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CEER Green Roof Project Erika Tokarz
A document Submitted to the
Department of Civil and Environmental Engineering Environmental Engineering
Villanova University Villanova, Pa May 2006 Introduction Since the release of the Clean Water Act (CWA) in 1972, more focus has been placed on improving the water quality of our nation’s waters. The main goal of the CWA was to minimize the discharge of pollutants to receiving water bodies. In 1984, the United States Environmental Protection Agency (US EPA) completed a report to Congress which declared that non-point source pollution was a leading cause of water quality impairment problems in the United States (US EPA 1984). Later in a 1988 report to Congress, the US EPA concluded that urban storm water was the fourth most extensive cause of water quality impairment in rivers, and the third most extensive source of water quality impairment in lakes (US EPA 1990). According to the 2000 National Water Quality Inventory, polluted runoff contributes to approximately 40% of our nation’s water body contamination (US EPA 2005). The Phase II Final Rule of the National Pollutant Discharge Elimination System (NPDES) published in 1990, intensified the push of NPDES permitting and therefore Best Management Practice (BMP) usage across the nation. Phase II was enacted to further reduce adverse impacts to water quality and aquatic habitat by controlling unregulated sources of storm water discharges (US EPA 2005). Small MS4’s are required to develop, implement, and enforce a storm water management program designed to reduce discharges of pollutants to the best possible levels to protect water quality, and to satisfy the water quality requirements of the CWA (US EPA 2005). It has been shown that urban storm water runoff effects water quality, water quantity, habitat and biological resources, public health, and the aesthetic appearance of urban waterways (US EPA 1999). In an effort to comply with NPDES permitting, Best Management Practices (BMPs) have collectively been used to manage storm water runoff effects. A storm water BMP is a “technique, measure, or structural control that is used for a given set of conditions to manage the quantity and improve the quality of storm water runoff in the most cost-effective manner (US EPA 1999b).” The main goals BMPs hope to accomplish are flow control, pollutant removal, and pollutant source reductions. Care must be taken when implementing a BMP design because each type has certain limitations based upon drainage area served, available land space, cost, pollutant removal efficiency, as well as a variety of site specific factors such as soil types, slopes, depth of the groundwater table, etc. (US EPA 1999). Some of the more used/researched BMPs include extended detention ponds, wet ponds, stormwater wetlands, infiltration trenches, infiltration basins, porous pavement, sand filters, and other vegetative practices. One BMP in particular is gaining more attention across the nation, Green Roofs. Green Roofs
Figure 1 - Atlanta, GA City Hall Green Roof Many urban areas are having difficulties in constructing BMPs due to the lack of space. Green Roofs have become the answer to many urban centers. They are practical because they can utilize thousands of square feet on building roofs that would otherwise be wasted space. Green roofs go by many names that include, but are not limited to, vegetated roofs, garden roofs, eco-roofs, and rooftop gardens. Regardless of the terminology used, they perform by reducing roof top runoff, which facilitates the removal of pollutant loads. Precipitation that falls on the green roof is absorbed, filtered, and retained by the soil media. This in turn is taken up by the plants. Evapotranspiration, due to the sun’s radiation, draws much of the water from the soil media and plant life. This greatly reduces, if not eliminates, the amount of stormwater leaving the site.
History of Green Roofs With the recent popularity of green roofs across the nation, many people assume the technology is new. In fact, green roofs have been around for thousands of years. One of the first notable appearances of green roofs occurred in the Hanging Gardens of Babylon around 500 BC. The site is considered one of the Seven Wonders of the World. The original design came from Iceland, where the people constructed houses with sod ceilings and walls. The country lacked other natural resources for construction purposes, so they made do with what resources they had. The sod houses then became popular among the rest of the Scandinavian people (Moran 2004). The ceilings were usually constructed of turf, but the walls were slightly more complicated. The first few layers of the wall were built up with stone. The rest was constructed by alternating strips of sod and turf. Any drift wood that was confiscated was also included in the design. Besides providing homes, the green roofs and walls provided extra insulation for the inhabitants during weather extremes. In Germany in the 18th century, green roofs took on a slightly different purpose. Instead of necessity for survival, green roofs were constructed to portray wealth and status among the German population (Moran 2004). They were constructed on the manor houses and castles of German’s finest. The trend then spread across Europe in the 19th century. The first signs of green roofs in the United States occurred in the 20th century in New York City (Moran 2004). Green roof designs continue to pop up all over the world. Even today, Germany still has the largest number of constructed green roofs. The benefits of using green roof technology has spurred the construction growth throughout the country. The green roof has gone from functional, to a decorative accessory, and back to functional.
Design and Function There are two types of green roofs: intensive and extensive. Intensive green roofs can accommodate large trees, shrubs, and well maintained gardens. They can be regularly accessed and use is encouraged. The intensive roof garden is designed with a minimum of a foot of soil depth, which can add 80 – 150 pounds per square foot of load to the building structure. The design also includes complex irrigation and drainage systems because annual precipitation can not feed the more intensive plant life. Regular maintenance for an intensive roof garden is required. The extensive green roof is more low key. It can accommodate many kinds of vegetative ground cover and grasses. Plants from the Sedum genus are usually used because they are hardy and colorful. Access and use of the roof by the public is generally restricted for an extensive roof garden. The extensive roof garden is designed with only one to five inches of soil depth, which can add 12 – 50 pounds per square foot (dry weight) depending on soil characteristics and the type of substrate. The design also includes a simple irrigation and drainage system. Maintenance on an extensive roof garden is minimal. The construction of roof gardens can be difficult, due to the many layers involved. See figure 1 below. The bottom layer is the roof construction. The roof construction must have a waterproofing layer that is durable enough to safeguard the structure over time with minimal maintenance. An example of a commonly used waterproofing agent is a fluid-applied rubberized asphalt waterproofing membrane (Wood 2004). Above the waterproofing layer is the moisture retention protection mat that retains a portion of the precipitation for future plant usage. Contained within the moisture retention protection mat is a root retardant that prevents plant roots from penetrating. Root barriers often contain copper sulfate to retard plant growth (Moran 2004). The next layer is the drainage layer. Various kinds of drainage layers are used by different contractors. Some layers have drainage channels that allow excess precipitation to collect and drain. Others contain small cups that collect excess precipitation that can be absorbed into the soil medium for plant use in the future. The water contained within the cups provides a moist, beneficial subsoil environment for the plants, without allowing fungus or root rot (Wood 2004). Next a filter fabric mat is installed to prevent soil particles from entering the drainage layer. The final layer consists of the soil medium. Ordinarily, good soil is very heavy due to its high clay content (Wood 2004). Because of the loads already associated with green roofs, lighter soil mediums are required. Soil medium used for green roofs is a combination of shale, pumice, sand, and organic matter. Care must be taken when preparing the soil mixture to prevent the export of pollutants. Mixes with large quantities of compost have been shown to export nitrogen and phosphorus. The USDA-Agricultural Research Service suggests providing the mix with a quality mature compost manufactured using industrial byproducts high in iron and manganese to reduce phosphorus solubility and
Figure 2 - Green Roof Layers (Source: American Wick Drain Corp.) increase heavy metal adsorption (Sherman 2005). The mix must promote hydrated plant life, but prevent over-saturation. The depth of the soil medium depends on the type of green roof under construction. The final step of the whole process is the selection and installation of the vegetation. Vegetation should be chosen for its ability to thrive in the local climate, withstand the harsh conditions of a roof, and imitate the surrounding landscape’s diversity (Wood 2004). It also needs to withstand direct radiation, drought, frost, and strong wind conditions. For an intensive green roof, trees, bushes, and other large plants are suitable. For an extensive green roof, smaller plants and grasses are more appropriate. Plants from the Sedum genus are
Figure 3 – Examples of the Sedum genus Left – Sedum adolphii Right – Sedum palmeri often chosen due to their modest soil requirements, and their resistance to droughts, the sun, and high wind conditions (Villarreal 2005). The success of a green roof depends on the successes of all its individual layers. During a precipitation event, water is absorbed into the soil medium pores and is stored in the drainage layer for future plant use. Runoff only occurs when the soil medium and the drainage layers have become fully saturated. This generally takes place after high intensity or long duration storm events. If runoff is produced by a precipitation event, it is generally not observable until several hours later.
Benefits of Implementation Green roof implementation provides a wide array of benefits for the user. Green Roofs (US EPA 2006):
Some research has shown that green roofs can also provide emotional benefits (Wood 2004). In high stress environments, employees can visit a green roof to take a break from the daily grind. The relaxing surroundings can improve their health and decrease employee absenteeism, thus making for a happier work environment. One of the most focused on benefits of green roofs is the reduction in storm water runoff. Precipitation that falls on traditional roof systems almost immediately runs off into the storm sewer system. Flat roofs can provide up to 0.2 inches of storage in rooftop depressions (Moran 2004), but the majority exits the area. The volume of runoff coming from roofs greatly increases the quantity and flow rate of storm water runoff from urban areas, and can play a part in flash flooding. Research done by North Carolina State University on two green roofs, concluded that green roofs function as excellent BMPs for water retention and peak flow reduction. Each of the green roofs under study retained the first 0.6 inches of rainfall over the area. The sites differed by roof size, soil medium depth, plant species, and precipitation amounts. The first site was 750 ft2 with a soil depth of 3 inches. The second roof was slightly larger at 1400 ft2 and a soil depth of 4 inches. The two sites retained 63% and 55% of the total rainfall amount. Average peak flow reduction for the two sites was 87% and 57% (Moran 2005). Water quality was also analyzed for one of the North Carolina sites. Since precipitation was the only water source for the roof, atmospheric deposition was the key pollutant, with emphasis on total nitrogen (TN) and total phosphorus (TP). Higher concentrations of TN and TP were seen in the green roof runoff, when compared to rainwater and their control roof. This was attributed to nutrients leaching from the soil matter, which was composed of 15% compost. Soil mixes for green roofs, especially in areas were quality is an issue, should have low concentrations of TN and TP to curb nutrient release. Similar volume reduction results were found for the green roof constructed on the Fencing Academy of Philadelphia. The site is 3000 ft2 with a soil depth of 2.74 inches. Runoff was negligible for storm events 0.6 inches and less. Runoff characteristics were modeled using rainfall records for 1994 from eastern Pennsylvania. The model predicted a 54% reduction in annual runoff volume (US EPA 2000).
Costs and Maintenance In terms of roofing construction costs, green roofs are generally more expensive in comparison to traditional roofing systems. Green roof costs are higher because they require more material and labor for installation. Currently in the United States, extensive green roofs cost approximately $8 per square foot (US EPA 2006). In comparison, traditional roofing costs are approximately $1.25 per square foot (US EPA 2006). Costs of green roofs reflect the fact that there are a limited number of contractors that are skilled in green roof construction within the United States Green roofs can be expensive in the short term, but in the long run green roofs can actually become cheaper than traditional roofing systems. The addition of a green roof will increase the property value, which allows building owners to charge higher rent fees. The insulation the green roof provides can reduce heating and cooling costs for the building. The green roof protects the original roof from ultraviolet degradation, which can extend the life of the original roof up to forty years (Wood 2004). This could save the building owner thousands of dollars in roof replacement costs (Wood 2004). Another, not so obvious benefit, is that green roofs can help ease stressful situations. This can lead to a boost in employee health and moral, which in turn leads to less absenteeism (Wood 2004). This could save companies thousands of dollars in lost income. Green roofs require less maintenance than traditional roofing systems. Therefore, owners will financially benefit from the proper installation of green roofing systems. Extensive green roof systems require minimal maintenance, which may include one annual weeding and watering (Wood 2004). Occasional clearing of the leaf litter and plant roots from the drain may be required. Intensive green roof systems require slightly more maintenance due to their complexity. Maintenance may include regular mowing, weeding, watering, and fertilizing (Wood 2004). Upon initial planting of the green roof, watering and fertilization may be required to help the plants become viable.
Villanova’s Green Roof Villanova University is currently in the preparation phases of building a green roof. The green roof is being built by the university’s Facilities Department, with assistance from the Water Resources Department. The green roof will be constructed on the Center for Engineering Education and Research (CEER) building, above the Holy Grounds coffee house. The estimated date of completion is the end of July 2006. The roof under consideration is approximately a 23 by 23 foot square. The specifications that are described below are the design criteria at this point, but are subject to change. The roof will contain one and a half inches of soil media, in an effort to retain the first half inch of every precipitation event. Around the border of the roof, a six inch rock berm will be constructed to contain the plant growth. For monitoring purposes, a rain gauge will be installed on the roof to measure the precipitation hitting the area. Temperature probes will be installed at the original roof layer, and on the top of the green roof to monitor the insulation effects of the roofing system. At the outflow pipe, a weir box, with a pressure transducer, will be installed to measure the flow leaving the site. Next to the green roof, a sign will be installed to provide information about what on-lookers are viewing. Once the design specifications are finalized, the construction process will begin by W.P. Hickman Systems, Inc. The construction process should take about two months to complete. Therefore, the process should commence in June 2006. Once planted, the green roof will take a few months to become completely viable. The green roof should be fully functioning by September 2006.
Works Cited
Moran, Amy Christine. “A North Carolina Field Study to Evaluate Green Roof Runoff Quantity, Runoff Quality, and Plant Growth.” North Carolina State University. 2004.
Sherman, Rhonda. “Compost Plays Key Role in Green Roof Mixes.” BioCycle. v 46. n 3. pg 29 – 34. March 2005.
US Environmental Protection Agency (US EPA): 1984. “Report to Congress: Nonpoint Source Pollution in the U.S.A.” Office of the Water Program Operations, Water Planning Division. Washington, D.C., U.S.A.
US Environmental Protection Agency (US EPA): 1990. “National Water Quality Inventory – 1988 Report to Congress.” Office of the Water Program Operations, Water Planning Division. Washington, D.C., U.S.A.
US Environmental Protection Agency (US EPA): 1999. “Introduction to the National Pretreatment Program.” Office of Wastewater Management. Washington, D.C., USA.
US Environmental Protection Agency (US EPA): 2005. “Stormwater Phase II Final Rule: An Overview.” Office of Water. Washington, D.C., U.S.A.
US Environmental Protection Agency (US EPA): 2006. “Green Roofs.” http://www.epa.gov/heatisland/strategies/greenroofs.html.
US Environmental Protection Agency (US EPA): 1999b. “Preliminary Data Summary of Urban Storm Water Best Management Practices.” Office of Water. Washington, D.C., USA.
Villarreal, Edgar L. and Lars Bengtsson. “Response of a Sedum Green-Roof to Individual Rain Events.” Ecological Engineering. v 25. pg 1-7. 2005.
Wood, Wyndham B. “Planning and Specifying a Garden Roof.” The Construction Specifier. v 57. no 9. pg 68–74. September 2004.
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