United States
Environmental Protection Agency
Office of Water (4203)
Washington, DC 20460
EPA-841-B-00-005D
October 2000
Vegetated Roof Cover
Philadelphia, Pennsylvania
Introduction
Vegetated roof covers on industrial and office
buildings have been used in Europe for more than
25 years to control runoff volume, improve air
and water quality, and promote energy
conservation. These systems, known as "green
roofs" or "extensive roof gardens," also have
aesthetic benefits. They typically include layers
of drainage material and planting media on a
high-quality waterproof membrane. These
systems use foliage and a lightweight soil mixture
to absorb, filter, and detain rainfall. Some of the
conditions responsible for the promotion and
acceptance of green roofs in Europe, which many
American cities face as well, are
- Widespread implementation of
storm water-related fees or taxes
- Laws requiring mitigation or compen-
sation for the elimination of open space
- Densely populated areas with high real
estate values
- Requirements to reduce loads on
combined sewer systems
(CSSs)
Project Area
The demonstration project was
installed on the roof of the Fencing
Academy of Philadelphia (Figure 1).
Like many urban areas on the East
Coast, Philadelphia experiences
frequent, small, high-intensity storm
events. These short-duration events
frequently overload and surcharge
sewer systems. In the Philadelphia
region, storms with 24-hour volumes
of 2 inches or less contribute 90
percent of all rainfall. Vegetated roof
covers are designed to control these
Key Concepts:
> Structural Control
> Retrofit Opportunity
> Volume Reduction
> Life Cycle Costs
LOW-IMPACT
DEVELOPMENT
CENTER
Project Benefits:
> Runoff Reduction
> Air & Water Quality
Improvement
> Aesthetics
> Energy Conservation
high-intensity storms by intercepting and retaining
water until the rainfall peak has passed, while also
allowing larger storm events to be safely
conveyed away from the building.
Vegetated roofs are complex structures that
require consideration of the load-bearing capacity
of roof decks, the moisture and root penetration
resistance of the roof membrane, hydraulics, and
wind shear.
The plants help recreate the hydrologic function
of open space in the following ways:
- Capturing and holding precipitation in the
plant foliage
- Absorbing water in the root zone
Figure 1. Fencing Academy of Philadelphia vegetated roof cover.
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- Slowing the velocity of direct runoff by
extending the flow path through the
vegetation
- Cooling the temperature of the air and
runoff. (Green roofs can be very effective
measures for reducing the "thermal shock"
caused by flash runoff from hot roof
surfaces.)
Project Description
The vegetated rooftop project at the Fencing
Academy of Philadelphia is a 3,000-square-foot
vegetated cover installed and monitored by
Roofscapes, Inc., on top of an existing structure
(Figure 1). The roof system was intended to
mimic the natural hydrologic processes of
interception, storage, and detention to control the
2-year, 24-hour storm event. The distinguishing
features of this system include
- Synthetic under-drain layer that promotes
rapid drainage of water from the surface of
the roof deck
- Thin, lightweight growth media that
permits installation on existing
conventional roofs without the need for
structural reinforcement
- Meadow-like setting of
perennial Sedum varieties
that have been selected to
withstand the range of
seasonal conditions typical
of the Mid-Atlantic region
without the need for
irrigation or regular
maintenance
The installed vegetated roof cover is
only 2.74 inches thick including the
drainage layer. The system weighs
less than 5 pounds per square foot
when dry and less than 17 pounds
per square foot when saturated. The
saturated moisture content of the
media is 45 percent by volume. The
saturated infiltration capacity is 3.5
inches per hour. Figure 2 shows the
components of the roof system.
The runoff characteristics of the roof were
simulated using rainfall records for 1994 from
eastern Pennsylvania. The model predicted a 54
percent reduction in annual runoff volume. The
model also predicted attenuation of 54 percent of
the 24-hour, 2-year Type II storm event and 38
percent of the 24-hour, 10-year Type II storm
event. Additionally, monitoring at a pilot-sized
project for real and synthetic storm events was
conducted for a period of 9 months at 14- and 28-
square-foot trays. The most intense storm
monitored was a 0.4-inch, 20-minute
thunderstorm. The storm event occurred after an
extended period of rainfall had fully saturated the
system. Figure 3 shows the runoff attenuation
effectiveness for this event. Although 44 inches
of rainfall was recorded during this period, only
15.5 inches of runoff was generated from the
trays. Runoff was negligible for storm events
with less than 0.6 inch of rainfall.
Project Summary and Benefits
This project showed that vegetated rooftop covers
can help to reduce peak runoff rates for a wide
range of storm events. The project also
demonstrated that existing structures can be
successfully retrofitted to help prevent CSS
surcharging in urban areas. Significant energy
Figure 2. Components of the vegetated roof cover.
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conservation benefits also are
associated with vegetated rooftop
covers. During the spring and
summer, temperatures on a
neighboring black tar roof varied
by as much as 90 °F, while the
variation under the 2.74-inch
vegetated cover was only 18 °F.
The vegetated cover also
insulates the roof in winter, and
the vegetation protects the roof
membrane from the elements.
Vegetated rooftop covers can
potentially extend the life of a
roof by 20 years or more.
References
0.18 -
n ifi
n 1 9
n m
n na
n nfi
0.04
0.02
n nn -
D Rainfall
• Runoff
ru m m m i-n
i-i
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5.0 mm
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Time (5-minute intervals)
Figure 3. Runoff attenuation efficiency for a 0.4-inch rainfall event with
saturated media.
Miller, C. 1998. Vegetated Roof Covers: A New
Method for Controlling Runoff in Urbanized
Areas. Pennsylvania Stormwater Management
Symposium, October 21-22, 1998, Villanova
University, Villanova, Pennsylvania.
Contact Information
Charlie Miller, P.E.
Roofscapes, Inc.
7114 McCallum Street
Philadelphia, PA 19119
(215)247-8784
cmiller@roofmeadow.com
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