White Paper
April 19, 2010
Green Roof
Rachel Hedge
Introduction
A major concern in our area is the water of the Chesapeake Bay or more specifically the
filtration of the water going into the bay. Water can go on many paths to the bay once it falls
from the sky as rain. Along the paths, the water picks up debris and many chemicals as it flows
toward a stream or creek, which will flow toward the bay. Debris such as litter and chemicals
like road salt are pollutants that can be stopped from running into the bay through lowering the
force of rain through plant material. An example of a way plant material can filter storm runoff
before reaching streams is through a green roof. A green roof is a roof that is covered in plants,
which reduces storm water run-off and lowers cooling costs. Green roofs protect the roof
membrane, which results in a longer material lifespan. It is estimated that green roofs will last up
to twice as long as conventional roofs, so there is a decrease in maintenance and it saves in
replacement costs(Peck, Green Roofs Benefits). Green roof systems are made up of drainage
layers, a filter cloth, growing media, and vegetation. Green roofs can either be extensive or
intensive. Intensive green roofs are essentially elevated parks. They can sustain shrubs, trees,
walkways and benches with their complex structural support, irrigation, drainage and root
protection layers. The foot or more of growing medium needed for an intensive green roof
creates a load of 80-150 pounds (36-68 kilograms) per square foot. Extensive green roofs are
relatively light at 15-50 pounds (7-23 kilograms) per square foot. They support hearty native
ground cover that requires little maintenance. Extensive green roofs usually exist solely for their
environmental benefits. Another benefit of having a green roof is that there is a potential of
earning more than 7 credits under the LEED certification.
Picture of the layers needed for a green roof. (J.V. Heidler Co.)
Cost benefit analysis
A green roof reduces the amount of roof drains
since the plants absorb most of the rain water on the roof. A
study using a Micro Axess Simulation model shows that a
green roof can retain 70-90% of the precipitation that falls
on them; in winter, they retain between 25-40% depending
on the growing medium. For example, a grass roof with a
growing medium layer of 1.6-7.9 inches can hold 3.9-5.9
inches of water(Peck, Green Roofs Benefits).
There are also many economic benefits through air quality and sound insulation. A green
roof will absorb heat by decreasing the air temperatures nearby through transportation and filter
the air moving across it(Peck, Green Roofs Benefits). Depending on the foliage type, 1 m2 (10.76
ft2) of grass roof can remove up to 2 kg (4.4 lbs) of airborne particulates from the air every
year(Peck, Green Roofs Benefits). A green roof can also provide a sound barrier via the soil,
plants, and trapped air(Peck, About Green Roofs). The sounds from Aberdeen proving ground,
airplanes, and traffic can be absorbed, reflected, or deflected because the substrate layer blocks
lower frequencies while the plants block higher frequencies. For example, a green roof with a
substrate layer of 4.7 inches can reduce sound by 40 decibels(Peck, Green Roofs Benefits).
Recommendation
An extensive roof using native plants is recommended for the engineering building. The slope of
roof does not need to be changed. A green roof is just an addition to the roof but with a few
moderations. The roof will need to be altered to be able to handle the “wet weight” (Miller)
which is the load bearing weight with the estimated water containment. The roof will also need
to bear the weight of people that maintain the plants and structure. The use of native shallowrooted grasses and mosses is also recommended to prevent an issue of invasive species. There
are two companies that are recommended by the National Institute of Building Sciences for
building green roofs, Tecta America and Roofscapes, Inc(Miller). Tecta America has a location
in Baltimore, Maryland(Tecta America), while Roofscapes, Inc. is located in Philadelphia,
Pennsylvania(Roofscapes, Inc).
Works Cited
J.V. Heidler Co. "Diagram of an extensive green roof system with insulation." 4 April 2010. Heidler.com,
Green Roof. 26 April 2010 <http://www.heidler.com/GreenRoofing.aspx>.
Miller, P.E., Charlie. Extensive Green Roofs. 17 May 2009. National Institute of Building Sciences. 26 April
2010 <http://www.wbdg.org/resources/greenroofs.php>.
Peck, Steven. About Green Roofs. 8 December 2009. 21 April 2010
<http://www.greenroofs.org/index.php/about-green-roofs>.
—. Green Roofs Benefits. 12 August 2009. 21 April 2010 <http://www.greenroofs.org/index.php/aboutgreen-roofs/green-roof-benefits>.
Roofscapes, Inc. Roofscapes, Inc. - Taking Green Roofs to the Next Level. Ed. Javelin Web Design. 2010.
26 April 2010 <http://www.roofmeadow.com/>.
Tecta America. Tecta America Roofersinc. 26 April 2010
<http://www.tectaamerica.com/component/k2/itemlist/user/93>.
United States Environmental Protection Agency. Heat Island Effect. 15 April 2010. 26 April 2010
<http://www.epa.gov/heatisld/>.