives
Many cities and communities have recognized the benefits of green roofing. For
instance, Chicago recognized these effects and in 2000, it embraced a greening initiative and has
implemented ordinances to install green roofs to reduce its stormwater runoff (Birch & Wachter,
S., 2008, p. 92).
Seattle is also a leader with their “Seattle Green Factor”. This initiative requires 30%
vegetation coverage for any new developments within their neighborhood or commercial
districts. This initiative focuses on decreasing the negative environmental impacts that have
resulted through the increased development. The remediation envisions a high-quality urban
landscape that provides increased biodiversity and other environmental benefits that are
mentioned here. This is a turning point initiative as it recognizes the fact that the urban landscape
is an important part of a functioning of the city, and not just there for aesthetic value (Birch &
Wachter, S., 2008).
Other cities that have implemented stormwater ordinances that include green roofs are
Portland, and Philadelphia. These cities allow tax breaks for some of their programs. These
programs save millions of dollars in stormwater management and storm drain system upgrades to
keep up with the runoff from the always increasing impervious surface area that comes when
buildings or streets are built (Weiler & Scholz-Barth, 2009).
18. GREEN ROOFING 18
There are many advantages from the additional vegetation. It will serve as a dust
collector and air cleaner. The vegetation will decrease noise pollution and increase habitat for
animals, birds, and insects. People have been fleeing the city to get to a pleasant place to live
and raise their families. This is the one of the main causes of urban sprawl (Birch & Wachter, S.,
2008). Greener spaces will increase the desire for people to live and work in the city. All of
these points will lead to a better ecology for the urban environment. When many cities
accomplish these goals, it will start reducing the global heat island effects through evaporative
cooling and water retention. While green roofs alone will not save the world, they can make a
significant contribution to reducing climate change.
Costs
As mentioned in the construction section, there are different types of green roofs.
Depending on the components, growing medium, membrane quality, drainage system, types and
quantity of plants, and use, the costs will vary greatly. An extensive roof will start around $5-
$10 per square foot and an intensive roof cost can start around $25-$40 per square foot
(Environmental Protection Agency, 2013). These costs can go up exponentially as different
options are included. The benefits of green roofing is that it will last up to 20 years more than a
conventional roof.
The annualized replacement costs for an extensive roof in the Los Angeles area is
reported to average between $1.03-1.66 per square foot (Environmental Protection Agency,
2013). The annualized costs for a conventional roof ran between $0.51-$1.74 per square foot
(Environmental Protection Agency, 2013). Annualized maintenance costs for either type of roof
ran about $0.75-$1.74 per square foot (Environmental Protection Agency, 2013). An important
19. GREEN ROOFING 19
note is that the intensive roof’s maintenance cost will be stable throughout its lifetime, while an
extensive roof‘s maintenance cost will drop once the roof is mature. These long-term benefits
will provide savings on a roof that is more enjoyable and longer lasting.
Savings
Specific savings vary depending on many variables including size of roof, use of the roof,
insulation, and other environmental considerations. There have been many success stories. For
instance, Chicago reports that their city hall saves about $3,600 annually in energy savings and
9,270 kWh from cooling savings and about 740 million Btu of saved heating (Environmental
Protection Agency, 2013).
Along with these direct and immediate savings, it reduces the need for constant upgrades
to sewage systems. Another benefit is that green roofs are more durable, often lasting about 20
years longer than a similar conventional roof (Werthmann, 2007).
Many cities are offering tax-breaks or remitting taxes altogether. This often will cover
the difference between the conventional roof and the green roof’s initial installation costs (Weiler
& Scholz-Barth, 2009). This will increase the savings benefits immediately.
Benefits
There are many benefits to green roofing. They include:
Reducing surface temperatures that allow the buildings to stay cooler
o Accomplished through evapotranspiraion and shading. Other factors that
influence this include: rooftop composition, geographic location, moisture
content, and solar exposure (Environmental Protection Agency, 2013)
20. GREEN ROOFING 20
o Lower temperatures low the formation of ground-level ozone-care must be
taken to ensure that volatile organic compound (VOC) producing plants
are avoided as these additional VOCs will add to ozone production
Creating green spaces to improve quality of life, citizen health, desirability of
urban living and working, increasing property values.
Introducing more mitigation factors for cleaner air, dust and particulate matter
(PM) filtering, and carbon dioxide reduction
o The EPA reports that a 1,000 square foot green roof can filter 40lbs of PM
annually- about the amount of CO2 emitted from 15 passenger cars
annually (Environmental Protection Agency, 2013)
Reducing energy needs by requiring air conditioners to run less
Lower long-term costs and increased durability; lowering annual costs as a roof
matures (Environmental Protection Agency, 2013)
Increased habitat for birds, animals, and insects (increased biodiversity)
Reduced stormwater runoff. Portland reported a 70% reduction over 15 months
(Environmental Protection Agency, 2013)
o Reduced costs of updating for increased flow resulting from increased
impervious construction and roofing
o Reduced erosion
o Reduced non-point source pollution
o Increased groundwater retention; the deeper and more extensive the green
roof, the more it collects
21. GREEN ROOFING 21
o Reduces peak runoff rates by up to 95% into stormwater systems during
intense storms (Environmental Protection Agency, 2013)
Will reduce noise
Can be used as a food producing source
Liabilities
There are a few negative aspects of green roofing that include:
Increased up front expenses
Required maintenance
Fire hazard if not kept up
Additional considerations that need to be calculated prior to installation or
building
o Each site needs to be evaluated separately
o Many disciplines of professionals need to be consulted prior to building.
These professionals will need to determine the infrastructural needs to
support the building and site program. They include:
Building architects
Landscape architects
Structural engineers
Civil engineers to calculate water retention capacity
Mechanical engineers to determine:
mass of the growing media and/or vegetation mass at
various moisture levels
22. GREEN ROOFING 22
how to incorporate the insulating values of the green roof
into the sizing of heating, cooling, and air-conditioning
systems.
Liability Mitigation
The liabilities are far outweighed by the benefits. The upfront costs can be offset by
municipal tax breaks or other incentives. Even if they are not, the long-term benefits and longer
life of the green roof offset this and surpass it as a long-term investment. The benefit to the
municipal stormwater makes this an effort worth investing in for cities.
The maintenance issues of having to water it regularly or weed the roof are something
that has to be a conscious decision to live with and plan for.
The fire hazards can be mitigated by using water heavy plants like succulents and
additional irrigation. Prior planning of grasses or other vegetation will take this into
consideration and can be alleviated by proper plant choice.
The technical considerations and professional requirements are something that are
included into the long-term costs. Cutting corners at the initial building phase will cause huge
issues later.
Conclusion
The green roofing initiative has been proven across Europe and in many cities across
North America. It has proven to lower the urban heat index, increase air quality, mitigate
stormwater runoff, and increase energy efficiencies while increasing urban beauty, providing
some biodiversity, lowering city noise, and improving the health of the citizens. Green roofs are
23. GREEN ROOFING 23
a sustainable environmental initiative that can assist in returning nature to cities in a way that
benefits consumers and businesses alike. The long-term financial benefits in the form of reduced
stormwater management systems will make this a feasible initiative for many cities.
24. GREEN ROOFING 24
References
Birch, E., & Wachter, S. (2008). City in the Twenty-First Century : Growing Greener Cities :
Urban Sustainability in the Twenty-first Century. University of Pennsylvania Press.
Retrieved February 18, 2015, from
http://site.ebrary.com/lib/apus/reader.action?docID=10641556
Carson, R. (1962). Silent Spring (40th Anniversary Edition ed.). New York, NY: Houghton
Mifflin Harcourt Publishing Company. Retrieved February 14, 2015
Cech. (2010). Principles of Water Resources/History, Development, Management, and Policy
(3rd ed.). Hoboken, NJ: John Wiley & Sons, Inc. doi: ISBN: 978-0-470-13631-7.
COTF.edu. (2004). Water Cycle. Retrieved February 20, 2015, from
http://www.cotf.edu/ete/modules/msese/earthsysflr/water.html
Dvorak, B. (2015). Green Roofs. Retrieved February 20, 2015, from Soil Science Society of
America: https://www.soils.org/discover-soils/soils-in-the-city/green-roofs
Environmental Protection Agency. (2013). Green Roof. Retrieved February 15, 2015, from Heat
Island Effect: http://www.epa.gov/heatisland/mitigation/greenroofs.htm
Environmental Science in Forestry. (n.d.). ESF Green Roof: Sustainability in Action. Retrieved
February 15, 2015, from http://www.esf.edu/sustainability/action/greenroof.htm
Weiler, S., & Scholz-Barth, K. (2009). Green Roof Systems: A Guide to the Planning, Design and
Construction of Landscapes Over Structure. John Wiley & Sons. Retrieved February 15,
2015, from
http://library.books24x7.com.ezproxy1.apus.edu/assetviewer.aspx?bookid=29517&chunk
id=999218983
25. GREEN ROOFING 25
Werthmann, C. (2007). Green Roof- A Case Study. Washington, D.C.: Princeton Architectural
Press. doi:SB419.5.W47 2007eb
26. GREEN ROOFING 26
Tables
Table 1
Runoff Coefficient
Runoff Coefficient (Weiler & Scholz-Barth, 2009)
Ground Cover or Land Use Runoff Coefficient (C)
Forests "0.05-0.25"
Lawns "0.10-0.35"
Cultivatedland "0.08-0.41"
Meadow "0.10-0.50"
Parks,cemeteries "0.10-0.30"
Unimprovedareas "0.10-0.30"
Pasture "0.12-0.62"
Pasture withmoderate grazing "0.10-0.30"
Bare earth "0.20-0.90"
Steepgrassedarea(2:1 slope) "0.50-0.70"
Residential areas "0.30-0.75"
Flatresidential areas,30%impervious "0.30-0.50"
Flatresidential areas,70% impervious "0.50-0.80"
Businessareas "0.50-0.95"
Flatcommercial/industrial area,90%impervious "0.50-0.90"
Asphaltorconcrete streets "0.70-0.95"
Brick streets "0.70-0.85"
Roofs "0.75-0.95"
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27. GREEN ROOFING 27
Table 2
Annual Runoff Water Loss
Table 2
Annual Runoff Water Loss (Weiler & Scholz-Barth, 2009)
MetropolitanArea
Water Loss (billion
gallons/year)
Atlanta,Georgia "56.9-132.8"
Boston,Massachusetts "43.9-102.5"
Philadelphia,Pennsylvania "25.3-59"
Washington,D.C. "23.8-55.6"
Nashville,Tennessee "17.3-40.5"
Charlotte,NorthCarolina "13.5-31.5"
Pittsburgh,Pennsylvania "13.5-31.5"
Houston,Texas "12.8-29.8"
Greensville,SouthCarolina "12.7-29.5"
Seattle,Washington "10.5-24.6"
Chicago,Illinois "10.2-23.7"
Raleigh-Durham/ChapelHill,NorthCarolina "9.4-21.9"
Orlando,Florida "9.2-21.5"
Minneapolis/St.Paul,Minnesota "9.0-21.1"
Detroit,Michigan "7.8-18.2"
Tampa, Florida "7.3-17"
Greensboro,NorthCarolina "6.7-15.7"
Dallas,Texas "6.2-14.4"
[2]
AmericanRivers,NRDC,SmartGrowthAmerica."PavingOurWayto Water Shortages:
How Sprawl Aggravatesthe Effectsof Drought."2002. p 2.
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