AT the Penn State Center for Green Roof Research in University Park, Pennsylvania, horticulturalists, plant physiologists and agricultural engineers are collaborating to study the many aspects of green roofs and their effects on the environment. From looking at broad questions of how green roofs mitigate storm water runoff to specifics in terms of modeling the evapotranspiration rates in these systems, the Center is dedicated to providing scientific answers for this fast growing sector of green technology. Fortunately for compost producers, the use of organic materials in this technology is proving essential.
A green roof can be either intensive or extensive based on the degree of maintenance required to grow plants in specially formulated green roof manufactured soils or “media” as it is often referred to in the industry. Intensive green roofs require a high degree of maintenance and can be thought of as “roof gardens”. Media depths range from approximately 18 to 213 cm (7-84 inches) depending on the plant species used. Typically media depth must be greater than 25 cm (approximately 10 inches) to support an array of plant life in these systems. The additional roof load capacity needed to support such media depths is between 300 - 1,500 kg/m2 (approximately 60-300 lbs/ft2 or psf; pounds per square foot) depending on a number of factors such as the slope of the roof, type of manufactured soil used, whether or not irrigation systems are installed, and human traffic calculations. For comparison, typical single family homes in mild climate regions of the U.S. have additional roof load capacities between 10 and 30 psf. An intensive green roof system would not be appropriate for these types of roof structures without significant structural adjustments.
In contrast, an extensive green roof system typically requires a media depth of only 5 to 20 cm (approximately 2-8 inches) and is more functional in nature compared to an intensive green roof system; functional in the sense that its primary focus is for implementation as a Best Management Practice or BMP for storm water management. Additional roof load capacity needed for an extensive green roof system varies between 90 and 250 kg/m2 (approximately 16-50 psf) depending on the same factors as outlined above for intensive systems). The studies carried out at Penn State all involve extensive green roof systems. It should be noted that load capacity ranges are generalized. Each green roof installation is unique and structural engineers should be consulted for the final load and design requirements.
HISTORY OF THE CENTER
The Center for Green Roof Research at Penn State began as an idea formed during a European trip that the Center's former director, Dr. David Beattie, an associate professor of Horticulture, took in the late 1990s. At the time, green roofs were not well known in North America despite over a century of usage in several European countries, mainly Germany. A car bumper manufacturer based in Malvern, Pennsylvania provided a much needed catalyst. JSP International Inc. approached Beattie and his colleague, current Center Director Dr. Robert Berghage, also an associate professor in horticulture, about additional markets for their porous expanded polypropylene or PEPP compressed plastic mats.
One possible application was as a layer in a built up green roof. Beattie and Berghage looked at two different species of plants, a sedum and a grass, planted in various mixtures of manufactured soils using the PEPP. Comparing temperatures on a conventional plastic sheeted roof, a gravel covered roof, and a green roof, they discovered just how well the green roof system did in terms of mitigating heat effects. During the summer of 2000 with an ambient temperature of 88°F, the temperature in the green roof media was 82°F. The plastic sheeting and gravel roof measured 140 and 118°F, respectively. In terms of the plant species used in the green roof system, sedum performed better than grass in terms of its ability to weather temperature and moisture extremes. Sedum, a relative of the cactus, grows low to the ground and doesn't shed much dead plant material. It also requires very little in the way of maintenance and nutrient requirements.
Because of the success of the initial partnership with JSP International Inc., the Center was able to obtain funding from other sources such as American Hydrotech and Carlisle SynTec, two green roof industry giants, and state and federal sources like the Pennsylvania Department of Environmental Protection (DEP) and the U.S. Environmental Protection Agency (EPA), respectively. The Center officially began in 2001 with a research site at the Russell E. Larson Agricultural Research Center in Rock Springs, PA, and a mission to demonstrate and promote green roof research, education, and technology transfer in the Northeastern U.S. The Center has become a leader in studying various media suitable for growing extensive green roof plants as well as providing quantifiable data necessary to answer critical questions about the effectiveness of green roofs in mitigating runoff from storm water events.
RECENT RESEARCH PROJECTS
One of the first studies carried out at the Center, and part of a master's thesis project by Julia (DeNardo) Hunt, a former graduate student in Agricultural and Biological Engineering (ABE), was an investigation into the mitigation effects green roofs have on storm water runoff. Dr. Albert Jarrett, a professor of Agricultural Engineering, was DeNardo's major advisor and supervised the study along with Beattie and Berghage. The research quantified the importance of green roofs in attenuating or reducing runoff.
According to Jarrett, “The benefits of green roofs in attenuating storm water runoff are clearest if one looks at four things: 1) runoff volume reduction, 2) peak runoff rate reduction, 3) overall runoff delay, and 4) peak runoff delay in these systems.” To look at these four variables, six buildings (4.65 m2; 48 ft2) were constructed; three with extensive green roof systems and three with conventional, standard asphalt roofs both with roof slopes of 1:12 (Figure 1). Each extensive green roof system consisted of a waterproof membrane and root barrier, a drainage layer, growth media, and vegetation (Figure 2).
The growth media had a depth of 7.8 cm (approximately 3 in.) and consisted of 12.5 percent sphagnum peat moss, 12.5 percent coir (coconut fiber), 15 percent perlite, and 60 percent hydrolite for a 75 percent mineral or inorganic material to 25 percent organic material ratio (v/v). Above the growth media was a 2.5 cm (1 in.) thick layer of PEPP into which vegetation (Sedum spurium) was transplanted.
The buildings at the Center for Green Roof Research had their own storm water collection systems installed complete with gutters, downspouts, and collection barrels.
The first event occurred on October 25, 2002, producing a cumulative rainfall total of 2.39 cm (.94 in). Green roof runoff total measured 1.93 cm (.76 in) for a runoff volume reduction of 0.46 cm (0.18 in) or 19 percent. The maximum rainfall intensity was measured at 6.6 mm/hr (0.26 in/hr) while the maximum runoff rate from the green roof was 4.1 mm/hr (0.16 in/hr) for an intensity reduction of 38 percent. In terms of delays, runoff from the green roof took 4 hours to begin and 1 hour from the time peak rainfall intensity was recorded to the time peak runoff occurred. Evidence such as this gives storm water engineers valuable information in planning storm water infrastructure for urban and suburban areas. Extra time to absolve rainfall events in these areas is critical to managing water quality and adds to the attractiveness of using green roof technology as a Best Management Practice or BMP in construction.
Other key findings from DeNardo's study were: 1) green roofs retained on average 6.5 mm (.26 in.) or approximately 45 percent of total rainfall during the period of study (two months), 2) peak runoff rates averaged 2.4 mm/hr (.09 in/hr) or 56 percent of the peak rainfall intensity, and 3) runoff from green roofs was delayed an average of 5.7 hours. Jarrett has since expanded on DeNardo's work developing models for the way green roofs respond in two typical storm water scenarios: extreme storm events and cumulative annual rainfall. “Understanding how green roofs can help us manage storm water begins by looking closely at these additional elements,” Jarrett comments. Two-year (2.6 in/day) and 100-year (5.3 in/day) extreme rain events were simulated. In addition, Jarrett used weather data from State College, PA for a 27 year period (1976 to 2003) to simulate how well a green roof would reduce the cumulative annual rainfall (mean annual rainfall = 40.3 inches). Results of Jarrett's modeling work showed that with a 2 year storm event in July with 5 dry days prior to the storm, green roofs could reduce the peak intensity of the runoff by 85 percent (compared to no green roof) and the volume of runoff by 61 percent. The 100 year rain event also in July and also with 5 dry days prior could reduce peak intensity of runoff by 60 percent and volume by 30 percent. Numbers such as these are the lifeblood of making technologies such as green roofs an attractive alternative to conventional storm water management strategies.
Other studies have followed and helped answer other critical research questions. Berghage and Beattie were interested in media depths and how different green roof plants might respond in these systems with varying drought situations. Christine Thuring, a former graduate student in the Department of Horticulture, with both Beattie and Berghage acting as her coadvisors, used this interest as part of her master's thesis project to investigate a suite of popular green roof plants grown in different types of media at varying depths and in different drought regimes. As mentioned above, plants in an extensive green roof system can be exposed to extreme temperatures and moisture levels. As extensive green roof systems are less deep than intensive systems, there is a tradeoff between using deeper media to conserve plant available water during these extremes and adding weight to the roof structure and hence cost to the project. Thuring's project looked at how five different plant species (Sedum album, S. sexangulare, Delosperma nubigenum, Dianthus deltoides, and Petrorhagia saxifraga) responded (when grown) in three different depths (3, 6, and 12 cm) of two different media (expanded shale and clay) in two different types of drought situations responded.
SWITCH TO COMPOST
The growth media for Thuring's study consisted of either 85 percent (by volume) expanded clay or shale as the mineral component and 15 percent (by volume) pelletized spent mushroom compost from Laurel Valley Soils in Avondale, PA as the organic component. The switch to composted organic material from sphagnum peat moss and coir fiber in the previous study was made to make use of a recycled waste product, spent mushroom compost, and promote sustainability issues in the region. Thuring adds that “compost usage in green roof media from a sustainability perspective makes a lot of sense and should be integrally tied to the makeup of green roof media mixes. Specifically, research in the area of water retention and the decomposition process by different composts as potential components of green roof media would be extremely beneficial.” Echoing Thuring's comments, Charles Friedrich, a licensed landscape architect for Carolina Stalite Company, in a paper presented at the Greening Rooftops For Sustainable Communities in Washington, DC last year, stated that compost “is a preferred source for the organic component in green roof system media because of its high nutrient and microbial count, and it is politically correct because of its recycling value.” However, he adds, “Care must be taken when selecting the source of compost; proper stability/maturity, particle size, and feedstock source of the product should be considered.”
The two inorganic substrates in Thuring's study were used because of their popularity in North American green roofs and do not reflect the Center's promotion of one over the other. Thuring notes that “locally available inorganic materials from regional tile or brick manufacturers can minimize transportation costs and provide ecologically sound choices in terms of green roof media.” All experiments were carried out in a 4 x 30 m polyethylene greenhouse tunnel with all media placed in propagation flats modified to achieve the desired depth requirements. Air circulation, temperature, and irrigation were monitored and regulated throughout the study.
Key findings from Thuring's study were: 1) media depth most affected the growth of all plants, 2) herbaceous species did not survive in 3 cm (1.2 in) depths of either media, nor in 6cm expanded shale when subjected to drought conditions, 3) plant growth under drought was better in the clay compared to the shale, and 4), irrigation in the first weeks after planting is beneficial for plant establishment and performance. Indeed, initial irrigation proved vital for the herbaceous taxa and valuable for long-term performance by the succulents (not including S. album). In terms of design implications, Thuring's study showed that media depth and type, as well as water availability, are important considerations when selecting species for extensive green roofs. Succulent species performed well in 6 cm (2.4 in.) media, but always did better in clay versus shale. As for herbaceous species, green roof media depth should exceed 6 cm (2.4 in.), however more research on the use of such plants, especially native species, is needed.
CURRENT AND FUTURE STUDIES AT THE CENTER
Dr. Shazia Husain, a plant physiologist and visiting doctoral scholar, has begun experiments that expand upon the knowledge gained by previous research described above. Specifically, Husain has three areas of focus; elucidating mechanisms of evapotranspiration in green roof plants, determining plant responses to various mixtures and particle sizes of inorganic substrates and compost, and investigating potential ozone effects on green roof plants.
Using lysimeters with temperature and moisture sensors attached, Husain is working to quantify evapotranspiraton rates in Crassulacean Acid Metabolism plants or CAM for short. Husain states, “These plants, mostly succulents, have stomates that open and close during different times of the day to maximize water retention and minimize water loss. Knowing the mechanisms that are involved with these CAM plants can help scientists understand more about which plants are best suited for green roof systems and how they function to attenuate storm water runoff.” In addition to studying the CAM mechanisms in these systems, acid buffering capacity, pH, electrical conductance, and nitrate levels are being monitored.
As there are numerous inorganic substances marketed for use in green roof media with equally numerous claims for their respective products' superiority, Husain is performing work on various expanded shales and clays of differing particle sizes. Additionally, Husain states, “Mixing these inorganics with organics such as compost is critical to rapid establishment of green roof vegetation. Approximately 20 percent organic material and 80 percent inorganic appears to be the best mix in terms of plant growth response to date,” Husain continues, but she cautions against hard and fast volumetric percentage recommendations. “Each system is unique and the goal of each green roof system must be considered.”
Husain is also working on the effects of ozone on green roof plants. While largely an unknown area of research in terms of green roofs, Husain hopes to build upon what is known about ozone formation and apply it to this “green” technology. According to the U.S. Environmental Protection Agency (EPA), ozone occurs in urban areas where expansive concrete, lack of vegetation, and fossil fuel burning are prevalent contributing to a ground level formation of ozone and localized temperature increases known as “heat islands”. Heat islands can affect vegetation growing in urban areas interfering with plants' abilities to grow and store food. The use of green roof technology has been suggested as a way to mitigate the effects of heat islands in metropolitan cities, but the mechanisms involved in how green roofs do this are still not completely understood.
Ozone (O3) is a colorless gas with a pungent odor. It's found in two layers of the atmosphere, the stratosphere and the troposphere. In the stratosphere, ozone provides a protective layer shielding the Earth from ultraviolet radiation's potentially harmful health effects. At ground level (the troposphere), ozone is a pollutant and contributes to the formation of smog (from www.epa.gov). Husain has proposed a number of experiments to look at the extreme effects of ozone on green roof plants to better understand how atmospheric conditions can be benefited by as well as cause harm to green roof systems.
In terms of future studies at the Center, a recent PhD candidate in Horticulture with experience performing bioremediation projects around the world has expressed interest in investigating the microbial community assemblages in green roof media. Nutrient cycling issues as well as the implications for green roofs to act as potential pollution treatment systems are anticipated as areas of exploration.
The Center just received additional funding from American Hydrotech in September of 2005. This funding has enabled the Center to effectively double its data gathering capacity at the Rock Springs research site. Known as the HB (Hydrotech Building), its roof now boasts two 1:12 slopes, one facing north and the other south, with an effective roof area of 27.9 m2 (288 ft2), equal to all of the original six buildings' roof areas combined.
In the final analysis according to Berghage, “Green roofs are an excellent technology for use in rural, suburban, and urban areas where attenuating storm water runoff is critical. Green roofs are aesthetically pleasing, and there is strong evidence that they effectively mitigate storm water runoff and heat island effects in large metropolitan areas.” But, Berghage adds, “in the U.S., regulatory agencies are sometimes slow to adapting new technologies and 'hard numbers' are necessary to make these same regulatory agencies take notice. Quantity right now in terms of how much green roofs can store and delay storm water and storm water runoff, respectively, is the driver for research in this area for the foreseeable future.”
The Center for Green Roof Research has come far in its short five year history. From an idea that started across the Atlantic to the reality of a research center located in central Pennsylvania, scientific curiosity is a powerful force. Thankfully, with scientists like Beattie and Berghage, who carried out the first experiments at the Center before it was officially called a “Center”, their collective curiosity appears to have been in large supply. And with engineering expertise from Jarrett, “hard numbers” were generated early to provide the quantification necessary to show green roofs could adequately mitigate storm water runoff. Graduate students have also proven to be an essential part of the Center's success with DeNardo, Thuring, and Rezaei answering essential and fundamental questions about this relatively new green technology in their theses. Husain's role as a plant physiologist at the Center will undoubtedly shed more light on the mechanisms of how plants in these systems work to slow storm water runoff as well as other benefits. The Center appears to be on a mission with a team of scientists dedicated to generating results that are going through the roof.
Drew Mather is a soil scientist with the USDA NRCS in Wyoming. He can be contacted via e-mail at firstname.lastname@example.org