Environmental Monitoring at a Constructed Green Roof in Prince George's County, Maryland

A green roof, or vegetated living roof cover, was installed during roof repair activities on a conventional flat roof at the Equipment Support Facility in Prince George's County, Maryland to demonstrate the practicability and benefits of vegetated roof covering in Maryland. Historically observed benefits of a green roof include: 1.) extend the life of the underlying roof waterproof membrane by shielding the roof surface from ultraviolet radiation, temperature extremes, and mechanical damage; 2.) Reduce the impact of stormwater by absorbing rainfall and retarding the release of runoff to storm drains and adjacent waterways; 3.) Improve the effectiveness of existing insulation, reducing energy costs in the building; and 4.) Restore the ecological and aesthetic value of open space to developed, urban areas, including reducing ambient air temperatures. A physical and economic feasibility study of the project was completed in 2001 and a structural analysis of the building verified the support load and provided an upper bound for the green roof design load. An economic analysis of approximate costs (construction and maintenance) for a conventional and a green roof installation described the benefits of increased roof membrane life versus the higher initial capital cost for of a vegetated roof covering.In Spring 2003, a 7,750 ft2 extensive green roof was constructed with five species of sedum plants, three inches of growth media, a layer of geotextile separation fabric, a polyester fleece “hydrogel” fabric layer, and roof insulation layers. To quantify the environmental benefits of the green roof, long-term environmental monitoring data were collected from April 2004 to November 2004 from both the green roof and an adjacent control roof of similar dimension and size. Stormwater runoff volume reduction and rooftop temperature reduction were hypothesized as the major environmental benefits of the green roof. Temperature loggers were placed on the green roof and the control roof to record ambient air and roof temperatures. Temperature loggers were also placed inside both the green roof and control roof buildings to document any significant changes in temperature. A rain gauge was placed on-site to record the rainfall in the vicinity of the green roof and the control roof. All runoff from the green roof and control roof was diverted into downspouts with a weir along and pressure transducers to record the volume and temperature of runoff from both the green and control roofs. This paper presents the results of the environmental monitoring data from April 2004 through June 2005.