May 23, 2007 | General

Storm Water Management, Green Roof Style

BioCycle May 2007, Vol. 48, No. 5, p. 42
Interest and investment are growing in use of green roof systems to manage storm water runoff.
Joshua Wachtel

WHEN Charlie Miller, founder and president of Roofscapes, Inc. in Philadelphia, first learned about green roofs over a decade ago, there was very little known in North America about this rooftop phenomenon. Miller had been working for a large environmental engineering firm and was hired to provide a new manual for the Commonwealth of Pennsylvania outlining the best and latest methods of storm water management. During the research and writing of that manual, Miller got interested in green roofs, which had been pioneered in Germany in the mid-1960s. Dr. Joachim Tourbier, a German friend and landscape architect, sent him a stack of technical books about green roofs, all in German, which Miller proceeded to translate and study.
“What I found was impressive,” says Miller. “It was a huge industry in Germany with lots of competitors and tens of millions of square feet of development.” He thought he’d hit on “the silver bullet” for dealing with a host of problems in municipal development. “I thought it would take off like wildfire,” he recalls.
As it has turned out, the development hasn’t been quite as swift as Miller originally hoped. Still, ten years of experience building roofs, testing products and systems, and educating architects, landscapers, engineers and builders about green roofs and gardens, Miller notes the change. “The difference between ten years ago and now is that every landscaper and architect in the U.S. knows what a green roof is. [And] it’s been more interesting in the last year or two.”
A 2005-2006 survey conducted by Green Roofs for Healthy Cities, the North American trade association, indicates a growth rate of over 25 percent from the 2004-2005 survey, representing more than 3 million square feet of green roofs installed in 2006. The findings are based on corporate members’ completed green roofs projects. “The green roof industry is growing rapidly in response to the pressing need for cleaner air, better storm water management, improved energy efficiency and more usable green space in our communities,” says Steven Peck, President of Green Roofs for Healthy Cities, based in Toronto, Ontario. The City of Chicago, which has policies in place to support green roofs and urban greening, implemented the most square feet of green roofs in 2006 of any city, followed closely by Washington, D.C. Over one million square feet of green roofs were completed in Chicago and another two million are in development.
Green roof infrastructure involves the use of technologies that incorporate drainage/filtering systems, quality waterproofing, root-repellency, engineered growing media and plants. The benefits of green roofs include a reduction in storm water run-off, better heat and sound insulation, energy savings, improved air quality and reduction in the urban heat island. Other benefits include increased park space, improved aesthetics, community gardening and biodiversity.
Green roofs are classified as extensive, semi-intensive, or intensive. Generally, extensive green roofs have six inches or less of growing medium, whereas intensive green roofs have greater than six inches of substrate. Semi-intensive green roofs can be defined as a hybrid between intensive and extensive green roofs, where at least 25 percent of the roof square footage is above or below the 6-inch threshold. Extensive green roofs provide many of the environmental benefits of intensive green roofs, but they are designed to be very low-maintenance with no public access. Semi- and intensive green roofs typically are designed for use by the public or building tenants as a park or relaxation area. However, they also require greater capital and maintenance investments than extensive green roofs.
Extensive green roofs with a media layer of 3- to 6-inches have a saturated weight of about 20 to 40 lbs/sq ft (assuming standard FLL-compliant (see sidebar) mineral based green media is used) and are ideal for the growth of drought-tolerant plants, particularly succulents like Sedum. Immediately after construction, green roofs need to be monitored regularly to ensure the vegetation thrives. During the first season, green roofs may need to be watered periodically if there is not sufficient precipitation. After the first season, extensive green roofs may only need to be inspected and lightly fertilized approximately once per year. (Intensive green roofs, on the other hand, need to be maintained as any other landscaped area.) Green roofs are less prone to leaking than conventional roofs. In most cases, detecting and fixing a leak under a green roof is no more difficult than doing the same for a conventional roof.
In general, the growing media on a green roof has to be able to absorb, hold and slowly release nutrients to sustain the plant life under extremely harsh conditions. Elements of the media must be prevented from segregating. Nonorganic materials like Styrofoam and vermiculite would tend to rise to the surface and wash away, while sand and other small, heavy mineral particles would tend to sink.
Finally, the media needs to both hold water and release it at a certain rate. This is the quality that makes green roofs so effective for storm water management. By absorbing water quickly in a storm, and then releasing it gradually, the intensity of runoff during a storm can be tempered. But at the same time, the growing media must hold onto enough moisture to keep the root systems of the plants healthy and to reduce or eliminate the need for irrigation. “We usually use mixtures with five percent organic matter,” says Miller. “Mixtures with more than 10 percent organic content tend to be too biologically active and degrade over time. Compost addition is the accepted method for nourishing plants in green roof mixes. Nutrients are released slowly and aid the development of media structure.”
The storm water management benefits of green roofs make then ideal for ultra-urban areas without consuming additional land. According to a U.S. Environmental Protection Agency fact sheet on green roofs (, a 2005 modeling study of Washington D.C. by Casey Trees and Limno-Tech found that green roofs on 20 percent of buildings over 10,000 square feet could add 23 million gallons of storage and reduce outflow to the storm sewer or combined sewer systems by an average of just under 300 million gallons/year. According to the authors, this would reduce the annual number of combined sewer overflow events in Washington D.C. by 15 percent.
One area where the benefits of storm water management have been applied is the big box store market (e.g., Wal-Mart, Home Depot). Since these stores carve out huge tracts of land, storm water management is a huge problem. “Developers are receiving pressure from municipal governments to do something about the imperviousness to water of these lots,” says Melissa Muroff of Roofscapes, noting these green roof projects can run into the tens or even hundreds of thousands of square feet. “We began to think, ‘What if we could consistently offer a lightweight green roof for significantly less than $10 per square foot?'”
This thought provided inspiration for the development of the Roofrug® – a product resulting from a collaboration between Roofscapes and Zeager Bros. Inc., a mulch and compost company based in Middletown, Pennsylvania. It imports and licenses a product made in Europe from recycled foam and plastic. Zeager found the product useful for improving drainage and safety of children’s playgrounds, but the product is also excellent for building green roofs, and can be provided for as little as $6.50 to $8/sq ft, not including waterproofing, says Muroff Zeager Bros., which started out as a sawmill in 1967, shut down its mill in 1990 and struck out in a new direction – selling wood by-products. It sells a variety of natural mulches, colored mulch and bark, leaf compost, Earthenrich compost made from biosolids and blends of topsoil and compost. One of its leading products is Woodcarpet®, a surface for children’s playgrounds made from wood fiber. When it comes to playgrounds, “drainage is critical,” says Ted Illjes. Manager of R&D at Zeager Bros. The company wanted an alternative way to upgrade the surfacing of playgrounds where its Woodcarpet was being used.
What Zeager Bros. hit upon ten years ago was a mat manufactured in Europe entirely from recycled postconsumer products. It is made by heat fusing ground nuggets of close cell foam trim-ends from a variety of industries, but most notably from the making of sneaker soles. The recycled foam mat (marketed by Zeager Bros. for playground use under the brand name Recbase or Duradrain) is placed on the ground and forms the base of a playground surface. It is covered by geotextile product and then the Woodcarpet. Since the foam mat is both resilient and water permeable, it allows for water drainage without degrading or compressing over time. And just as the system is great for drainage beneath swing sets, it turns out that it is also useful as a sublayer on green roofs.
The recycled foam layer is a key element in the Roofrug. The assembly is slightly more complicated than the one used for playgrounds. Strips of the foam mat are placed on the roof on top of the waterproofing layer. These strips have small V-shaped channels pressed into the bottoms of them. Additionally, triangular drainage conduits are integrated into the assembly to facilitate drainage during large storms. A thin, white geotextile separation layer made out of plastic from recycled soda bottles spun into a filament is fused on top of the foam. This layer is water permeable and separates the foam mat from the media. Last come the plants and a wind erosion layer.
Roofscapes recently installed a 74,000 square foot Roofrug on a Wal-Mart in Chicago. It is four-inches thick and has a maximum weight, when saturated, of 20 lbs/sq ft. The plant cover consists of flowering succulent plants called Sedum. Wal-Mart will be voluntarily monitoring the green roof as a service to the City of Chicago. The company is measuring runoff rates and volumes; energy flow across the roof, both into and out of the building; and rooftop temperature.
Since compost is such an essential component of green roof soil mixes, it’s natural that composters have entered the field and are finding a market niche. In 2001, Joe DiNorscia, General Manager of Laurel Valley Soils ( based in Avondale, Pennsylvania, a provider of mushroom compost for 25 years, got a phone call from Ed Snodgrass of Emory Knoll Farms, a sustainable farm supplying green roof plants based in Maryland ( Snodgrass was asking about the possibility of obtaining a soil medium that would be appropriate for use on green roofs.
Laurel Valley was just starting to diversify its product line, using its compost to produce different types of soils and amendments. The timing coincided with the emerging interest and investments in green roofs. Laurel Valley decided get involved, and began to develop a proprietary line of green roof mixes called “rooflite™.”
DiNorscia says when Laurel Valley first got involved with green roofs, “It was a fad – the ‘in thing’.” Laurel Valley started out providing soil for relatively small 500 to 2,000 sq ft roof projects. But as the projects grew to 30,000 and 40,000 square foot roofs, DiNorscia sensed a change in the market. “Companies were needing to provide storm water management plans in new construction, and this was becoming a bigger factor in the decision to build green roofs,” he says.
With the success of its green roof media line – and concurrent developments in the green roof industry – Laurel Valley formed a new company, Skyland USA, dedicated exclusively to making and marketing Rooflite ( Laurel Valley Soils provides the compost component – 20 percent by volume and 10 percent or less by weight – from its mushroom composting operation. For the mineral component, Skyland experimented with various materials before settling on a unique ceramic product called HydRocks™, a lightweight, porous, durable and absorbent material manufactured by Garick Corporation of Cleveland, Ohio. HydRocks is produced by fully calcinating clay at temperatures in excess of 2000°F and helps maintain pH levels in the growing medium, according to the Garick website.
In May 2006, Skyland announced a formal partnership with Garick, which owns Paygro of Ohio, a company initially developed in 1972 to compost cattle manure from the Ohio Feed Lot. The purpose of the Skyland/Garick partnership is to make Rooflite available on a national basis. The agreement “melds the production and technical know-how that Skyland brings to the table with the logistics and customer service strengths of The Garick Company,” according to DiNorscia. “We are providing a brand name mix that meets the ASTM and FLL standards for the green roofing industry.”
DiNorscia adds that media for about 100 jobs – around 500,000 sq ft of roofs – has been provided since the partnership began. When he considers how much green roof development there has been in Germany, DiNorscia sees the potential for tremendous growth in the green roof business in North America. “There’s no question it’s more expensive to install a green roof than a conventional roof,” he says. “But when you look at the longevity of the roof, the benefits in storm water management and energy savings for cooling, is it really more expensive?”
Joshua Wachtel is a freelance writer for BioCycle and In Business (
Page 44
GERMANY has taken the lead on setting standards for green roof growing media. The German FFL- Greenroof Guidelines (ForschungsgesellschaftLandschaftsentwicklung Landschaftsbau e.V., Guideline for the Planning, execution and Upkeep of Green Roof Sites, Release 2002) are universally accepted as the scientific foundation for green roofs worldwide. They define requirements and testing procedures for all green roof components. Requirements for growing media are related to: Environmental and plant compatibility; Particle size distribution; Maximum water holding capacity; Air-filled porosity (at maximum water holding capacity); Water permeability (saturated hydraulic conductivity); pH value; Salt, organic matter and nutrient content; Limitations of contamination; Fire and frost resistance; Structural stability.
In 2000, a Green Roof Task Force was convened by ASTM (American Society for Testing and Materials) to create green roof standards. Five standards have been published by ASTM to date (; search on green roofs. Other standards are in development. Several of the approved standards address green roof media, e.g., “Saturated Water Permeability of Granular Drainage Media,” “Water Capture and Media Retention of Geocomposite Drain Layers, “Maximum Meida Desnity for Dead Load Analysis.”
Green Roofs For Healthy Cities recently convened a subcommittee to determine performance-based specifications for growing mediums as they relate to the requirements of a North American marketplace. Jennifer Sprout, Director of Local Market Development for the Association, is overseeing the subcommittee’s developments (jsprout@ The U.S. Green Building Council also has recently added green roofs to its LEED (Leadership in Energy and Environmental Design) program standards, says Charlie Miller of Roofscapes, who is active in both USGBC and ASTM standards development work.
Page 46
LAST month, the U.S. Environmental Protection Agency (EPA) signed an agreement with four national organizations to promote the use of green infrastructure to capture polluted storm water runoff from paved roads and buildings to minimize sewer overflows and treat storm runoff. Stephen Johnson, EPA Administrator, signed the national “statement of intent” in Pittsburgh with representatives from the Association of State and Interstate Water Pollution Control Agencies (ASIWPCA), Low Impact Development Center, Natural Resources Defense Council (NRDC) and National Association of Clean Water Agencies (NACWA). The agreement pushes EPA and state permitting agencies to explore new methods or incentives to incorporate the use of green infrastructure into the requirements of National Pollutant Discharge Elimination System (NPDES) permits. It also directs EPA to develop guidance materials explaining how regulatory and enforcement officials should evaluate and “credit” the use of such approaches in meeting Clean Water Act requirements.
The statement released by EPA recognizes principles and practices long advocated by BioCycle readers. It states: “Through green infrastructure techniques, storm water and its pollutants are managed using natural systems to help absorb, infiltrate, evaporate or reuse excess storm water instead of using traditional infrastructure that collects, stores and transports water through large, buried sewer systems … Roofs that are covered with vegetation and plantings, tree boxes, rain gardens and pocket wetlands are just a few examples of common green infrastructure approaches. Water is treated as an important resource rather than a waste product.” More details are available at

Sign up