reducing urban heat islands: green roofs
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Reducing Urban Heat IslandsCompendium of Strategies
Green Roofs
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Acknowledgements
Reducing Urban Heat Islands: Compendium of Strategiesdescribes the
causes and impacts o summertime urban heat islands and promotes
strategies or lowering temperatures in U.S. communities. This compendium
was developed by the Climate Protection Partnership Division in the U.S.
Environmental Protection Agencys Oce o Atmospheric Programs. Eva
Wong managed its overall development. Kathleen Hogan, Julie Rosenberg,
and Andrea Denny provided editorial support. Numerous EPA sta in
oces throughout the Agency contributed content and provided reviews.
Subject area experts rom other organizations around the United States and
Canada also committed their time to provide technical eedback.
Under contracts 68-W-02-029 and EP-C-06-003, Perrin Quarles Associates,
Inc. provided technical and administrative support or the entire
compendium, and Eastern Research Group, Inc. provided graphics and
production services.
PositvEnergy provided support in preparing the Trees and Vegetation, Cool
Roos, and UHI Activities chapters under contract PO #2W-0361-SATX.
Experts who helped shape this chapter include:
Ryan Bell, Robert Berghage, Hitesh Doshi, Robert Goo, David Hitchcock,
Megan Lewis, Tom Liptan, Karen Liu, Greg McPherson, Dave Nowak, Steven
Peck, Katrin Scholz-Barth, Je Sonne, Benjamin Taube, Linda Velazquez,
Kathy Wol, Jim Yarbrough, and Barry Zalph.
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Contents
Green Roos 1
1. How It Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2
2. Green Roo Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Extensive Green Roos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
2.2 Intensive Green Roos . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
3. Benets and Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Benets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
3.3 Benet-Cost Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
4. Other Factors to Consider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
4.1 Site Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
4.2 Installation and Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
( L o w - P r o l e / E c o r o o s ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
(High-Prole/Roo Gardens) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
4.3 Fire Saety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
5. Green Roo Initiatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
6. Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Endnotes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
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Green Roos
Green roos are an emerging technol
ogy that can help communities miti
gate urban heat islands. A green roo
is a vegetative layer grown on a rootop.
As with trees and vegetation elsewhere,
vegetation on a green roo shades suraces
and removes heat rom the air through
evapotranspiration. These two mechanisms
reduce temperatures o the roo suraceand the surrounding air. The surace o a
vegetated rootop can be cooler than the
ambient air, whereas conventional rootop
suraces can exceed ambient air tempera
tures by up to 90F (50C).2 Green roos
can be installed on a wide range o build
ings, including industrial, educational,
and government acilities; oces; other
commercial property; and residences. This
chapter reviews:
How green roos work to mitigateheat islands
What types o green roos are available The benets and costs o green roos Other actors to consider in using this
mitigation strategy
Initiatives used to promote green roos Tools and resources to urther explore
this technology.
Opportunities to Expand Use oGreen Roos in Urban Areas
Most U.S. cities have signicant opportunities to
increase the use o green roos. As part o EPAs
Urban Heat Island Pilot Project, the Lawrence
Berkeley National Laboratory conducted analysesto estimate baseline land use and tree cover in
ormation or the pilot program cities.1 Figure 1
shows the percentage o roo cover in our o these
urban areas: roos account or 20 to 25 percent o
land cover. Even though not all these areas will be
likely candidates or installing a green roo, there
is a large opportunity to use green roos or heat
island mitigation.
0 5 10 15 20 25
Chicago
Houston
Sacramento
Salt Lake City
Percent Coverage
Figure 1: Roo Cover Statistics or Four U.S. Cities
(Below Tree Canopy)
3
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1 How It Works
With regard to urban heat islands, green
roos work by shading roo suraces and
through evapotranspiration. Using green
roos throughout a city can help reduce
surace urban heat islands and cool the air.
Shading.The plants o a green roo and
the associated growing medium, a specially
engineered soil, block sunlight rom reach
ing the underlying roo membrane. Though
trees and vines may not be common on
green roos, they indicate how other vege
tation on green roos shade suraces below
them. For example, the amount o sunlight
transmitted through the canopy o a tree
will vary by species. In the summertime,generally only 10 to 30 percent o the suns
energy reaches the area below a tree, with
Green Roo Market
In the United States demand and
interest in green roos has grown
tremendously. A survey o Green
Roos or Healthy Cities membersound that 25 percent more square
eet o green roong were installed
in the United States in 2005 than in
2004.3A Green Roos Project Data
base, available at , estimated
a total o 6.6 million square eet
(614,000 m2) o completed or ongo
ing green roo projects in the United
States as o June 2007. Germany,
widely considered a leader in green
roo research, technology, and usage,
has had decades o experience with
green roos. An estimated 10 percent
o all fat roos in Germany are roo
top gardens.4,5
Figure 2: Intensive Green Roo in
Frankurt, Germany
Germany has long been a leader in green roos; an
intensive green roo covers much o this building
in Frankurt.
WikimediaCommons
the remainder being absorbed by leavesand used or photosynthesis and some
being refected back into the atmosphere.
In winter, the range o sunlight transmit
ted through a tree is much wider10 to 80
percentbecause evergreen and decidu
ous trees have dierent wintertime oliage,
with deciduous trees losing the leaves and
allowing more sunlight through.6
Shading reduces surace temperatures
below the plants. These cooler suraces,in turn, reduce the heat transmitted into
buildings or re-emitted into the atmo
sphere. For example, a multi-month study
measured maximum surace temperature
reductions due to shade trees ranging rom
20 to 45F (11-25 C) or walls and roos
at two buildings.7Another study examined
the eects o vines on wall temperatures,
and ound reductions o up to 36F (20C).8
Furthermore, the growing medium o a
green roo itsel protects the underlyinglayers rom exposure to wind and ultravio
let radiation.
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Evapotranspiration. Plants absorb water
through their roots and emit it through their
leavesthis movement o water is called
transpiration. Evaporation, the conversion
o water rom a liquid to a gas, also occurs
rom the suraces o vegetation and the sur
rounding growing medium. Together, the
processes o evaporation and transpiration
are reerred to as evapotranspiration. Evapo
transpiration cools the air by using heat
rom the air to evaporate water.
Figure 3: Evapotranspiration and
Shading on a Green Roo
Evapotranspiration
Shading
Indoor
Thermal mass
KarenLiu
Plant shade reduces the sunlight that reaches the
roo. Evapotranspiration urther cools a green roo
by using heat to evaporate water rom the growing
medium and plant suraces
Green roo temperatures depend on the
roos composition, moisture content o the
growing medium, geographic location, so
lar exposure, and other site-specic actors.
Through shading and evapotranspiration,
most green roo suraces stay cooler than
conventional rootops under summertime
conditions. Numerous communities and
research centers have compared surace
temperatures between green and conventional roos. For example:
Chicago compared summertime suracetemperatures on a green roo with a
neighboring building. On an August
day in the early aternoon, with tem
peratures in the 90s, the green roo
surace temperature ranged rom 91 to
119F (33 to 48C), while the dark, con
ventional roo o the adjacent building
was 169F (76C). The near-surace air
temperature above the green roo was
about 7F (4C) cooler than that over
the conventional roo.9
A similar study in Florida ound thatthe average maximum surace tempera
ture o a green roo was 86F (30C)
while the adjacent light-colored roo
was 134F (57C).10
Reduced surace temperatures help build
ings stay cooler because less heat fows
through the roo and into the building. In
addition, lower green roo temperaturesresult in less heat transer to the air above
the roo, which can help keep urban air
temperatures lower as well. Some analyses
have attempted to quantiy the potential
temperature reductions over a broad area
rom widespread adoption o green roo
technology. A modeling study or Toronto,
Canada, or example, predicted that adding
green roos to 50 percent o the available
suraces downtown would cool the entire
city by 0.2 to 1.4F (0.1 to 0.8C). Irrigatingthese roos could urther reduce tempera
tures by about 3.5F (2C) and extend a 1
to 2F (0.5-1C) cooled area over a larger
geographic region. The simulation showed
that, especially with sucient moisture or
evaporative cooling, green roos could play
a role in reducing atmospheric urban heat
islands.11
A similar study in New York City modeled
air temperature reductions two meters, or6.5 eet, above the roo surace based on a
scenario assuming 100 percent conversion
o all available roos area to green roos.
The model results estimated a temperature
reduction o about 0.4F (0.2C) or the city
as a whole, averaged over all times o the
day. The model projected that temperatures
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Figure 4: Temperature Dierences between a Green and Conventional Roo
NationalCenterofExcellence/ASU
On a typical day, the Chicago City Hall green roo measures almost 80F (40C) cooler than the neighboring conventional roo.
at three oclock in the aternoon would
be reduced 0.8F (0.4C). The researchers
also evaluated, in detail, six areas withinthe city. The area with the highest 24-hour
average reduction in temperature had a
change o 1.1F (0.6C), and the reductions
at three oclock in the aternoon in those
six areas ranged rom 0.8F (0.4C) to 1.8F
(1.0C).12
2 Green Roo Types
A green roo can be as simple as a 2-inch
(5 cm) covering o hardy, alpine-likegroundcover, generally termed an exten
sive system, or as complex as a ully ac
cessible park complete with trees, called an
intensive system.
2.1 Extensive Green Roos
For the simpler, lighter weight extensive
green roo system, plant selections typi
cally include sedumssucculent, hardy
plantsand other vegetation generally suit
able or an alpine environment. The concept is to design a rugged green roo that
needs little maintenance or human interven
tion once it is established. Plants adapted to
extreme climates oten make good choices
and may not require permanent irrigation
systems. Overall, because o their light
weight, extensive systems will require the
least amount o added structural support,
which improves their cost-eectiveness
when retrotting an existing structure.
Extensive green roos have been grown on
roos with slopes o 30 or more, which
would equal a ratio o rise to run o 7:12 or
greater. (In contrast, a low-sloped roo with
a ratio o rise to run o 2:12 would have a
slope o 9.5.) The slope determines i the
roo will need additional support to hold
the growing medium and other parts o the
vegetative layer in place. Steeper roos may
retain less stormwater than an equivalent,fatter roo.
2.2 Intensive Green Roos
An intensive green roois like a conven
tional garden, or park, with almost no limit
on the type o available plants, including
large trees and shrubs. Building owners or
managers oten install these roos to save
energy and provide a garden environment
or the building occupants or the general
public to enjoy. Compared to extensive
green roos, intensive green roos are
heavier and require a higher initial invest
ment and more maintenance over the long
term than extensive roos. They generally
require more structural support to ac
commodate the weight o the additional
growing medium and public use. Intensive
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Figure 5: Combination Extensive/ Figure 6: Fords Dearborn Truck Plant: An
Intensive Green RooThe Rootop Example o an Extensive Green Roo
Garden on Chicagos City Hall
J.DavidMattox/CityofManhattan,
Kansa
s
Fords Dearborn Truck Plant in Michigan covers 10.4
acres (42,100 m2) and is anticipated to reduce the
buildings energy costs by 7 percent.15
GreenRoofsforHealthyCities/ww
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The photograph provides an example o a
combination extensive/intensive green roo on
Chicagos City Hall.
systems also need to employ irrigationin the Trees and Vegetation chapter andare briefy described here in the context o
systems, which can use rainwater captured
rom the roo or another source.
3 Benefts and Costs
Green roos provide many o the same
benets that trees and other ground level
vegetation provide. Green roos have an
advantage, though, in that they can be
used in dense, built-up areas that may nothave space or planting at the ground level.
The benets o vegetation were discussed
Green Roos and Green
Walls
In addition to green roos, build
ing owners can install green walls,
sometimes reerred to as living wallsor vertical gardens. These walls can
involve placing trellises or cables in
ront o exterior walls and allowing
vines to grow up them, or can be
more elaborate, with plants actually
incorporated into the wall.13
green roos.
3.1 Benets
Reduced Energy Use. Green roos can
save energy needed to cool and heat
the buildings they shelter. When green
roos are wet, they absorb and store large
amounts o heat, which reduces tempera
ture fuctuations. When dry, green roo lay
ers act as an insulator, decreasing the fow
o heat through the roo, thereby reducing
the cooling energy needed to reduce build
ing interior temperatures. In the winter,
this insulating eect means that less heat
rom inside the building is lost through the
roo, which reduces heating needs. In the
summertime, green roo vegetation reduces
roo surace temperatures and ambient air
temperatures, thus lowering cooling energy
demand. The insulating properties o green
roos vary as they are dynamic systems that
change throughout the year, particularly
with regard to water storage. As with cool
roos, discussed in the Cool Roo chapter,
green roos should not be used as a substi
tute or insulation.
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Figure 7: Green Wall in Huntsville, Alabama
This 2,000-square oot (190 m2) green wall on a store
in Huntsville, Alabama, is one o the largest in North
America. 14
EOA/ElmslieOslerArchitect
Green Roo TypesChanging Nomenclature?
The term low prole has been used
in place o extensive to describe
green roos that are lighter weight,
shallower, and simpler. Similarly,
high prole or deep prole hasbeen used instead o intensive to
describe a heavier, more complex
green roo system with deeper soil.
Figure 8 compares the average daily fow
o heat through a dark, conventional roo
and an extensive green roo in Ottawa,
Canada. During the spring and summer,
rom May to September 2001, the energy
demand needed to remove heat that fowedthrough the conventional roo was six to
eight kilowatt hours (kWh) a day, while the
green roos energy demand rom heat fow
was less than 1.5 kWh a day, a reduction o
more than 75 percent. In contrast, during
the all and winter months, rom Novem
ber 2000 through March 2001, heat fow
through the green roo was only slightly
less than the reerence roo in all months
except January, so that the energy demand
rom both roos was relatively similar.
During this time, snow had accumulated,
and the temperatures o both roos stayed
about the same.16
Although green roos can save energy both
in summer and winter, the specic savings
will depend on the local climate and individ
ual building and roo characteristics, such as
size, use, and insulation. For example:
Chicago estimates that its City Hallgreen roo project could provide cool
ing savings o approximately 9,270
kWh per year and heating savings o
740 million Btus.18This translates into
annual, building-level energy savings o
about $3,600.
A Canadian study modeled the heatingand cooling energy savings o a roughly
32,000- square oot (2,980 m2) green
roo on a one-story commercial build
ing in Toronto.19The analysis estimated
that the green roo could save about 6
percent o total cooling and 10 percent
o heating energy usage, respectively,or about 21,000 kWh total. The study
noted that the cooling energy savings
would be greater in lower latitudes.
For instance, when the authors ran the
same simulation or Santa Barbara, Cali
ornia, the cooling savings increased to
10 percent.
A study in central Florida measuredyear-round energy savings rom a green
roo. By the roos second summer, the
average rate o heat transer, or fux,through the green roo was more than
40 percent less than or the adjacent
light-colored roo. The reduced heat
fux was roughly estimated to lower
summertime energy consumption o
the 3,300 square oot (1,000 m2) project
building by approximately 2.0 kWh per
day.20 Under winter heating conditions,
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Figure 8: Comparison o Average Daily Energy Demand Due to Heat Flow Through an Extensive
Green versus Conventional Roo in Ottawa, Canada17
876
HeatFlow(kWh)543210 Kar
enLiu
Nov Dec Jan Feb Mar Apr May Jun Jul Aug SepThis chart shows the average daily energy demand due to observed heat ow through a green and conventional roo.
The period o evaluation was November 22, 2000, through September 30, 2001.
when the outdoor air temperature was
less than 55F (13C), the heat fux was
almost 50 percent less or the green
roo than or the conventional roo.21
Reduced Air Pollution and Greenhouse
Gas Emissions.As described in the
Trees and Vegetation chapter, vegetation
removes air pollutants and greenhouse
gas emissions through dry deposition and
carbon sequestration and storage. The
reduced energy demand rom green roos
also reduces air pollution and greenhouse
gas emissions associated with energy
production. Further, because ground-level
ozone orms more readily with the rise in
air temperatures, green roos help slow
the ormation o ground-level ozone by
lowering air temperatures. As with treesand vegetation, when selecting vegetation
or a green roo, building owners in areas
with poor air quality may want to consider
the volatile organic compound (VOC)
emissions rom certain plant species, as
VOCs are a ground-level ozone pre-cursor.
Plant suraces can remove certain pollut
ants rom the air through dry deposition.
A green roo can remove particulate mat
ter (PM) and gaseous pollutants, includ
ing nitrogen oxides (NOX), sulur dioxide
(SO2), carbon monoxide (CO), and ground-
level ozone (O3) rom the air. Many studies
have investigated the potential air pollutantremoval o green roos:
Researchers estimate that a 1,000-squareoot (93 m2) green roo can remove
about 40 pounds o PM rom the air in a
year, while also producing oxygen and
removing carbon dioxide (CO2) rom
the atmosphere.22 Forty pounds o PM
is roughly how much 15 passenger cars
will emit in a year o typical driving.23
A modeling study or Washington, D.C.,examined the potential air quality benets o installing green roos on 20
percent o total roo surace or buildings
with roos greater than 10,000 square eet
(930 m2). Under this scenario, green roos
would cover about 20 million square
eet (almost 2 million m2) and remove,
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annually, about 6.0 tons o O3 and almost
6 tons o PM o less than 10 microns
(PM10), or the equivalent o the pollutants
that could be absorbed by about 25,000
to 33,000 street trees.24
A similar study or the midtown areao Toronto modeled various greenroo scenarios and compared pollutant
reductions with existing baseline urban
tree and shrub benets. One scenario
involved green roos on fat roo surac
es, representing 20 percent o midtown
roos in total, such as commercial, high-
rise residential, and institutional build
ings. In that scenario, the green roos
removed about 10 to almost 20 percent
o the pollution that existing trees andshrubs remove, depending on the pol
lutant examined. I green roos were
added to all available suraces across
midtown Toronto, the model predicted
that green roos collective perormance
would increase to between roughly 25
and 45 percent o the reductions cur
rently obtained by existing vegetation.25
Vegetation and the growing medium on
green roos also can store carbon. Becausemany o the plants are small and the grow
ing medium layer is relatively thin, green
roos tend not to have as large a carbon
storage capacity as trees or urban orests.
Improved Human Health and Comort.
Green roos, by reducing heat transer
through the roo o a building, can improve
indoor comort and reduce heat stress as
sociated with heat waves. The use o cool
roos (see Cool Roo chapter) providessimilar indoor air temperature benets.
These improvements in building comort
can yield human health benets, particu
larly in non-air conditioned buildings.
Enhanced Stormwater Management
and Water Quality.Another key benet
o green roos is that they can reduce and
Figure 9: Green Roo on Seattle Public Library
Municipal buildings, such as this public library in
Seattle, have oten been used to demonstrate the
benets o green roos and the easibility o the
technology.
GreenRoofsforHealthyCities/w
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slow stormwater runo in the urban en
vironment. The plants and growing me
dium o a green roo, in the same manner
as other natural suraces and vegetation,
absorb water that would otherwise become
runo. The amount o rainall retained by
a green roo will depend primarily on the
depth o the growing medium and may also
be aected by the roo slope. Studies have
shown that extensive roos will typically
capture between 50 and nearly 100 percento incoming rain, depending on the amount
o growing medium used, the density o
vegetation, the intensity o an individual
rainstorm, and the requency o local rain
events.26An intensive green roo, with thick
er layers o growing medium, will capture
more rainall under comparable conditions
than an extensive roo. Field study results
below help illustrate these ndings:
A North Carolina study o actual greenroo perormance ound that test green
roos reduced runo rom peak rainall
events by more than 75 percent and that
the roos temporarily stored and then
released, through evapotranspiration,
more than 60 percent o all rainall.27
A Canadian green roo demonstration
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measured signicant reductions in
runo over a six-month period, with
steep reductions in ve o the six
months, and then lower reductions in
one month that had many large rain
events, which did not allow the grow
ing medium to dry out between events.
Overall, this project showed the green
roo reduced runo by more than 50
percent.28
A green roo demonstration projectin Portland, Oregon, examined runo
reductions over a 15-month period. In
that study, a green roo with about our
inches (10 cm) o growing medium
reduced runo by almost 70 percent.29
In addition, the authors noted that theretention rate appeared to increase over
time, which might be related to matur
ing vegetation. Because o the benets
in controlling stormwater, Portland has
approved green roos (or eco-roos)
as a technique to help meet stormwater
management requirements or new devel
opment and redevelopment projects. 30
Stormwater retention will vary with local
conditions, and communities generallyconsider this when projecting the poten
tial stormwater benets o green roos in
their area.
Even when a green roo does not retain all
the water rom a storm, it can detain runo
or later release and reduce the runo
rate. For example, the same Portland study
demonstrated that the green roo reduced
peak run-o rates by 95 percent during an
intense storm.31The North Carolina studyound that average peak runo rates rom
the green roos were roughly 75-85 percent
less than average peak rainall rates, so that
even when rain was alling on average at
Various research projects are un
derway to continue monitoring
pollutants in stormwater runoff
from green roofs, such as those at
Pennsylvania State Universitys Green
Roo Research Center, North Carolina
State Universitys Greenroo Research
program, the Green Roo Test Plots
research at the Chicago Center or
Green Technology, and Portland, Or
egons Eco-Roo program.
about 1.5 inches/hr (42 mm/hr), it ran o
the green roo at less than 0.25 inches/hr
(6 mm/hr).32 Reduced rates o runo can
help communities minimize fooding and
combined sewer overfow (CSO) events.*
The plants and growing medium o a
green roo not only retain and delay
the release o stormwater but also act
as a lter. Findings rom various studies
demonstrate the ability o green roos to
remove pollutants and highlight the need
to select growing media careully to avoid
elevated levels o certain pollutants, which
may initially leach rom organic materials.
A 2005 Canadian report synthesized past
studies on this issue.33 It noted that sev
eral studies rom Europe had ound that
green roos can bind and retain signicant
levels o pollutants, with one study stat
ing that green roos could remove up to
95 percent o the cadmium, copper, and
lead rom stormwater runo. The study
also summarized ndings rom a moni
toring program on a green roo in York,
Ontario, which ound decreased pollutant
concentrations compared to a control roo.
The reductions ranged rom 80 to almost
* Combined sewer systems are single-pipe systems that carr y sewage and stormwater runo together; when they overow during heavy rain, they dis
charge directly into surace waters.
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95 percent or several pollutants, such as
suspended solids, copper, and polycyclic
aromatic hydrocarbons. The same study,
however, ound increased concentrations
o nitrogen and phosphorous.
Recent research in Pennsylvania ound im
proved pH in green roo runo compared
to a conventional roo, as well as reduc
tions in total nitrate loadings based on the
reduced amount o stormwater rom the
green roo. The concentration o other pol
lutants in the green roo runo, in contrast,
was generally higher than concentrations
rom a conventional roo.34
As with the eld study in York, Ontario,
research in North Carolina ound increases
in total nitrogen and total phosphorous,
which the authors attributed to certain
compost materials in the roo substrate.35
Research in Portland and Toronto ound
that phosphorous levels appeared to de
crease over time as the green roo vegeta
tion matured and the phosphorous in the
initial substrate leached during rainall
events.36,37A German study also revealed
that a green roo retained more phosphate
as it matured, with retention percentage in
creasing rom about 26 percent in the rst
year to about 80 percent in the ourth.38
Enhanced Quality o Lie. Green roos
can provide many o the same quality o
lie benets as other urban greenery. People
in taller, neighboring buildings may enjoy
looking down at a rootop garden. Allowing
public access to rootop gardens provides
residents another green space to enjoy.
Finally, some researchers are evaluating the
potential or green roos to provide a sae
habitat or rare or endangered species, re
moving them rom ground-level predators.39
3.2 Costs
The costs o green roos vary depending
on the components, such as the growing
medium, type o roong membrane, drain
age system, use o encing or railings, and
type and quantity o plants. A 2001 reportestimated that initial costs start at $10
per square oot (0.09 m2) or the simpler,
extensive roo and $25 per square oot or
intensive roos.40 Other estimates assume
$15 to $20 per square oot. Costs in Germa
ny, where green roos are more prevalent,
range rom $8 to $15 per square oot.41
Prices in the United States may decline as
market demand and contractor experience
increase.
Initial green roo costs are more than those
o most conventional and cool roo tech
nologies (see Cool Roos chapter). Green
roos have a longer expected lie, though,
than most roong products, so the total
annualized costs o a green roo may be
closer to those o conventional and cool
roos. Los Angeles estimated that to retro
t a building with an extensive green roo
would cost rom $1.03-$1.66 per square
oot, on an annualized basis, while aconventional re-roong would range rom
$0.51-$1.74 per square oot.42
In addition to construction costs, a build
ing owner incurs maintenance costs to
care or the plants on a green roo. Al
though the level o care depends on plant
selection, most o the expenses arise in
the rst years ater installation, as the
plants establish themselves and mature.
For either an intensive or extensive roo,maintenance costs may range rom $0.75
to $1.50 per square oot. The costs o
maintaining an extensive roo decrease
ater the plants cover the entire roo,
whereas maintenance costs will remain
more constant or an intensive roo.43
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3.3 Benet-Cost Considerations
Although a green roo might have higher
initial costs than most conventional or
cool roos, a ull lie-cycle analysis can
identiy how the roo benets the build
ing owner. In many cases, these benetsjustiy the cost o green roos in densely
populated areas. In addition, a building
owner can directly benet rom reduced
energy use, reduced stormwater manage
ment ees, and increased roo lie. Finally,
the widespread adoption o green roos
may provide signicant, indirect net ben
ets to the community.
Although ew detailed, ull lie-cycle analy
ses exist, researchers and communitiesare beginning to invest in these evalua
tions. A report on the use o green roos
in New York City outlined one ramework
or a cost-benet analysis o green roos.44
The ramework incorporates both private
and public benets and costs (see Table
1). Under most hypothetical scenarios,
a green roo project yields net benets
when assessed with public benets, such
as reduced temperature and stormwater.
Table 1: Benet-Cost Elements or Green Roos
Under a high-perormance scenario that
generally assumes reduced costs rom
widespread adoption o green roo tech
nology and a mature market, an owner
would achieve net benets based on pri
vate benets alone.
A University o Michigan study compared
the expected costs o conventional roos
with the cost o a 21,000-square-oot (1,950
m2) green roo and all its benets, such
as stormwater management and improved
public health rom the NOXabsorption. The
green roo would cost $464,000 to install
versus $335,000 or a conventional roo
in 2006 dollars. However, over its lietime,
the green roo would save about $200,000.
Nearly two-thirds o these savings would
come rom reduced energy needs or the
building with the green roo.45
Portland, Oregon, meanwhile, has begun a
comprehensive cost-benet analysis o its
current eco-roo program, as the city plans
to expand green roo coverage rom 6
acres (24,300 m2) in 2007 to over 40 acres
(162,000 m2) in 2012.46
Benets/Costs Energy, Hydrology, and UHI Benets Other Benets
Private Benets
Reduced energy use
Extended service lie
Noise reductionAesthetic valueFood production
Public Benets
Reduced temperature
Reduced stormwater
Reduced installation costs (rom widespread
technology use)
Reduced air pollutantsReduced greenhouse gasesHuman health benets
Private Costs
Installation
Architecture/Engineering
Maintenance
N/A
Public Costs Program administration N/A
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Cool and Green Roos: Diferent Options or
Diferent Motivations
Cool and green roos both help to mitigate urban heat islands. The two technologieshave dierent cost and perormance implications, though, and the motivations or
selecting one or the other are typically dierent.
Cool roos generally have a minimal incremental
cost compared to their conventional equivalent.
Depending on the type o product (e.g., asphalt
shingle, concrete tile), costs can range roughly
between $0.50 to $6.00 per square oot. Costs can
vary greatly, though, depending on the size o the
job, ease o access to the roo, and local market
actors. The initial cost o a green roo, on the otherhand, is much higher, starting rom $10 per square
oot or the basic, extensive green roo.
Both cool and green roos lower surace and air temperatures and reduce summer
time peak and overall energy demand. The extent o the energy savings varies de
pending on actors including the local climate, attic ventilation and insulation levels,
andparticularly or green roosthe design and
maintenance o the roo.
Green roos provide additional benets, including
reducing and ltering stormwater runo, absorbingpollutants and CO2, providing natural habitat and
a sound barrier, and potentially serving as a recre
ational green space and having aesthetic value.
Communities or building owners with limited
budgets, who are primarily interested in energy savings or reducing peak energy
demand, generally ocus on cool roos. Whereas others, who can consider lie-cycle
costs and public benets, and who are interested in broader environmental impacts,
particularly improving stormwater management, may choose to install green roos.
Sustainability leaders, such as Chicago, recognize the value and opportunity or
both cool and green roo technologies and are supporting eorts to encourage
both options.
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Energy Savings and
Green Roos
For building owners and communities primarily interested in saving
energy, cool roos and other energy
eciency measures are generally
more cost-eective than green roos.
(See the Cool Roo chapter and the
ENERGY STAR website
or inorma
tion about a wide array o cost-e
ective energy ecient products and
practices.) Green roos provide ben
ets beyond energy savings, though,which is why they are attractive to
diverse interest groups and sustain-
ability advocates.
Figure 10: Green, or Eco, Roo in
Portland, Oregon
This apartment building in Portland, Oregon, is
among the 6 acres (24,300 m2) o green roos in the
city, as o 2007. Many roos remain candidates to
become green roos.
PortlandBureauofEnvironmentalServices
4 Other Factors to Consider
4.1 Site Characteristics
Recommendations or ideal site charac
teristics vary and oten depend on project
or program objectives. For example, Chi
cago and New York City are ocusing on
hot spot areas, which are oten ound in
dense, built up urban cores. Green roos
may be the only option to provide an e
ective amount o vegetation in these older
city centers that have vast amounts o
impervious cover and ew opportunities to
retroactively plant shade vegetation. Fur
ther, entities interested in providing recre
ational space or improving aesthetics may
also ocus on high density areas that arevisible rom adjoining or near by buildings.
On the other hand, stakeholders ocused
on saving energy and managing stormwater
oten target low-to-medium rise buildings
that have a large roo area. These sites, such
as the Fords Dearborn Truck Plant in Michi
gan, may be ound in less developed areas.
From a structural standpoint, existing roos
with concrete structural systems likely will
require the least amount o intervention;
roos with steel deck can require the most.
Installing a green roo on a fat or low-
sloped roo generally will be easier than
installing one on a steep-sloped roo. Also,
green roos tend to be easier to design into
new rather than existing buildings, given
that loads and other requirements can be
included in the design process. However,
retrot installations are becoming increas
ingly common in the expanding greenroo market. Many existing buildings, such
as low-sloped residential and commercial
buildings with large roo areas, can be
modied without signicant disruption
when replacing an old roo. For example,
projects at Carnegie Mellon University,
Tobyhanna Army Depot, and the Albemarle
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The Green Roo Continuum
The decision to install an extensive
or intensive green roo depends onavailable resources and the building
owners goals or how the roo will
be used. For example, someone with
a limited budget who desires minimal
maintenance and is mainly interested
in the energy and environmental ben
ets o a green roo, would most like
ly install an extensive green roo. On
the other hand, someone who wants
to create an accessible garden and
is able to maintain the green space,will probably install a more intensive
green roo. Many green roos incorpo
rate a combination o extensive and
intensive green roo eatures. These
semi-extensive or semi-intensive
green roos lie within the continuum
o green roo types, with extensive
and intensive at each end o the
spectrum.
County, Virginia, oce building have high
lighted the ease o replacing stone-ballast
on existing roos with vegetative layers.47
4.2 Installation and Maintenance
Whether extensive, intensive, or somewhere in between, green roos generally
consist o the same basic components.48
From the top layer down (see Figure 12),
these include:
Vegetation.The choice o vegetationdepends on the type o roo (extensive
or intensive), building design, local
climate, available sunlight, irrigation
requirements, anticipated roo use, and
similar actors: Extensive green roo plants are
typically hardy perennials. They are
preerably shallow-rooting, sel-
generating plants that spread rap
idly and require minimal nutrients.
They should tolerate sun, wind,
and extreme temperature fuctua
tions. Succulents, such as sedums,
are well adapted or green roos
because they are drought-resistant
and their high water content makesthem re resistant. Sedums come
Figure 11: A Green Roo Replaces a Stone Ballast Roo
Albemarle County, Virginia, replaced the stone ballast
roo on its county ofce building with a green roo in
2005. The project received money rom the Chesapeake
Bay Program through the Virginia Department o
Conservation and Recreation.
GregorPatsch/AlbemarleCounty
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Figure 12: Typical Layers o a Green Roo
KarenLiu/NationalResearchCouncilofCanada
in a wide variety o sizes, textures,
and colors. Building owners also
can ensure that the selected plants
suit USDA plant hardiness zones or
their area.49
Intensive green roos have deeper
growing media, which allows them
to incorporate larger plants, in
cluding shrubs, bushes, and trees,
in their design. Most intensivegreen roos also have irrigation
systems that can support a wide
variety o plants.
A lightweight, engineeredgrowing me-dium may or may not include soil as
the primary organic matter. The plant
ing media used in green roo systems
are usually engineered to provide the
best support or plants with the lightest
weight and can be tailored to maximize
water retention without water-logging
the plants. A growing medium should
ideally last as long as the roo it will
cover. Typically, the growing medium
will consist primarily o lightweight
inorganic mineral materials (at least 80
percent) and up to 20 percent organic
materials like topsoil.50 Extensive green
roos use up to roughly 6 inches (15
cm) o growing medium51 while inten
sive green roos use 8 inches (20 cm)
or more.52
Aflter membrane is usually a geotextile that allows excess water rom
the growing medium to fow out, while
preventing the ne particles rom wash
ing away and clogging the roo drain.
Adrainage layerhelps the excess water rom the growing medium to fow tothe roo drain, which prevents over
loading the roo and provides a good
air-moisture balance in the growing
medium. Some drainage layers take the
orm o egg crates to allow or some
water storage.
Aroot barriercan protect the roomembrane rom aggressive plant roots,
which may penetrate the waterproong
layer and cause leaks.
Awaterproofng/roofng membraneprotects the building rom water pen
etration. Any roong membrane can be
used in green roos, although single-ply
waterproong membranes are gener
ally thicker and more durable on green
roos than on conventional ones. Some
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membranes are naturally protected
rom root penetration, while others
require a root barrier.
Acover boardis a thin, semi-rigid boardthat provides protection, separation, and
support or a roong membrane. Thermal insulation can be installed
either above or below the membrane
o a green roo. The insulation value o
the growing medium in a green roo
increases as its moisture content de
creases. However, green roos are not
a substitute or conventional insula
tion; using the recommended insula
tion levels or ones local climate helps
conserve energy.
Avapor barrieris typically a plastic oroil sheet that resists passage o mois
ture through the ceiling.
Building and roo structural sup-port.The components o a green roo
weigh more than conventional roong
materials, and thus the roo requires
support panels. Not only are the roo
ing membranes and other materials
heavier on a green roo, but the roo
design also must account or the weighto water-saturated plants and growing
medium. An extensive roo typically
weighs rom 15-30 pounds per square
oot, although the range will depend on
the depth o the growing medium and
other site-specic actors.53An inten
sive roo can weigh much more, with
signicantly greater depth o growing
medium, more extensive vegetation, and
people using the space. Building own
ers must ensure that the structure cansupport the green roo even when ully
saturated, in addition to meeting build
ing code requirements or snow and
wind loads. Reinorcing roo supports
on existing buildings adds to the proj
ect cost but can usually be worked into
building retrot or renovation plans. It
is oten easier to put green roos on new
Inverted Roo Membrane
Assemblies (IRMAs) and
Green Roos
Inverted roo membrane assemblies
(IRMAs) have insulation above the
waterproong membrane, as opposed
to conventional roos, which have
insulation below the membrane. This
design protects the membrane and
prolongs the lie o the roo. A green
roo that has insulation between its
vegetative layer and the waterproong
membrane is an IRMA, with the veg
etation protecting the membrane andweighing down the insulation. More
conventional IRMAs use concrete pav
ers or stones or ballast. These IRMAs
oten make good candidates or green
roo retrots, as the conventional bal
last can be replaced with the neces
sary green roo layers.
buildings, as the requirements or the
added roo load can be included as parto the initial design parameters, and the
cost or the upgrade is usually minimal.
Although both extensive and intensive
green roos share these basic components,
their characteristics vary (see Figure 13).
Most important, the intensive green roos
are likely to require more structural sup
port and enhanced irrigation systems to
support the wider variety o plants, in
creased weight loads, and desired publicaccess. However, intensive roos will prob
ably also retain more rainall and support
more species.54
In addition, any green roo generally will
require some ongoing maintenance. Ex
tensive green roos not designed or pub
lic access have ewer obligations. For an
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intensive roo, maintenance can be continu
ous, similar to a traditional garden, because
aesthetics will be more important.
For either roo, early weed control is im
portant to ensure that the installed plants
have a chance to spread and to minimize
the opportunity or invasive weeds to
take root. According to a ederal guide on
green roos,55 weeding might be necessary
monthly or quarterly or the rst two years
and might be reduced to only once a year
in many cases ater the plants have ully
covered the roo. The guide also lists other
important maintenance activities including:
Fertilize. Given the thin layer ogrowing medium, building owners or
managers might need to apply a slow
release ertilizer once a year to avoid
soil acidity, especially when the plants
are rst establishing themselves.
Irrigate.An ideal green roo couldrely on natural irrigation, especially or
extensive roos. However, some green
roos might require irrigation based
on local climate and the stage o plant
growth or a particular project. Irriga
tion might also be needed to reduce
re risks or to increase evaporative
cooling. Almost all intensive green
roos need irrigation systems. Extensive
green roos, however, may only need
them during plant establishment. For
large, extensive green roos, building
owners oten install a drip irrigation
system, which is generally inexpensive
and saves the time and eort o having
someone manually water the roo. Replant. Over time, some level o
replanting or addition to the growing
medium might be necessary.
Figure 13: Comparison o Common Features o Extensive and Intensive Roos
Extensive vs. Intensive
(Low-Prole/Ecoroos) (High-Prole/Roo Gardens)
Low growth media: 2 6" > 6"-15" and deeper Lightweight: 13 50 lbs/s Heavier weights: 50+ lbs/s Low growing plants: 1" 24" H Trees, shrubs and more Less variety o plants: Alpine types, Huge variety o plant selection/
succulents, herbs, some grasses architectural eatures depending
and mosses on loads, design & budget
Usually non-accessible and Designed or human recreation:non-recreational gardening, socializing, etc.
Slopes up to 30 & higher Relatively fat Less expensive: $5-$25/s More expensive: $25-$40+/s Low water requirements Irrigation usually necessary Low maintenance Higher maintenance
LindaVelasquez
There is no ofcial denition o an extensive or intensive green roo. This chart is not meant to strictly dene these
green roo types and instead aims to describe the general characteristics o roos at each end o the continuum.
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Figure 14: A Modular Green Roo on a
Sloped Residential Roo
This home in Arizona shows a modular green roo
on a steep-sloped roo.
Greenroofs
.comandKellyLuckett/Gr
eenRoofBlocks
Modular Green Roo Systems
Some green roo systems use modular
components. These components are
generally plastic trays a ew eet long
(~0.5-1 m) on each side and several
inches (~10-20 cm) deep. They are
lled like fowerpots with growing
media and the desired plants and
placed directly on top o the existing roo. The grid o trays covers the
roos surace to provide benets simi
lar to built-in green roos. Moving or
replacing individual modules is poten
tially easier than changing or repair
ing parts o a non-modular green roo.
Modular roos, however, are relatively
new, and have not been as widely
studied as non-modular roos.
Clean Gutters. Similar to conventionalroos, clean gutters decrease the risk o
standing water and leaks. It is also nec
essary to keep drains and gutters clear
o plant growth to prevent blockage.
In addition to routine maintenance, green
roos may require repairs over time, al
though the expected lie o a green roo is
about twice that o a conventional roo.56
I correctly installed, the membrane under
the vegetation o a green roo is expected
to last 30 to 50 years.
4.3 Fire Saety
Green roos, when saturated with water,
can retard the spread o re,57 but dry
plants on a green roo can be a re hazard.
The most common ways to increase re
saety are to:
Avoid grasses and plants that coulddry up in summer and instead use reresistant plants, like sedums, and a
growing medium that is low in organic
material content.
Construct re breaks on the roo2-oot (0.6 m) widths o concrete or
gravel at 130-oot (40 m) intervals.
Another precaution that some practitioners
recommend is to install sprinkler irrigation
systems and connect them to a re alarm.
5 Green Roo Initiatives
Green roo research eorts are growing with
an increasing number o universities oer
ing courses or developing centers ocused
on improving our understanding o green
roo technology. Many communities are also
taking action by encouraging or sponsor
ing green roo projects. These initiatives are
typically motivated by various environmental
concerns, mainly stormwater management,but also the desire to reduce urban heat
islands and enhance the urban ecosystem.
Many o these eorts involve a single dem
onstration or showcase project as a highly
visible means to promote green roo technol
ogy, such as the green roo on Atlantas City
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Many green roo projects are motivated
not by government policies but by a de
sire to show a commitment to sustain-
able design and the environment.
Hall patio. Some cities such as Chicago, Port
land, Seattle, and Toronto have been develop
ing more coordinated programs and policies
to promote green roos. The Heat Island
Reduction Activities chapter provides many
examples demonstrating the wide range o
green roo eorts. Table 2 identies some o
the research activity and options available or
taking action to advance green roos.
Green building programs in many com
munities provide another opportunity to
encourage green roo installation. The U.S.
Green Building Council (USGBC) Leader
ship in Energy and Environmental Design
(LEED) Rating System (see ) and Green Globes operated by the
Green Building Initiative (GBI) in the
United States (see ), are
two rating systems that communities are
using. These and other systems give credit
or a broad range o building and devel
opment techniques that save energy and
protect the environment. Green roos can
achieve credit under multiple categories
such as stormwater management, heat
island mitigation, water eciency, energy
and atmosphere, materials and resources,
and innovation and designdepending onhow they are constructed.
Figure 15: A Newly Installed Green Roo in New York City
2008J.BurlingChaseforSustainableSouthBronx
Initiatives to install green roos in urban areas reduce urban heat islands and can help to create jobs in the local
economy, such as this roo installed by graduates o Sustainable South Bronxs Bronx Environmental Stewardship
Training (BEST) program.
GREEN ROOFS DRAFT 19
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Table 2: Examples o Green Roo Initiatives
Type o Initiative Description Links to Examples
Research University
programs
- Michigan State University began its
Green Roo Research Program in 2000 to assist with the design and study o
Fords Rouge Plant. The program has since expanded and now investigates
green roo plant selection among other topics.
- Penn State
Universitys Center or Green Roo Research studies the energy savings,
stormwater retention and ltration, and other benets o green roos.
- North Carolina State University has
extensive green roo test sites in Goldsboro and Kinston, North Carolina, as
part o the Biological and Agricultural Engineering Program.
- The University o Central Florida ocuses
primarily on stormwater management, which has led to its investigations o
green roos.
- The British Columbia Institute o
Technologys Centre or the Advancement o Green Roo Technology collabo
rates with industry to support and improve the deployment o green roos.
Voluntary eorts Demonstration
projects
- For background inormation on Chicago
City Halls green roo, see the Links section o this site.
This site provides an overview o the Atlanta City Hall green roo demon
stration project.
Incentives - Chicago has sponsored a green roo
grant program or several years. Grants o up to $5,000 each were available
in the application cycle that ended in January 2008. See the Department oEnvironment page and browse under Initiatives and Programs.
- Torontos green
roo incentive program oers grants o Canadian $50 per square meter
or eligible projects, up to a total o $10,000 or single-amily homes and
$100,000 or all other buildings.
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http://www.hrt.msu.edu/greenroofhttp://hortweb.cas.psu.edu/research/greenroofcenterhttp://www.bae.ncsu.edu/greenroofshttp://www.stormwater.ucf.edu/http://commons.bcit.ca/greenroof/http://www.chicagogreenroofs.org/http://www.atlantaga.gov/mayor/energyconservationgreenroof.aspxhttp://egov.cityofchicago.org/http://www.toronto.ca/greenroofs/incentiveprogram.htmhttp://www.toronto.ca/greenroofs/incentiveprogram.htmhttp://egov.cityofchicago.org/http://www.atlantaga.gov/mayor/energyconservationgreenroof.aspxhttp://www.chicagogreenroofs.org/http://commons.bcit.ca/greenroof/http://www.stormwater.ucf.edu/http://www.bae.ncsu.edu/greenroofshttp://hortweb.cas.psu.edu/research/greenroofcenterhttp://www.hrt.msu.edu/greenroof -
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Table 2: Examples o Green Roo Initiatives (Continued)
Type o Initiative Description Links to Examples
Voluntary eorts Incentives - Portland, Or
egon, oers grants, workshops, and other technical assistance to support
green roos.
- The Hous
ton Downtown Management District (HDMD) Vertical Gardens Matching
Grant initiative is intended to assist in the acilitation o wall cover plantings
and exceptional landscaping on blank walls, parking garages, and side
walks; improving overall aesthetics, pedestrian comort, and air quality; and
reducing the heat island eect.
Outreach &
education
- EPAs Heat Island Reduction Initiative pro
vides inormation on the temperature, energy, and air quality impacts rom
green roos and other heat island mitigation strategies.
- EPAs Oce o Water highlights design options, including green roos, that reduce
stormwater runo and water pollution.
- Green Roos or Healthy Cities hosts a series o
green roo design and implementation workshops throughout North America.
Policy eorts Density bonus
provisions in
zoning codes
- Document that highlights eorts o Chicago; Seattle; Portland, Or
egon; Toronto; and Waterloo, Ontario, to encourage green roo installations
by oering density bonus incentives in their zoning codes.
GREEN ROOFS DRAFT 21
http://www.portlandonline.com/bes/index.cfm?c=43077http://www.houstondowntown.com/Home/Business/DoingBusiness/DevelopmentAssistance/Development%20Assistance.PDFhttp://www.houstondowntown.com/Home/Business/DoingBusiness/DevelopmentAssistance/Development%20Assistance.PDFhttp://www.houstondowntown.com/Home/Business/DoingBusiness/DevelopmentAssistance/Development%20Assistance.PDFhttp://www.epa.gov/heatisland/http://cfpub.epa.gov/npdes/home.cfm?program_id=298http://www.greenroofs.org/http://commons.bcit.ca/greenroof/publications/2006_regulations.pdfhttp://commons.bcit.ca/greenroof/publications/2006_regulations.pdfhttp://commons.bcit.ca/greenroof/publications/2006_regulations.pdfhttp://commons.bcit.ca/greenroof/publications/2006_regulations.pdfhttp://commons.bcit.ca/greenroof/publications/2006_regulations.pdfhttp://www.greenroofs.org/http://cfpub.epa.gov/npdes/home.cfm?program_id=298http://www.epa.gov/heatisland/http://www.houstondowntown.com/Home/Business/DoingBusiness/DevelopmentAssistance/Development%20Assistance.PDFhttp://www.portlandonline.com/bes/index.cfm?c=43077 -
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6 Resources
Table 3 lists some guidance documents and
organizations that promote green roos.
Table 3: Green Roo Resources
Name Description Web Link
Guidance Documents
U.S. Department o Energy
Federal Technology Alert:
Green Roos
DOEs Energy Efciency and Renewable Energy pro
gram publishes technology alerts and developed
this primer on green roo technology.
Green Roos as Urban
Ecosystems: Ecological
Structures, Functions, and
Services
The journal Bioscience November 2007 issue contains
this comprehensive article summarizing the research
on green roos and their costs and benets.
National Roong Contrac
tors Association Green
Roo Systems Manual
The NRCA has recently released a guidebook or
sale that ocuses on the waterproong needs o
green roos.
Los Angeles Green Roo
Resources Guide
The City o Los Angeles developed this guide as a
resource or individuals and groups interested in
developing green roos in Los Angeles. This guide
includes inormation on how to plan, design, and
maintain a green roo.
Other Resources
Green Roos or Healthy
Cities
Green Roos or Healthy Cities oers resources
on green roo installation, benets, projects, andtraining. This group also publishes the Green Roof
Infrastructure Monitor.
Greenroos.com Greenroos.com provides green roo industry
resources, including how-tos, plant lists, reerences,
and an international database o green roo projects.
Chicago Green Roo
Program
Chicagos Green Roo Program has online inorma
tion on building green roos in Chicago, including
an aerial map o completed and planned projects,
requently asked questions, eatured projects, and
links to other resources.
REDUCING URBAN HEAT ISLANDS DRAFT22
http://www.nrel.gov/docs/fy04osti/http://www.aibs.org/bioscience-press-releases/resources/11-07.pdfhttp://www.aibs.org/bioscience-press-releases/resources/11-07.pdfhttp://www.aibs.org/bioscience-press-releases/resources/11-07.pdfhttp://www.nrca.net/rp/pubstore/http://www.fypower.org/pdf/LA_GreenRoofsResourceGuide.pdfhttp://www.fypower.org/pdf/LA_GreenRoofsResourceGuide.pdfhttp://www.fypower.org/pdf/LA_GreenRoofsResourceGuide.pdfhttp://www.greenroofs.org/http://www.greenroofs.com/http://www.chicagogreenroofs.org/http://www.chicagogreenroofs.org/http://www.greenroofs.com/http://www.greenroofs.org/http://www.fypower.org/pdf/LA_GreenRoofsResourceGuide.pdfhttp://www.fypower.org/pdf/LA_GreenRoofsResourceGuide.pdfhttp://www.fypower.org/pdf/LA_GreenRoofsResourceGuide.pdfhttp://www.nrca.net/rp/pubstore/http://www.aibs.org/bioscience-press-releases/resources/11-07.pdfhttp://www.aibs.org/bioscience-press-releases/resources/11-07.pdfhttp://www.aibs.org/bioscience-press-releases/resources/11-07.pdfhttp://www.nrel.gov/docs/fy04osti/ -
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Endnotes
1 Rose, L.S., H. Akbari, and H. Taha. 2003. Characterizing the Fabric o the Urban Environment: A
Case Study o Greater Houston, Texas. LBNL-51448, January 2003.
2 Liu, K. and B. Baskaran. 2003. Thermal perormance o green roos through eld evaluation.
National Research Council o Canada-46412.
3 Final Report, Green Roo Industry Survey, 2006, prepared by Green Roos or
Healthy Cities, April 2007. Retrieved 7 Dec. 2007 rom .
4 The Green Roo Research Program at MSU. Retrieved 7 August 2007 rom .
5 Peck, S.W., C. Callaghan et al. 1999. Greenback, rom Green Roos: Forging a New Industry in
Canada. Prepared or Canada Mortgage and Housing Corporation.
6 Huang, J., H. Akbari, and H. Taha. 1990. The Wind-Shielding and Shading Eects o Trees on
Residential Heating and Cooling Requirements. ASHRAE Winter Meeting, American Society o
Heating, Rerigerating and Air-Conditioning Engineers. Atlanta, Georgia.
7 Akbari, H., D. Kurn, S. Bretz, and J. Hanord. 1997. Peak power and cooling energy savings oshade trees. Energy and Buildings. 25:139-148.
8 Sandier, S. and B. Givoni. 2002. Thermal Eects o Vines on Wall TemperaturesComparing
Laboratory and Field Collected Data. SOLAR 2002, Proceedings o the Annual Conerence o the
American Solar Energy Society. Reno, NV.
9 Department o Energy 2004. Federal Technology Alert: Green Roos. DOE/EE-0298, Washington, D.C.
10 Cummings, J., C. Withers, J. Sonne, D. Parker, and R. Vieira. 2007. UCF Recommissioning, Green
Roong Technology, and Building Science Training; Final Report. FSEC-CR-1718-07. Retrieved
18 December 2007 rom .
11 Liu, K. and B. Bass. 2005. Perormance o Green Roo Systems. National Research Council
Canada, Report No. NRCC-47705, Toronto, Canada.12 Rosenzweig, C., W. Solecki et al. 2006. Mitigating New York Citys Heat Island with Urban For
estry, Living Roos, and Light Suraces. Sixth Symposium on the Urban Environment and Forum
on Managing our Physical and Natural Resources, American Meteorological Society, January 31,
2006, Atlanta, GA.
13 Bass, B. and B. Baskaran. 2003. Evaluating Rootop and Vertical Gardens as an Adaptation Strate
gy or Urban Areas. National Research Council Canada, Report No. NRCC-46737, Toronto, Canada.
14 McKeough, T. Room to Improve. New York Times. 21 Feb. 2008. Retrieved 10 Mar. 2008 rom
.
15 Ford Motor Company. Ford Installs Worlds Largest Living Roo on New Truck Plant. Retrieved 2
August 2007 rom .
16 Liu, K. 2002. A National Research Council Canada Study Evaluates Green Roo Systems Thermal
Perormances. Proessional Roong.
17 Bass, B. and B. Baskaran. 2003. Evaluating Rootop and Vertical Gardens as an Adaptation Strate
gy or Urban Areas. National Research Council Canada, Report No. NRCC-46737, Toronto, Canada.
18 Department o Environment. Chicago City Hall green roo project. Retrieved 18 October 2007
rom .
GREEN ROOFS DRAFT 23
http://www.greenroofs.org/storage/2006grhcsurveyresults.pdfhttp://www.greenroofs.org/storage/2006grhcsurveyresults.pdfhttp://www.hrt.msu.edu/faculty/Rowe/Green_roof.htmhttp://www.hrt.msu.edu/faculty/Rowe/Green_roof.htmhttp://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1718-07.pdfhttp://www.nytimes.com/2008/02/21/garden/21room.htmlhttp://media.ford.com/newsroom/release_display.cfm?release=15555http://egov.cityofchicago.org/http://egov.cityofchicago.org/http://media.ford.com/newsroom/release_display.cfm?release=15555http://www.nytimes.com/2008/02/21/garden/21room.htmlhttp://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1718-07.pdfhttp://www.hrt.msu.edu/faculty/Rowe/Green_roof.htmhttp://www.hrt.msu.edu/faculty/Rowe/Green_roof.htmhttp://www.greenroofs.org/storage/2006grhcsurveyresults.pdfhttp://www.greenroofs.org/storage/2006grhcsurveyresults.pdf -
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19 Bass, B. and B. Baskaran. 2003. Evaluating Rootop and Vertical Gardens as an Adaptation Strate
gy or Urban Areas. National Research Council Canada, Report No. NRCC-46737, Toronto, Canada.
20 Cummings, J., C. Withers, J. Sonne, D. Parker, and R. Vieira. 2007. UCF Recommissioning, Green
Roong Technology, and Building Science Training; Final Report. FSEC-CR-1718-07. Retrieved
18 Dec. 2007 rom .
21
Sonne, J. Energy Perormance Aspects o a Florida Green Roo, Fiteenth Symposium on Improving Building Systems in Hot and Humid Climates, July 24-26, 2006 Orlando, FL.
22 Peck, S. and M. Kuhn. 2003. Design Guidelines or Green Roos. Canada Mortgage and Housing
Corporation, Ottawa, and the Ontario Association o Architects, Toronto.
23 This comparison assumes each car will produce 0.1g o PM per mile (based on new ederal
standards that would limit PM emissions to this level or lower in passenger vehicles), and that
each car is driven 12,500 miles (20,000 km) in a year, which was the average mileage or a car
in America in 2004. See U.S. Department o Transportation Federal Highway Administration.
Annual Vehicle Distance Traveled in Miles and Related Data-2004. Highway Statistics 2004.
October 2005. Retrieved October 19, 2007 rom .
24 Casey Trees Endowment Fund and Limno-Tech, Inc. 2005. Re-Greening Washington, D.C.: A GreenRoo Vision Based on Quantiying Storm Water and Air Quality Benets. Washington, D.C.
25 Currie, B.A. and B. Bass. 2005. Estimates o Air Pollution Mitigation with Green Plants and
Green Roos Using the UFORE Model. Sixth Biennial Canadian Society or Ecological Economics
(CANSEE) Conerence, October 27-29, 2005, Toronto, Canada.
26 VanWoert, N.D., D.B. Rowe, J.A. Andresen, C.L. Rugh, R.T. Fernandez, and L. Xiao. 2005. Green
Roo Stormwater Retention: Eects o Roo Surace, Slope, and Media Depth. Journal o Envi
ronmental Quality 34:1036-1044.
27 Moran, A., B. Hunt et al. 2004. A North Carolina Field Study to Evaluate Greenroo Runo
Quantity, Runo Quality, and Plant Growth. Paper Presented at Green Roos or Healthy Cities
Conerence, Portland, OR, June 2004.
28 Liu, K. 2003. Engineering perormance o rootop gardens though eld evaluation. National
Research Council Canada, Report No. NRCC-46294, Ontario, Canada.
29 Hutchinson, D., P. Abrams et al. 2003. Stormwater Monitoring Two Ecoroos in Portland, Or
egon, USA. Proceedings o Greening Rootops or Sustainable Communities, 2003, Chicago, IL.
30 Portland. 2002. City o Portland EcoRoo Program Questions and Answers. Bureau o Environ
mental Services, Oce o Sustainable Development, City o Portland, Oregon, PL 0203, Port
land, OR.
31 Hutchinson, D., P. Abrams et al. 2003. Stormwater Monitoring Two Ecoroos in Portland, Or
egon, USA. Proceedings o Greening Rootops or Sustainable Communities, 2003, Chicago, IL.
32 Moran, A., B. Hunt et al. 2004. A North Carolina Field Study to Evaluate Greenroo Runo
Quantity, Runo Quality, and Plant Growth. Paper Presented at Green Roos or Healthy Cities
Conerence, Portland, OR, June 2004.
33 Banting, D., H. Doshi, J. Li, and P. Missios. 2005. Report on the Environmental Benets and
Costs o Green Roo Technology or the City o Toronto. Department o Architectural Science,
Ryerson University.
34 Berghage, R., D. Beattie, A. Jarrett, and T. OConner. 2007. Greenroo Runo Water Quality. Fith
Annual Greening Rootops or Sustainable Communities Conerence, April 29-May 1, 2007.
REDUCING URBAN HEAT ISLANDS DRAFT24
http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1718-07.pdfhttp://www.fhwa.dot.gov/policy/ohim/hs04/htm/vm1.htmhttp://www.fhwa.dot.gov/policy/ohim/hs04/htm/vm1.htmhttp://www.fhwa.dot.gov/policy/ohim/hs04/htm/vm1.htmhttp://www.fhwa.dot.gov/policy/ohim/hs04/htm/vm1.htmhttp://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1718-07.pdf -
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35 Moran, A., B. Hunt et al. 2004. A North Carolina Field Study to Evaluate Greenroo Runo
Quantity, Runo Quality, and Plant Growth. Paper Presented at Green Roos or Healthy Cities
Conerence, Portland, OR, June 2004.
36 Hutchinson, D., P. Abrams et al. 2003. Stormwater Monitoring Two Ecoroos in Portland, Or
egon, USA. Proceedings o Greening Rootops or Sustainable Communities, 2003, Chicago, IL.
37
Van Seters, T., L. Rocha, and G. MacMillan. 2007. Evaluation o the Runo Quantity and QualityPerormance o an Existing Green Roo in Toronto, Ontario. Fith Annual Greening Rootops or
Sustainable Communities Conerence, April 29-May 1, 2007.
38 Kohler, M. and M. Schmidt. 2003. Study on Extensive Green Roos in Berlin. Translated by S.
Cacanindin. Retrieved 27 April 2006 rom .
39 Banting, D., H. Doshi, J. Li, and P. Missios. 2005. Report on the Environmental Benets and
Costs o Green Roo Technology or the City o Toronto. Department o Architectural Science,
Ryerson University.
40 Peck, S. and M. Kuhn. 2001. Design Guidelines or Green Roos. National Research Council
Canada, Toronto, Canada.
41 Scholz-Barth, K. 2001. Green Roos: Stormwater Management rom the Top Down. Environmen
tal Design & Construction.
42 City o Los Angeles, Environmental Aairs Department. 2006. Green RoosCooling Los Ange
les (A Resource Guide). Los Angeles, CA.
43 Peck, S. and M. Kuhn. 2001. Design Guidelines or Green Roos. National Research Council
Canada, Toronto, Canada.
44 Rosenzweig, C., S. Gan, and L. Parshall (Eds.). 2006. Green Roos in the New York Metropoli
tan Region: Research Report. Columbia University Center or Climate Systems Research and
NASA Goddard Institute or Space Studies. New York. 59 pages.
45 Clark, C., P. Adriaens, and F.B. Talbot. 2007. Green Roo Valuation: A Probabilistic Analysis o
Environmental Benets.
46 Personal correspondence with Tom Liptan, Portland Bureau o Environmental Services, 18 De
cember 2007.
47 In 2005, Carnegie Mellon University replaced a stone ballast roo on Hamerschlag Hall with a
green roo . When Albemarle County
replaced a stone ballast roo with a green roo in 2007, it did not have to modiy the roo be
cause the saturated vegetative layer weighed about the same as the stones and the underlying
membrane and insulation remained the same . In 2006, Tobyhanna Army Depot replaced a stone ballast roo
with a modular green roo. The roo had already been designed with capacity to support an
extra foor, so no modication was required to install a green roo .
48 Peck, S. and M. Kuhn. 2001. Design Guidelines or Green Roos. National Research Council
Canada, Toronto, Canada.
49 Department o Energy. 2004. Federal Technology Alert: Green Roos. DOE/EE-0298, Washington, D.C.
50 Beattie, D., and R. Bergharge. 2004. Green Roo Media Characteristics: The Basics. In Greening
Rootops or Sustainable Communities, Portland, Oregon, June 2004.
51 Scholz-Barth, K. 2001. Green Roos: Stormwater Management rom the Top Down. Environmen
tal Design & Construction.
GREEN ROOFS DRAFT 25
http://www.roofmeadow.com/http://www.greenroofs.com/projects/pview.php?id=292http://www.albemarle.org/department.asp?department=planning&relpage=8660http://www.albemarle.org/department.asp?department=planning&relpage=8660http://aec.army.mil/usaec/publicaffairs/update/win07/win0709.htmlhttp://aec.army.mil/usaec/publicaffairs/update/win07/win0709.htmlhttp://aec.army.mil/usaec/publicaffairs/update/win07/win0709.htmlhttp://aec.army.mil/usaec/publicaffairs/update/win07/win0709.htmlhttp://www.albemarle.org/department.asp?department=planning&relpage=8660http://www.albemarle.org/department.asp?department=planning&relpage=8660http://www.albemarle.org/department.asp?department=planning&relpage=8660http://www.greenroofs.com/projects/pview.php?id=292http://www.roofmeadow.com/ -
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52 Department o Energy. 2004. Federal Technology Alert: Green Roos. DOE/EE-0298, Washington, D.C.
53 Department o Energy. 2004. Federal Technology Alert: Green Roos. DOE/EE-0298, Washington, D.C.
54 Coman, R. Vegetated Roof Systems: Design, Productivity, Retention, Habitat, and Sustainability
in Green Roof and Ecoroof Technology. (Doctoral Dissertation, The Ohio State University, 2007.)
55 Department o Energy. 2004. Federal Technology Alert: Green Roos. DOE/EE-0298, Washington, D.C.
56 See, e.g., Department o Energy (2004). Federal Technology Alert: Green Roos. DOE/EE-0298,
Washington, D.C.; and City o Los Angeles, Environmental Aairs Department (2006). Green
RoosCooling Los Angeles (A Resource Guide). Los Angeles, CA.
57 Peck, S. and M. Kuhn. 2001. Design Guidelines or Green Roos. National Research Council
Canada, Toronto, Canada.