Water Resources Program  |  Green Roof -- Monitoring

Green Roof
Plant Types
& Benefits
& Care
Green Roof Monitoring Monitoring Results

200 Gallon Runoff Collection System

A key benefit to green roof systems is their ability to reduce the quantity of stormwater runoff through membrane absorption and plant evapotranspiration.  To quantify this, a 200 gallon plastic tank has been set up to collect rain runoff from approximately 1/6 of the green roof surface area.  Runoff in the tank is measured and compared to rain gage data after each storm to calculate the amount of rainfall retained by the green roof.   When the data collection process is completed, we plan to find a suitable way to reuse this water (i.e. irrigation).

The thermal properties of a green roof system also provide many benefits including reduced heating and cooling costs, and extended rooftop durability. Thermal sensors have been placed on both the green roof and an adjacent conventional roof to measure and compare temperature data between the two roofs.  HOBO data recorders will be used to continuously measure and record (every 15 minutes) a vertical temperature profile on each roof.  This profile consists of the following thermal levels:

            1. Outside ambient air temperature (10' above the roof surface)
            2. Radiated heat temperature (1' above roof surface)
            3. Rooftop surface temperature (directly in soil/rocks)
            4. Roof underside temperature
            5. Inside ambient air temperature (3' below underside of roof)
            6. Climate control influence temperature (just above office space insulation)

HOBO Temperature Sensor on Green Roof   HOBO Temperature Sensor on Conventional Roof
     HOBO Temperature Sensor on Green Roof                      HOBO Temperature Sensor on Conventional Roof

Comparing and analyzing this rooftop data will allow us to better quantify the environmental and economic benefits of a green roof system.  Water quality tests were conducted by a University of Virginia Engineering student this spring when the roof was partially covered with plants. County staff plans to evaluate new water quality results to determine the roof's ability to treat stormwater. As vegetation (Sedum) coverage increases, we hope to see an improvement in both the stormwater quality and retention capacity, and the thermal benefits. We look forward to updating this website with results from our analysis as soon as possible.  Please contact Gregor Patsch  434-296-5816 with any questions.

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