Interdisciplinary Green Roof Research Group

Green roofs mitigate urban stormwater in several different ways. First, plants evapotransipire moisture back into the atmosphere during photosynthesis. Extensive green roofs can recycled up to 45% of rainfall back in the atmosphere through evapotranspiration. (Kolb, 2004). This helps to reduce the total annual volume of runoff in urban areas from 60% to 100% depending upon environmental conditions. (DeNardo et al., 2005; Liescke, 1998; Moran et al., 2004; Rowe et al., 2003; Vanwoert et al., 2005). 

Green roofs can also delay runoff from rooftops depending upon the amount of moisture present in the growth media. Rain events over 0.5” usually produce runoff from green roofs. The range in time before runoff occurs from green roofs can vary from 95 minutes to 4 hours compared to conventional rooftops where runoff can occur nearly instantly (Liu, 2003; Moran er al., 2004). This delay in runoff helps to prevent flooding of storm sewers and in older urban areas, the overflowing of combined sewers systems as well.

Green roofs also increase the life span of roof membranes by protecting them from exposure to ultraviolet radiation. Green roofs shade and moderate membranes, which reduces the stress of daily expansions and contractions of membranes. Connelly and Liu found a green roof’s diurnal fluctuation was only 3°C where a conventional roof experienced a fluctuation of 50°C (Connelly and Liu, 2005).

Green roofs provide shade and an insulating effect which results in energy savings and mitigation of the urban heat island effect. The transpiration and media depth can reduce solar energy gain by up to 90% compared with conventional rooftops. Temperatures of conventional rooftops can reach 70°C where green roofs may only reach 25°C during a sunny afternoon in the summer (Liu and Baskaran, 2003).

Green roofs provide habitat for microorganisms, insects and birds as most extensive green roofs are inaccessible to the public. It is also a potential way to restore native plant species to an area where deeper substrates can be applied. (Brenneisen, 2003; Dewey and colleagues, 2004)  

Research found that humans who had a view of nature has beneficial health effects and experienced greater job satisfaction than those who had no natural view. These benefits not only can improve health and worker productivity for residence but also increase the value of the building, especially where views to green are significant. (Kaplan and colleagues, 1988; Ulrich, 1984)

Green roofs can alleviate airborne contaminants and particulate matter through filtration of plants. After particulate matter is captured by green roof vegetation, the particles are washed away into the soil by rainwater and the plants can absorb some of the pollutants. (Liesecke and Borgwardt, 1997; Yok Tan and Sia, 2005)

Green roofs can reduce noise levels by up to 40 db compare to conventional roofs. Because the substrate and vegetation of green roof can absorb sound waves, green roofs buffer sound where the hard surface of conventional roof membranes can only reflect sound waves. (Dunnett and Kingsbury, 2004; Peck and Kuhn, 2001)