March 28, 2005 | General


BioCycle March 2005, Vol. 46, No. 3, p. 29
New options for storm water and improving water quality are offered by increased installation of green roofs throughout the nation.
Rhonda Sherman

STORM WATER runoff has been identified by the U.S. EPA as one of the major sources of water quality impairment throughout the United States. Recent rules developed by EPA’s National Pollutant Discharge Elimination System (NPDES) emphasize use of best management practices (BMPs) to improve runoff quality. Green roofs are a new option for storm water BMPs – offering the potential to convert thousands of square feet of idle space on rooftops to retention areas.
Green roofs provide peak flow reduction of rooftop runoff and roof surface temperatures. Because a green roof absorbs most of the ultraviolet light striking it, the roof lasts much longer than a standard roof. It also acts as providing an insulator, lowering building heating and cooling costs, as well as an added living space for residents in congested spaces.
Green roofs provide hydrologic control of small, frequently-occurring storms and their effects on ecosystems downstream. They also offer combined sewer overflow mitigation, watershed pollutant load management, and compliance with the NPDES Phase II Final Rule.
Research on the role of green roofs as a storm water BMP has been limited. The City of Portland, Oregon began its EcoRoof program in 1996, and collected data during a 15-month period during 2002 and 2003. The monitoring data found that the water retention of an extensive green roof with a 4- to 5-inch media depth was 69 percent of total rainfall. Peak flow reductions of 80 percent were observed. Research conducted at Michigan State University has shown that 66 percent of the precipitation was retained by an extensive green roof studied over an average of 24 rainfall events. (This data was included in a paper given by Bill Hunt (see below) and colleagues at a 2004 green roof conference.)
Much green roof research on storm water retention and water quality in the United States is being conducted at North Carolina State University (NCSU). By discovering the extent to which green roofs can be used as nutrient reduction BMPs, the research engineers can determine what removal efficiency to assign to them. Since 2002, some NCSU professors and students have been conducting research on green roofs. At three sites (Kinston, Goldsboro, and Raleigh), they are studying four parameters: 1) Storm water runoff reduction, both volume and peak; 2) Water quality, to determine if less nitrogen and phosphorus are leaving green roofs; 3) Plant survival, to see which grow best on green roofs in the Southeast; and 4) Temperature of green roof runoff, to see if it increases or decreases (this is important to nearby trout waters because the fish will experience thermal shock if the water is too warm).
All three of the roofs under study have extensive systems with three to four inches of plant growth media consisting of 55 percent expanded slate, 30 percent sand, and 15 percent compost that was donated by Carolina Stalite. Researchers found that the total volume of runoff was reduced for all three of the green roofs studied. The roofs captured 55 to 70 percent of rainfall and released it back to the atmosphere. Flatter roofs held more water. Two roofs with pitches of three percent or less retained 70 percent of rainfall, while a roof with an eight percent pitch held 55 percent.
Researchers also studied peak flow mitigation. All three roofs reduced peak runoff by over 50 percent from storms with a one to three inch rainfall. The Kinston green roof had an 87 percent peak flow reduction and the green roof in Goldsboro had a 78 percent reduction. NCSU researchers originally hypothesized that green roofs would improve storm water runoff water quality. They analyzed samples of green roof runoff, control roof runoff and rainfall for nutrients. Results showed that N and P are leaching from the green roof soil media.
Compost in the soil mix appears to have served as an additional source of N and P to the runoff. To test this theory, a small column study was conducted at NCSU to ascertain the leaching effects of N and P from three different green roof soil mixes. Their results indicated that if less compost is used in the soil mix, less N and P leaches out. In response to the study results, the soil mix manufacturer who supplied materials for the three green roofs under study by NCSU changed its recipe so that it contains less compost.
“We found that higher amounts of compost in the soil mix release more nutrients,” says Bill Hunt, Assistant Professor and Extension Specialist with NCSU’s Department of Biological & Agricultural Engineering. “We are interested in seeing if after three years, are the nutrients flushed out and have reached a steady state in runoff release or do they keep on leaching?” Researchers are looking forward to answering that question this spring, when one of the green roofs under study turns three years old.
“We have seen that richer compost media produces faster plant growth on green roofs,” says Hunt, “but plant growth needs to be balanced with water quality. If the nutrient releases reach equilibrium, we could balance those amounts with better plant growth. Pollutants coming off the green roofs under study are two to three times greater than regular roofs. The concentrations of nitrogen and phosphorus in runoff from green roofs that are one to two years old are so high that they offset the water retention value.” Some soil scientists projected that after 2.5 to 3 years, the runoff nutrient concentrations would not be as high, so Hunt is hoping that net nutrient loading will taper off.
Green roofs continue to sprout up slowly in North Carolina. Dr. Hunt does not expect green roofs to be used extensively in the state because of the cost. Bioretention areas or water reuse systems can be installed for a fraction of the cost of constructing green roofs. However, Hunt sees a lot of potential for green roof use in areas of North America with intensive land use such as Portland, Chicago, Atlanta, Washington, D.C., Toronto and Vancouver. “Portland and Chicago are currently the leading cities with green roofs,” says Hunt, “but Atlanta and Washington D.C. are emerging in this arena.”
ERTH Products of Peachtree City, Georgia is capitalizing on Atlanta’s movement towards green roof use. ERTH’s history of compost utilization projects positioned them to take a leading role in green roof installations. ERTH initially worked on writing compost specifications for golf course green root zone mixes. Compost research conducted by Clemson and Auburn universities enabled them to get approved for U.S. Golf Association (USGA) root mixes for golf greens. Then ERTH started working with structural soils and obtained a license to produce and sell the CU-Structural Soil/Cornell University Urban Tree Mix. This mix can be compacted to legal density to ensure pavement integrity, yet still possess the physical and organic properties vital for root growth beyond the confines of the tree pit.
When green roof work got started in Atlanta, the firm was called in because of its work with engineered soils and soil mixes. It has since installed a number of green roofs in the Atlanta area, including the Arthur Blank Family Foundation Building, which recently became Georgia’s first gold-certified LEED building (LEED is the Leadership in Energy and Environmental Design green building rating system developed by the U.S. Green Building Council).
Extensive green roof systems are characterized by low weight, lower capital cost, less plant diversity, and minimal maintenance requirements as compared to intensive green roofs. The plant growing media is primarily sand-based, with expanded clay or slate, and compost. Expanded clay or slate are products that have been heated (2,000°F) and expanded so they are porous like lava rocks. These inorganic products possess exceptional water and nutrient holding capacity while providing superior aeration and drainage for optimum plant growth.
The depth of the plant growth media is between two to six inches on extensive green roof systems. The weight specifications for these roofs are 16 to 35 pounds per square foot, so it is important to use a light-weight soil media. “You almost need to be an engineer when designing these types of soil mixes because you are working with so many materials of different sizes, weights and measures,” says Wayne King, Sr., who manages ERTH Products. “Engineered soils in these types of projects play a very important role other than just providing the media for the plant palette. It is important that the soil media be designed to meet the demanding physical, chemical and biological performance requirements associated with storm water. Properties such as moisture retention, maximum water capacity, hydraulic conductivity, porosity, absorption, sedimentation and filtration, maximum/minim dead weight, drainage efficiency all must be considered for each type of green roof design.”
King notes that intensive green roofs are less restrictive in regards to weight of materials. The soil ranges from approximately 7-inches to 7-feet, with a saturated weight ranging from 60 to 200 pounds per square foot.
ERTH uses four different methods of distributing its soil mix on rooftops, depending on how high they are off the ground and the degree of difficulty in reaching them. They often use a pneumatic blower truck with a 300 foot hose. This is a more expensive option, but works well for hard to reach places because the hose extends so far from the truck. For one job, they had to run the hose through the building and up several flights of stairs to reach the roof.
Sometimes the soil mix is put into one cubic yard “super sacks” and delivered to rooftops via a crane. For other jobs, palletized 30-pound bag products are transported to the roof in a freight elevator. They also like to use conveyors to distribute the soil media to reduce compaction.
King stresses that the physical, chemical and biological properties of compost should be examined. ERTH Products initially focused on the physical properties of its soil mix because the U.S. Golf Association primarily concentrates on size distribution and physical properties of USGA root zone mixes. But now that ERTH is addressing water quality through its green roof, erosion control, and storm water projects, the chemical and biological properties of the soil mixes must be considered. “You need to not only be concerned about good plant growth, but how quickly and how much of the nutrients leach out of the soil media,” says King.
In a Green Roof Soil Media Laboratory study performed at North Carolina State University that examined the leaching effects of nitrogen and phosphorus leached from three different green roof soil mixes, ERTH’s product came out on top. The researcher concluded that the ERTH HydRocks soil mix holds the most promise for green roof growing media because it leached the smallest amount of nitrogen and phosphorus in the laboratory study. King says that according to experts at USDA-Agricultural Research Service, “success lies in providing a quality mature compost manufactured using industrial byproducts high in iron (Fe) and manganese (Mn) to reduce phosphorus solubility and increase heavy metal adsorption by organic amendments. Most nutrients in compost are in a plant available form and the nitrogen release rates will closely match that of the needs of the plant. Nutrient leaching is a balance game and every attempt should be made not to apply any more nutrients than what is needed for the plants specified.”
ERTH has installed mostly intensive green roof systems, since 80 to 90 percent of green roofs going up in Atlanta are of that type. The Woodward Academy installation in College Park, Georgia was challenging because it had to be engineer-certified prior to application. This requirement was because the soil growing media and the plants had to weigh less than 28 pounds per square foot, saturated. The firm’s soil mix was chosen because it is so lightweight and absorbent. The material (which consists of expanded clay, sand, and compost made of peanut hulls and biosolids) holds in nutrients and stays together. ERTH had used expanded slate in its mixes, but discovered that expanded clay is lighter and more absorbent than slate. “The clay is even lighter than slate when it is saturated,” says King, “so although it holds more water than slate, the expanded clay still weighs less.”
The green roof at the Woodward Academy was installed using a container system supplied by Green Tech. The boxes are 4-feet by 4-feet and about 7-inches in depth, and are connected to a grid system. “We used a pneumatic blower truck to put the media into the boxes,” says King. “In this case, it was critical that we did not fill the boxes to the top because we were limited to a 25 lbs/sq.ft. saturated weight, which isn’t very much. One question that came up during the design of the green roof was what amount of weight a saturated sedum plant added when it is fully matured. I went to the experts and learned that it is about 2 lbs/sq.ft. In this case, because of the stringent weight requirements, we needed to have some wiggle room and managed that by not filling the boxes all the way to the top.”
Organic matter is a significant major component of the soil. King recommends that the percentage of organic matter in the final mix of an extensive green roof should fall in the range of five to 10 percent by weight, which is about 10 to 20 percent by volume. Intensive green roofs can be somewhat higher depending on the variety and use of the plant material.
The challenge in green roof growing media installations is balancing plant growth needs with nutrient leaching. Compost manufacturers need to produce a consistent product that contains the least amount of nitrogen and phosphorus needed for plant growth because of nutrient release concerns. If green roof specifications call for small amounts of compost due to nutrient concerns, it is imperative that compost companies provide high quality compost that will help plants grow quickly and stay healthy.
Rhonda Sherman is a freelance writer and faculty member at a university in Raleigh, North Carolina. She is installing a green roof at her Compost Training Facility (see faculty/sherman). For more information on NCSU’s green roof research, go to http://
LUSH GRASS and succulents surround the Council Chamber atop Seattle’s new City Hall, creating an evolving garden roofscape as plants change color with the seasons. Directly across, the 12th floor terrace of Seattle’s new Justice Center offers views of its own garden roof. … They represent important environmental and sustainability components in the city’s new Downtown Civic Campus, a project implemented under the Sustainable Building Policy, as part of the city’s Environmental Management plan. That’s how Environmental Design + Construction begins its article called Sustainability With A View,” which also makes these points:
Life-cycle analysis indicates that green roofs greatly extend the life of the roofs’ waterproof membranes by providing protection from ultraviolet degradation, temperature extremes and mechanical damage. Both garden roofs require minimal maintenance and can reduce annual storm water runoff by 50 percent to 75 percent. The Justice Center garden roof is 8,500 sq ft; the City Hall roof is 13,000 sq ft. Green roofs consist of a multilayered waterproofing membrane system supplied by American Hydrotech, Inc. of Chicago. The membrane (Monolithic Membrane 6125) is a hot fluid-applied, rubberized asphalt. Water retention and drainage are provided by Floradrain 40, lightweight panels made of 100 percent recycled polyethylene. A geotextile filter sheet (Systemfilter SF) helps prevent loss of soil, mulch and compost while allowing moisture flow. Soils included mix of pine bark, pumice, compost, sand, peat and nutrients.
About landscape design, Matthew Suhadolnik of SvR Design Company explains: “Flowing, naturalistic patterns have been created using groundcover plants of varying textures and in subtle shades of blue, gray and green.” Adds Marcia West of Gustafson Guthrie Nichol Ltd. “We elected to mingle different textures and colors, so that the roof becomes one integrated carpet of plantings when you look down on it – with squares and patches of different plantings that blend together at the edges.” – J.G.
GREEN roofs fall into two basic design categories – intensive and extensive. Characteristics of intensive green roofs are: Soil base ranging in depth from 8- to 24-inches with a saturated weight increase between 60 to 200 lbs/sq.ft.; More diverse plant selection that can include trees and shrubs; More demanding maintenance requirements, especially watering.
Extensive green roofs use a shallower layer of media – between 2- to 6-inches – with a weight increase of 16- to 35 lbs/sq. ft. when fully saturated. Notes Wayne King of ERTH Products: “Extensive green roof growing media includes a mineral base of sand, gravel, expanded clay or slate, organic matter and some soil. Plants must be low and hardy as they typically are only watered and fertilized until they are well established.”
Both intensive and extensive green roofs are good for reducing storm water runoff, although intensive designs are more expensive and are mostly done over parking garages that are heavily reinforced structurally, explains King. On the other hand, most roof designs can accommodate extensive green roof installations.
Wayne King
ONE MAIN benefit from green roofs is quality of life. We’re greening the neighborhood, contributing to its livability and aesthetics. In the Atlanta, Georgia region – where my composting operation ERTH Products is located – we’re looking at the reduction of the urban heat island effect. At the same time, green roofs can play a critical role in urban storm water management. At a green roof conference held in Portland, Oregon last year, it was pointed out that if half of the downtown area in Portland (219 acres) all had green roofs, we’d see an estimated 66 million gallons of water retained annually. At the same time, this would eliminate combined sewer overflows by 17 million gallons, reducing storm water discharges between 11 to 15 percent. You could see that in heavily urbanized areas where there are a lot of rooftops – particularly urban areas challenged by combined sewer overflows that lead to contamination of surface waters – installation of green roofs can play a significant role in the amount of water that can be retained after storm events.
ERTH Products, LLC provides engineered soil mixes for green roofs. We have found that a heavily vegetated green roof can hold between 4- to 6-inches of water. The water stored in the growing media is released through evaporation and evapotranspiration. Based on our experiences with green roof installations over the past several years, we have gained several important insights into use of green roofs as a storm water Best Management Practice (BMP):
-Typical storm water management designs use a 100-year storm event as a baseline. At that point, BMPs like green roofs go out the window. It is important to design BMPs around more frequent storm events typical for a region vs. 100-year events. Then you can really start seeing applications for green roofs, bioretention ponds, etc.
-A paradigm shift is taking place with regard to how soils are valued in an engineered landscape. Traditionally, soils are viewed as something cheap, with most money allocated to the plant palette, i.e., the vegetative component. When considering BMPs for tools like green roofs, the soil component includes inorganic and organic materials that have a profound influence on performance. The soil matrix determines factors such as water holding capacity, density, degree of drainage and fertility for vegetation. In short, we have higher expectations for these soils.
-The demanding performance properties of the soil mixes being used in storm water management projects and green roofs are beginning to have an effect on the way specifications are being written. Prescriptive specifications which tend to only identify specific green roof components and manufacturers are now shifting toward a performance based approach that is not biased toward a particular product, component or assembly type. Performance specifications provide the owner/developer a tool to verify, through independent laboratory tests that a proposed green roof system will perform as required.
ACCORDING to Bill Hunt, Assistant Professor in North Carolina State University’s Biological & Agricultural Engineering Department, much of his teaching, research and outreach has focused on innovative storm water treatment practices such as wetlands, bioretention areas, permeable pavements and – most recently – green roofs. In 2001, Hunt and a colleague, the late Mike Regans, visited Stuttgart, Germany, to learn about green roofs. After visiting with a few German green roof firms, Regans and Hunt established the first two research green roofs in the Southeast, located in Kinston and Goldsboro, NC. These roofs, along with others added since 2002, have been researched by NCSU faculty for pollutant removal and runoff reduction benefits. Hunt has collaborated with colleagues at Michigan State University and Pennsylvania State University who are also conducting research on these innovative systems. He views green roofs as one of several tools that designers can use to mitigate storm water runoff.
FOR 2004, the Green Roofs for Healthy Cities organization named these winners in its “Awards of Excellence” ratings in the following categories:
Intensive Residential – Located on the 19th and 28th floor of the Solaire Building in New York City’s Battery Park, two green roofs (5,000 sq ft and 4,800 sq ft) have become an integral part of the low-impact design objectives of this Gold LEED-rated building. Dense strands of bamboo trees were planted in the center to provide a windscreen throughout the year and shade green roof paths and benches.
Intensive Institutional – Two years ago, a 17,250 sq ft “Roofmeadow” was installed on the Oaklyn Branch Library in Evansville, Indiana. The earth sheltered structure blends the roof with the landscape to create a native meadow prairie.
Intensive Industrial/Commercial – Rebuilding Soldier Field in Chicago to restore the prominent park setting has led to approximately 17 acres of reclaimed public waterfront parkland.
Extensive Residential – The Island House is a single family residence on one of the Thousands Islands in the St. Lawrence river between New York State and Ontario. The architects wanted to retain the openness of this agrarian landscape while providing clients with privacy and river views. The green roof is integrated both into the site and the building concept – complementing notion of building roof and ground plane.
Extensive Institutional – Engineered by Roofscapes, Inc., this 6,000 sq ft Roofmeadow green roof was installed on a wellness center in central Pennsylvania – an integral part of a green building. Challenges included: Stabilizing vegetation on a steep slope; Detecting leaks on the sloped surface; and Securing waterproofing at the gapped fascia.
Extensive Industrial/Commercial – Covering 454,000 sq ft atop Ford’s truck assembly plant, green roof is a significant component of a 600-acre storm water management system designed by architect William McDonough. Other objectives are habitat establishment at roof level, reduction in ambient temperatures, and protection of roof membrane.
(To review Green Roof Infrastructure Monitor,

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