Running head: Green roofing 1

Green Roofing

Brian Rasmussen

American Military University

Green roofing 2

Table of ContentsGreen Roofing..............................................................................................................................................4

Description of a Green Roof........................................................................................................................4

Construction.................................................................................................................................................4

1. Extensive Roof. ..............................................................................................................................5

2. Intensive Roof..................................................................................................................................6

Layers............................................................................................................................................................7

1. Vegetation........................................................................................................................................7

a. Extensive green roof. ................................................................................................................7

b. Intensive green roof. .................................................................................................................8

2. Growing Medium............................................................................................................................8

3. Drainage Layer. ............................................................................................................................8

4. Root Barrier....................................................................................................................................9

a. IRMAs. ......................................................................................................................................9

5. Waterproofing Membrane.............................................................................................................9

6. Cover Board....................................................................................................................................9

7. Thermal Insulation.........................................................................................................................9

Hydrologic Cycle........................................................................................................................................10

1. Precipitation..................................................................................................................................10

2. Runoff............................................................................................................................................11

3. Surface and groundwater storage...............................................................................................11

4. Evaporation/transpiration...........................................................................................................11

5. Condensation.................................................................................................................................11

Stormwater.................................................................................................................................................11

Computing Water Discharge................................................................................................................13

Determining Flow...................................................................................................................................14

Peak Runoff Rate...................................................................................................................................14

Ecology........................................................................................................................................................16

Green Roof Initiatives................................................................................................................................16

Costs............................................................................................................................................................17

Savings........................................................................................................................................................18

Benefits........................................................................................................................................................19

Liabilities....................................................................................................................................................20

Liability Mitigation....................................................................................................................................21

Green roofing 3

Conclusion..................................................................................................................................................22

References...................................................................................................................................................23

Tables..........................................................................................................................................................25

Table 1.....................................................................................................................................................25

Runoff Coefficient...................................................................................................................................25

Table 2.....................................................................................................................................................26

Annual Runoff Water Loss.....................................................................................................................26

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

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

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

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 (Environmental Protection Agency, 2013). Additional

(Weiler & Scholz-Barth, 2009)

Green roofing 8

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)

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).

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

Green roofing 10

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)

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

Hydrologic Cycle

Green roofing 11

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).

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

Green roofing 12

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.

(Environmental Science in Forestry, n.d.)

Green Roof as Part of

Stormwater Management Plan

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):

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).

Green roofing 14

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.

The hydraulic gradient formula looks like this (Cech, 2010, p. 122):

i=dh /dl

Where i=hydraulic gradient

dh=change∈elevationbetween two points at the top of the grade

dl=distancebetween 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.

Green roofing 15

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.)

I=rainfall intensity (inches perhour )

This indicates the intended storm frequency and how long the water will be concentrated in the

drainage medium

A=Area∈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.

Green roofing 16

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

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

Green roofing 17

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).

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

Green roofing 18

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

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

Green roofing 19

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)

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

Green roofing 20

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

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

Green roofing 21

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

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

Green roofing 22

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

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.

Green roofing 23

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&chunkid=999218983

Werthmann, C. (2007). Green Roof- A Case Study. Washington, D.C.: Princeton Architectural

Press. doi:SB419.5.W47 2007eb

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Green roofing 25

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"Cultivated land "0.08-0.41"Meadow "0.10-0.50"Parks, cemeteries "0.10-0.30"Unimproved areas "0.10-0.30"Pasture "0.12-0.62"Pasture with moderate grazing "0.10-0.30"Bare earth "0.20-0.90"Steep grassed area (2:1 slope) "0.50-0.70"Residential areas "0.30-0.75"Flat residential areas, 30% impervious "0.30-0.50"Flat residential areas, 70% impervious "0.50-0.80"Business areas "0.50-0.95"Flat commercial/industrial area, 90% impervious "0.50-0.90"

Asphalt or concrete streets "0.70-0.95"Brick streets "0.70-0.85"Roofs "0.75-0.95"

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Green roofing 26

Table 2Annual Runoff Water Loss

Table 2

Annual Runoff Water Loss (Weiler & Scholz-Barth, 2009)

Metropolitan Area 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, North Carolina "13.5-31.5"Pittsburgh, Pennsylvania "13.5-31.5"Houston, Texas "12.8-29.8"Greensville, South Carolina "12.7-29.5"Seattle, Washington "10.5-24.6"Chicago, Illinois "10.2-23.7"Raleigh-Durham/Chapel Hill, North Carolina "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, North Carolina "6.7-15.7"Dallas, Texas "6.2-14.4"[2]American Rivers, NRDC, Smart Growth America. "Paving Our Way to Water Shortages: How Sprawl Aggravates the Effects of Drought." 2002. p 2.

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