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December 2007
guest feature

New Stormwater Regulations Promote Green Roofing and Waterproofing
By William F. Foley, CSI, CCPR, Waterproofing and Greenroof Design Consultant

Publisher's Note: This is an updated article originally published in the Summer 2007 issue of Applicator Magazine, published by the Sealant, Waterproofing & Restoration Institute.


The Stormwater Management Problem

One only needs to listen to the news or read the local paper to find all too common stories of flooding and property damage caused by seemingly routine rainfall events. The increased frequency of these events indicates that current stormwater management regulations are inadequate.

The Phillips Eco-Enterprise Center, Minneapolis, MN, above, is an excellent example of stormwater management on a greenroof with native plantings using ZinCo FD-60 material; Images by Corrie Zoll courtesy of The Green Institute.

For environmental reasons we need to consider roofs and plazas part of the overall stormwater management program in most densely populated areas in North America. This inclusion will require a paradigm shift in roofing and waterproofing design from getting water off of our buildings as quickly as possible to slowing and retaining water flow on our rooftops. This will also require new designs, new systems and increased skill and dedication to fulfill this purpose.

In addition to the more obvious effects as flooding and erosion, rainfall also washes over impervious surfaces so that the initial flush of runoff can carry high concentrations of pollutants to nearby drinking water supplies, waterways, beaches and properties. Pollution washed from the land surface by rainfall is called “non-point” source pollution.

Local stormwater management programs seek to protect properties and aquatic resources from damages caused by increased volume, frequency and peak rate of stormwater runoff. Further, these programs seek to protect those resources from increased “non-point” source pollution carried by stormwater runoff.

Traditional Solutions

Traditional stormwater treatment practices fall into five major categories: stormwater ponds, stormwater wetlands, infiltration practices, filtering practices, and open channels. Within each category, there are several design variations. Stormwater filtering systems include surface sand filters, perimeter filters, organic filters, underground filters, pocket sand filters, rain gardens and bioretention. Stormwater infiltration systems include infiltration trenches, shallow infiltration basins and porous pavement.

Green Roof Contribution to an Integrated Stormwater Management Plan

Many municipalities and states in the U.S. are now introducing new stormwater regulations that limit impervious surfaces (such as roofs and paved areas) and encourage and even reward building developers for the inclusion of greenroofs that are recognized as pervious surface area.

The hydrologic response of a green roof bears closer resemblance to a lawn or meadow than impervious surface. The green roof system is composed of multiple layers including waterproofing, a drainage layer, engineered planting media, and specially selected plants. Vegetated roof covers can be optimized to achieve water quantity and quality benefits. Through the appropriate selection of materials, vegetated covers can provide rainfall retention and detention functions.

Left: ZinCo drainage with expanded shale and filter sheet; Middle and Right: The Phillips Eco-Enterprise Center; PEEC images by Corrie Zoll courtesy of The Green Institute.

Unlike conventional roofing, green roofs promote retention, slow release, and evapotranspiration of precipitation. This stormwater management technique is very effective in reducing the volume and velocity of stormwater runoff from roofs.

Water Quantity Control

The use of green roofs to improve urban stormwater management is one of the most important benefits this technology offers. Green roofs attenuate peak flows and reduce the total volume of stormwater runoff by retaining rain water in the growing media and returning a portion of it back to the atmosphere through evapotranspiration. The runoff flows leaving the green rooftop, and ultimately draining to receiving waters, are significantly decreased and delayed relative to peak flows in the receiving water system. This helps to minimize downstream channel erosion, flooding risk, and negative impacts on aquatic ecosystems.

Studies undertaken in North America and Europe indicate average percent retention rate reported in these studies was 60%, ranging from a low of 39% to a high of 72%. Substrate thickness had a positive influence on rainfall retention in studies where more than one substrate thickness was monitored.

The ability of greenroofs to attenuate runoff peak flows has also been demonstrated in several monitoring studies. A monitoring study of a flat extensive greenroof undertaken between 2002 and 2003 in Portland, Oregon (Hutchinson et al., 2003) found that peak precipitation run-on rates ranged from 0.041 to 0.193 ft3/sec while flow rates running off the greenroof were only 0.008 to 0.012 ft3/sec. The Toronto and Region Conservation Authority (2006) reported peak flow reduction rates of 50% for runoff events greater than 40 mm and 87% for events between 10 and 29 mm. Rain from smaller events was often completely retained by the green roof. The chart below from the American Society of Landscape Architects test greenroof in Washington, D.C. shows the relationship of rainfall to roof retention atop their headquarter greenroof (2006):

Source:  American Society of Landscape Architects (ASLA); see the ASLA Headquarters Green Roof in The Greenroof Projects Database.

Green roofs also delay runoff peaks, which helps to ensure that peak discharges to receiving waters do not occur when the stream is being heavily impacted by stormwater flows from other impervious surfaces. A study of an extensive green roof in Toronto by the National Research Council of Canada reported runoff delays relative to a conventional roof of between 20 and 40 minutes on average (Liu and Minor, 2005) and found average runoff delays of 30 minutes on a green roof with a thicker growing medium.

Improved Runoff Quality

Green roofs help to decrease the total load of contaminants conveyed to receiving water systems in two key ways: (i) reducing contaminant concentrations and (ii) reducing total runoff volumes from the roof.

Research has shown that runoff from a green roof tends to have lower concentrations of contaminants such as aromatic hydrocarbons, heavy metals, and suspended solids. This occurs because the green roof material does not leach these contaminants into runoff the way a conventional roofing surface does, and also because the green roof soil and plants remove atmospherically deposited contaminants though various chemical, physical and biological processes.

When rain falls on a green roof, the impact is absorbed by the surface soil or substrate, before it percolates slowly through the media, past a geotextile filter and into the drainage cell. On a conventional roof, the impact of rain and flow of rainwater on the hard surface leaches pollutants from the roofing material that were not deposited atmospherically.

Several studies have shown that leaching of pollutants from conventional roofing materials can result in very high runoff concentrations of metals, polycyclic aromatic hydrocarbons (PAHs) and other pollutants. PAHs are chemical compounds that can be point source pollutants (e.g. oil spill) or non-point source (e.g. atmospheric deposition) and are one of the most widespread organic pollutants. Some of them are known or suspected carcinogens, and are linked to other health problems. They are primarily formed by incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, tobacco, or incense. Tar also contains PAHs. (Wikipedia, 2007) Atmospheric deposition is another major source of contaminants in rooftop runoff.

In a roof garden, soil adsorption, plant uptake, microbial activity, and filtration are all processes which either remove contaminants from the runoff or prevent atmospherically deposited contaminants from entering runoff. The retention of runoff by greenroofs discussed earlier translates into a significant decrease in the volume of water discharged to receiving water systems. These lower flow volumes in turn result in reduced contaminant loads in runoff from the roof.

Hydrologic System Impact

In older municipalities in which combined sewer systems are used, green roofs can be an important stormwater management technology as they do not take up valuable space at ground level. The implementation of green roofs in built up areas helps to reduce overall flows to storm sewers and minimizes combined sewer overflows. When combined sewer systems overflow from wastewater treatment facilities raw sewage dumping occurs, including human waste directly into rivers, lakes and bays. Greenroofs are another way to reduce this incidence and translates into fewer beach closures and the overall improved health and aesthetic value of rivers, waterfronts and coastlines.

Stormwater Management Drivers for Green Roof Infrastructure

Besides stormwater management the private and public benefits and incentives to build greenroofs are many. Green roofs offer public benefits including; aesthetics, waste diversion (less refuse from re-roofs), reduction in urban heat island effect, improved air quality, improved quality of life and local job creation. Private owner benefits of green roofs include: improved energy efficiency, increased membrane durability, fire retardation, noise reduction, and marketability of green roof amenities.

Stormwater management is probably the single most important driver in green roof infrastructure growth. Many local stormwater management regulations are beginning recognize green roofs as part of an integrated system of stormwater flow reduction and treatment. The key benefit for green roof proponents is that when a green roof is integrated into stormwater design of a building and site it is very difficult to value engineer the system off the project or change the design criteria. To do so would cause a cost increase in other systems or perhaps affect the area for occupied space or available leased space by decreasing the allowable building footprint.

Examples of local and state stormwater legislation that incorporate green roof technology can be found in Philadelphia, PA; Minneapolis, MN, Seattle, WA and Portland, OR. It is certain that many other state and local stormwater management authorities have moved in that direction.

Another key driver that involves stormwater management affecting greenroof infrastructure growth is LEED®-NC. Under the USGBC LEED® program many more buildings are seeking LEED® certification. Greenroofs can contribute to LEED® in credits in up to as many as 15 different categories. Greenroofs contribute via stormwater management in two specific LEED® categories:

Stormwater Design: Quantity Control (SS Credit 6.1), 1 point. A Green Roof System stores stormwater in growing medium and in strategically placed water stormwater retention/drainage composites to reduce runoff and promote evapotranspiration.

Left: This example of a drain flow restrictor uses ZinCo FD-60 drainage with a separate Inspection Chamber KS-12 on a greenroof.  Right: This ZinCo FD-25 greenroof material section shows how the growing medium holds water, the drainage composite holds water and the thick fabric layer just beneath the drainage composite is a moisture retention mat.

Stormwater Design: Quality Control (SS Credit 6.2), 1 point. A Green Roof System filters stormwater with microbial activity, absorption in the growing medium, filtering through the growing medium and filter fabrics and through plant uptake and evapotranspiration.

Many government agencies such as the GSA are making LEED® certification a requirement thus encouraging greenroof stormwater design consideration. Similarly, some municipalities such as New York City and Washington, D.C. have adopted new statutes. “Local Law 86” in New York City requires that all municipal buildings or municipally funded (many private) projects meet LEED®-NC Silver Certification. In Washington, D.C. “The Green Building Act of 2006” requires all new construction, public and private over a certain size to meet LEED®-NC Silver Certification. This has already greatly impacted the number of greenroofs in planning and design in those cities.

As contractors, consultants, designers and manufacturers in the industry of waterproofing we must all be very excited about this new opportunity for green roofs and the link to stormwater management. Stormwater management designs must have the best waterproofing systems installed by the most qualified applicators due to the critical nature of storing and slowing the flow of water above occupied space. Many of us have been disappointed at times over the promise of green roof designs that were value engineered due to cost overruns or budget constraints.

The stormwater management regulated and designed systems must be built to meet the expectation of the overall stormwater management system. Therefore, the opportunity to design high quality, integrated systems that will be built with much more frequency is upon us. As others in the green roof industry have been quoted, “We are finally at the tipping point where the green roof market in the North America is about to explode.”

William F Foley, CSI, CCPR, is the Managing Partner of Enviro-Plane Consulting, LLC. In 2006, Enviro-Plane Consulting, LLC was founded in part to promote, specify and help to maintain building envelope infrastructure as it relates to environmentally sustainable and energy efficient building envelope design and implementation. Bill has been active in commercial construction contracting for more than 25 years. For the past 18 years Bill has worked in building envelope materials product representation, new product development and marketing strategy.

In 1994, Bill earned Certified Construction Product Representative (CCPR) from Construction Specifications Institute and has served in various positions there. Bill has authored numerous articles appearing in industry magazines, and since 2003 Bill has been directly involved in the advancement of building envelope technology helping to develop, register and deliver air barrier and green roof AIA programs all over the East Coast. Contact Bill at: 610.390.1068 or visit: www.enviroplane.com.

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