![]() ¹ Company names are included for clarity and do not imply endorsement of these products by the authors or Penn State. We have used an ENKA¹ product with a layer of spunbonded geotextile on its top to keep the growth media from entering the drainage layer. There are a number of products available and some folks are simply placing a 0.5-inch layer of pea-gravel between the roof membrane and green-roof media to serve as the drainage layer. The drainage layer is usually a thin layer (about 0.5 inches thick) of plastic netting or small chambers that receive the water flowing downward through the media and then channel this excess water to the edge or outflow location of the roof where a downspout takes the water to ground level. The drainage layer is an open, highly drainable material that quickly channels gravitational water to the down-slope edge of the roof. Green Roof Components Roof Membraneīefore a green roof can be installed, the roof must be covered with an impermeable membrane that will not let roots penetrate. Though the structural integrity of a roof should always be evaluated by a qualified engineer, we have found that if a building is designed and built to present-day codes, an extensive green roof can typically be placed on the roof without any additional reinforcement. Careful selection of media and willingness to do some of the installation can greatly reduce this amount. Commercially designed and installed green roofs often cost $20 to $30/ ft2. The two most frequent concerns expressed about using green roofs is the cost and the possible need to provide additional reinforcement to the structure. Finally, green roofs intercept stormwater before it runs off a roof, which addresses the stormwater issue at the source rather than after the runoff has been collected. These benefits, in combination with the limited open space in cities, make green roofs a practical method for easing the pressure on storm drainage systems. In Pennsylvania 50 to 60% of the rain falling on a green roof will be captured and evapotranspired back to the atmosphere, thus never entering the stormwater system. The stormwater benefits offered by green roofs include delaying the runoff peak and decreasing the peak rate of runoff from the building simulating runoff characteristics expected from a meadow. The general design of a green roof consists of four distinct layers: an impermeable roof membrane and root barrier, a drainage layer, lightweight growth media, and adapted vegetation, see Figure 1. Extensive green roof advocates have claimed numerous benefits including improved air quality, stormwater attenuation, reduction of the "heat island effect," extended roof life, and aesthetic value. This fact sheet will focus on extensive green roofs.Įxtensive green roofs are a surface treatment for rooftops, typically less than 6 inches in depth, involving the addition of growth media and plants to create a sustainable green space on a flat or nearly flat roof. Green roofs are typically characterized as intensive (having 6 to 24 inches of media and large vegetation) or extensive (having 3 to 6 inches of media and smaller vegetation). This captured water is then evapotranspired back to the atmosphere through the green roof vegetation. Green roofs are unique in that they have the ability to capture and retain a volume (depth) of rain from each rainstorm. Research at Penn State has shown that green roofs provide considerable reduction in the volume of stormwater leaving a developed site and they reduce the peak rates of runoff. ![]() The Pennsylvania Stormwater manual recommends vegetated roofs (green roofs) as a stormwater low impact development (LID) practice.
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