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
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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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