Green
Roofs
One of the goals of Boiler Green Initiativel is to install a green
roof on an existing building on campus that is being
replaced.
Flat roofs are especially amenable to the green roof system, so a
building with a flat roof or section of flat roof that is scheduled
for repair is the ideal choice.
The Armory will be having a 7392 sq. ft. area of the roof repaired
in summer 2008. Our goal is to install a green roof system
over the entire area.
We are also planning on having a green patio on the terrace of the
Mann building.
Boiler Green Initiative, working with the physical facilities
department, will monitor temperature on and just below the roof and
water quality and flow to determine the green roof’s
performance. Due to the inaccessibility of the roof, data
from monitoring equipment must be obtained remotely and will be
posted and stored on the club’s
website.
Green Roofs
Green roofs are a vegetated cover placed on top of a roof.
They range from mossy vegetation on a growing medium,i.e. extensive
green roofs to full scale gardens, meadows or trees on specialized
structures on the tops of buildings, i.e. intensive green roofs.
Benefits
The US Green Building Council recommends installing green roofs to
reduce stormwater runoff and urban heat island effects (US Green
Building Council 2000). In addition, green roofs improve
energy efficiency, are aesthetically pleasing, and extend the life
of the roof itself by protecting the membrane from extreme
temperatures and weather.
Urban development replaces forests and agricultural fields where
stormwater is normally absorbed and is allowed to flow naturally
into streams and water bodies. Impermeable surfaces such as
parking lots, roads, sidewalks and traditional roofs cause higher
volumes of runoff at faster rates during storm events which cause
stream degradation, erosion and faster transport of pollutants
(Carter and Jackson 2007). Green roofs decrease runoff by
holding some of the water in the soil medium during rainstorms, the
vegetation can utilize some of the water and, when rainfall is
light enough, roofs can store all of the water, releasing it later
through evapotranspiration rather than letting it run off the
surface. They have been shown to reduce runoff by as much as
82.8% on average, compared to 48.7% for gravel roofs (VanWoert,
Rowe et al. 2005). Other studies have shown retention rates
of 69% during heavy storms and 100% for warm weather storms
(Hutchinson, Abrams et al. 2003). Carter and Jackson (2007)
have shown a wide range of positive retention rates for varying
scales of green roof installations on a city scale. Reduced
runoff can improve aquatic life in receiving streams and lower the
load for storm sewers and stormwater treatment systems (Bengtsson,
Grahn et al. 2005).
Similarly the widespread use of green roofs has been hypothesized
to reduce the urban heat island effect due to the
vegetation’s low solar absorbance and insulation properties
(Saiz, Kennedy et al. 2006). More heat is absorbed while
shading is provided for a building on which a GR is installed in
the warmer months of the year. Not only do green roofs
decrease cooling needs for a building by more than 75% during the
spring and summer months, they also decrease heating needs somewhat
since they act as an insulator (Liu 2002). In an
environmental Life Cycle Assessment (LCA) by Kosareo and Ries
(2007) energy savings more than compensated for the initial cost of
materials of green roofs installed in Pittsburgh, PA. In
addition to the economic benefit of reduced energy use, the life
expectancy of the roof increases. Since the vegetation
reduces the amount of UV light from reaching the roof’s
surface, the life expectancy of the roof at least triples.
The extended life expectancy is also due to lower temperature
fluctuations at the roof’s surface. This will lower
costs over the life-span of the roof (Wong, Tay et al.
2003).
The plants on a green roof not only increase the aesthetic value of
the building, but also provides an increase in biodiversity (Getter
and Rowe 2006). Since green roofs usually are not open to the
public, they become an ideal habitat for birds, insects, spiders,
and many other animals and microorganisms. A study in
Switzerland found that green roofs also provide an excellent
environment for rare and endangered species, since there is little
human disruption (Brenneisen 2003). Green roofs have also
been used to help restore native vegetation to an area. This
is especially important, as native animal species rely on native
vegetation to survive and out-compete exotic invasive
species. Another role that green roofs can play is to provide
a safe habitat for endangered plants.
Design
Green roofs are composed of waterproofing, root barrier, drainage
mat, water retention fleece, planting substrate, and vegetative
material. Figure 1 is a representative extensive green
roof.
Figure 1: Typical extensive
green roof from Kosareo and Ries (2002).
Extensive green roofs can be retrofitted to any roof which has been
resurfaced in the last 5 years. Designs for intensive green
roofs will need to be incorporated to building plans so that the
structure is adequate for the added weight of the soil and its
water retaining capacity. Green roofs are typically installed
on flat roofs but can be installed on roofs with up to a 30°
slope.
References
-Bengtsson, L., L. Grahn, et al. (2005).
"Hydrological Function of a Thin Extensive Green Roof in Southern
Sweden." Nordic
Hydrology 36(3):259-268.
-Brenneisen, S. (2003). The
Benefits of Biodiversity from Green Roofs: Key Design
Consequences. 1st North
American Green Roof Conference: Greening Roofs for Sustainability,
Chicago, IL.
-Carter, T. and C. R. Jackson (2007). "Vegetated Roofs for
Stormwater Management at Multiple Spatial Scales."
Landscape and Urban
Planning 80(1-2): 84-94.
-Getter, K. and D. B. Rowe (2006). "The Role of Extensive Green
Roofs in Sustainable Development." HortScience 41(5): 1276-1285.
-Hutchinson, D., P. Abrams, et al. (2003). Stormwater Monitoring
Two Ecoroofs in Portland, Oregon USA. Greening Rooftops for Sustainable
Communities. Chicago
IL, www.greenroofs.ca.
-Kosareo, L. and R. Ries (2007). "Comparative Environmental Life
Cycle Assessment of Green Roofs." Building and Environment
42: 2606-2613.
-Liu, K. K. Y. (2002). "Energy Efficiency and Environmental
Benefits of Rooftop Gardens." Construction Canada 44(2): 17, 20-23.
-Saiz, S., C. Kennedy, et al. (2006). "Comparative Life Cycle
Assessment of Standard and Green Roofs." Environmental Science and
Technology 40: 4312-4316.
-US Green Building Council (2000). LEED Green Building Rating
System Version 2.0. USGBC.
-VanWoert, N., D. B. Rowe, et al. (2005). "Green Roof Stormwater
Retention: Effects of Roof Surface, Slope and Media Depth."
Journal of Environmental
Quality 34: 1036-1044.
-Wong, N. H., S. F. Tay, et al. (2003). "Lifecycle cost analysis of
rooftop gardens in Singapore." Building and Environment
38(3): 499-509.