Green Roof Testing at Virginia Tech

Architecture and Urban Studies at Virginia Tech students have been researching the effects of green roofs and controlling urban runoff. Early results were recently released from the 2 year project. Not only do green roofs provide shade and help cool buildings by the extra insulation, they also reduce the force of storm water runoff. The vegetation acts like a sponge absorbing and filtering water that would normally rush into city streets and into the overburdened sewers.

Elizabeth Grant, assistant professor of architecture and lead investigator for the project said “We want to determine the effectiveness of various depths and types of green roof systems.” She went on to say “before designing these systems architects need to be able to answer this question, how well do they work?”

Different Depths for Platforms

By using 8 by 8 foot elevated platforms, the students determined that green roofs reduced and slowed runoff during the first 5 months. The plywood platforms were covered with white reflective thermoplastic roof membranes and three different depths of modular green roof covering; 6 inches deep, 4.25 inches standard and light at 2.5 inches. Also included in the study was a control platform and a 4.25 inch depth of growing medium, but no plants. A weather station recorded air temperature, humidity, radiation, rainfall and wind speed and direction.

49 storms were recorded and results were the deep, standard, light and medium only platforms retained more water than the control platform. Overall, the deeper systems held more than the shallow ones. The depth and other properties of the growing medium may be as critical as the vegetation to reducing storm water runoff.

The study will continue to evaluate how the weather and various types of green roofs reduce urban runoff. If you would like to know more about this project please visit

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