When it comes to green roofs, the public generally envisions either the sod roofs of pioneer cabins or the earth and grass-covered roofs of the 70s residential “earth ship” era. A new generation of lightweight green roof systems is poised to update those impressions.
CONVENTIONAL GREEN ROOFSIn general, a green roof is topped with growing media and plants. Many green roof system designs in the United States share some similarities with the more common Inverted Membrane Roof Assembly (IRMA), also called a Protected Membrane Roof or plaza roof. In both cases, the roof membrane is typically installed directly over the roof deck. As is typical in an IRMA system, rigid insulation can be installed over the roof membrane as part of a green roof assembly to protect the membrane from the ravages of dynamic temperatures typical in a roof environment, while doubling as membrane protection from landscaping activities. The IRMA system prototype works well with green roof assemblies because an IRMA roof is typically topped with pavers to provide ballast and protect the insulation, a green roof substitutes a living ecosystem system for the pavers, while the insulation and roof membrane configuration are essentially the same proven assembly.
Green roofs of the past were often simply comprised of a deep layer of soil, often a foot or more in depth, planted with conventional landscaping over an IRMA roof. These intensive green roofs are quite heavy, with superimposed structural loads exceeding 100 pounds per square foot. As a result of these structural loads and the associated costs, intensive green roofs have been used on only a very small segment of the U.S. building market, the primary motivation for their use typically either aesthetic, providing a more appealing alternative to a conventional roof in a highly visible location, or functional, allowing the roof to serve as plaza or green space on a dense urban site. Until quite recently, Lord, Aeck & Sargent’s green roof experience was limited to intensive green roofs, ranging from Zoo Atlanta’s Action Resource Center, completed in the 1990s, to the University of Michigan’s Cyclotron Building Addition, completed in 2005.
GREEN ROOF BENEFITSGreen roofs provide the benefits of an IRMA roof’s protected roof membrane including reduced thermal shock and cycling, elimination of exposure to ultraviolet radiation, and protection from traffic and abrasion. Green roofs can also provide the aesthetic and functional benefits previously mentioned.
However, there has been a shift toward the selection of green roofs because of their ecological benefits. This has come about because of the convergence of an increasing awareness of the importance of ecological design with newly available lightweight and less costly green roof systems alternatives. The primary ecological benefits are reduced Urban Heat Island Effect and natural storm water management. While the surface temperatures for a conventional roof can exceed 120 degrees Fahrenheit, green roof temperatures closely reflect surrounding ambient temperatures. This reduces the Urban Heat Island Effect as well as heat gain through the roof. Green roofs can also reduce the volume of stormwater runoff, delay the peak discharge, extend the roof lifecycle, and cool and filter the runoff compared with a conventional roof. And, they provide insect and bird habitats.
Finally, and maybe most importantly, green roofs provide a metaphor for ecological design, replicating some of the natural systems and processes that would otherwise be lost in creating a building. Green roofs provide a way for buildings to demonstrate ecological responsibility in a tactile, understandable way to building visitors and occupants.
EXTENSIVE GREEN ROOF SYSTEMSExtensive, or low-profile, green roof systems were developed to provide many of the benefits of a conventional green roof at a fraction of the weight and cost. These modern lightweight green roof systems were pioneered in Germany to provide a solution to stormwater challenges on dense urban sites. Industry figures suggest that 10 percent of German roofs are now greened (Penn State Center for Green Roof Research, http://hortweb.cas.psu.edu/research/greenroofcenter/history.html).
Rather than the 8- to 12-inch or greater soil depth of an intensive system, extensive green roofs contain a soil depth of 6 inches or less, which is comprised of an often proprietary lightweight engineered growing media. Like a conventional IRMA roof, extensive green roof systems usually begin with a waterproof membrane applied directly to the roof deck and topped with insulation. However, a root barrier is added to prevent plant roots from penetrating the waterproof membrane. The membrane, root barrier and insulation are topped with a drainage and/or water retention layer, a filter fabric, a lightweight engineered growing media and the plant material. Given the shallow growing media depth of an extensive roof system, specialized, drought-tolerant plantings with shallow root systems are required.
The specific details of an extensive green roof will vary to meet the constraints and parameters of a given application. At over 40,000 square feet, Lord, Aeck & Sargent recently completed the largest steep-slope extensive green roof in the U.S. (http://www.greenroofs.com/projects/), the Gwinnett Environmental & Heritage Center. To better understand the process of creating an extensive green roof, we’ll explore the design, specification and construction of this project.
THE GWINNETT ENVIRONMENTAL & HERITAGE CENTERA collaborative effort of Gwinnett County, the University of Georgia, the Gwinnett County Public Schools, and the Gwinnett Environmental & Heritage Center Foundation, the Gwinnett Environmental & Heritage Center (GEHC) serves as a model of ecological design for the Southeast. Amid the sprawling development in this north-Atlanta metropolitan area, Gwinnett County had the foresight to protect a swath of land to provide a permanent public green space. The vision was born for an educational facility that would teach the region’s youth about the importance of water, telling the story of the powerful impact water has had on our history, our everyday lives, and the remarkable water management challenges that we face into the future.
PROJECT OVERVIEWFrom its earliest inception, the design included an extensive green roof supported by wood-framed trusses. The county’s program also included interpretive exhibits about the role of water from both a cultural and environmental context, and the desire for the building itself to serve as an interpretive exhibit. The building design consists of a series of adjacent linear gable structures running east to west. The two-story structure is built in two wings with a single bay bridge spanning across a natural ravine between. The ravine became the site of an innovative recirculating water feature that was developed to provide cooling for the building.
GREEN ROOF DESIGN AND SPECIFICATIONTo minimize structural loads on the building’s wood structure, an extensive green roof system was selected that resulted in a 35-pound-per-square-foot design load. The roof is supported with dimensional lumber trusses on 4-foot centers, spanned by 4-by-4 wood purlins and a Tectum roof deck.
The building’s roof geometry posed some challenges, with the undulating gable form creating interior valleys, and flat overhangs requiring careful planning in order to channel roof drainage. Interior valleys were cricketed with a low slope to exterior overhang areas. Overhangs were developed to direct water with tapered insulation to a virtual gutter area just outside the exterior walls, but interior from the roof edge. Outlets through the roof are spaced at column centerlines where copper rain chains direct the runoff to bioswales, providing an interpretive element for building visitors.
It was necessary to develop the specifications for a hard bid format that would facilitate the Design-Bid-Build project delivery process required by many public projects. A hot fluid-applied rubberized asphalt membrane was selected for its longevity. This type of roofing is squeegeed onto roofing felts forming a single, continuous membrane. The rubberized membrane is protected by a modified bitumen cap sheet and root barrier, and topped with extruded polystyrene insulation.
An exposed Tectum roof deck was specified for its interior sound attenuation and high recycled content. The decking was detailed with tapered insulation as necessary to provide the desired drainage, and topped with -inch thick DensDeck sheathing material specifically made to accept membrane roofing.
With the IRMA system details determined, design of the extensive green roof system components posed the next challenge. The unique aspects of the project included a 4:12 gable roof slope, presenting soil stabilization concerns, and the desire to utilize native plants for the roof vegetation. As we recommend for all of our institutional clients, our desire was to have the entire system, including the membrane roof system as well as the soil and vegetation, covered by a single-source, 20-year warranty. This means that, should the membrane fail during the warranty period, a single entity will be responsible for removing the over burden, repairing the membrane and replacing the overburden and landscaping. We believe this is an essential component to making green roofs acceptable to institutions whose operating funds come through the public sector.
Through this research process, we became familiar with the uncertainties for delivery of extensive green roof systems. Many roofing manufacturers are embracing complete green roof systems, including their conventional roofing products and also the specialized landscape components, and have technical expertise to help in the roof design. While this “one stop” service offered by roofing manufacturers has the benefit of convenience, it also presents two primary problems. First, reliance upon a proprietary green roof landscape specification puts decisions about individual components, including the landscaping and growing media, in the hands of the roofing manufacturer instead of the design professionals. Second, use of a proprietary system limits the bid process to only those manufacturers that can provide a comprehensive system. There are several roofing manufacturers whose products meet this project’s demanding membrane requirements, but in order to be able to furnish bids meeting the specified warranty requirements, these manufacturers must also support alternative delivery methods for the growing media and landscape components by teaming green roof landscape specialty consultants with installers. In order to promote competitive bidding, we chose to structure the landscape portions of our specification in a performance format, identifying only generic descriptions of each individual component’s requirements, to allow for a number of different delivery methods.
Requirements for root barrier, drainage mats, engineered growth media and accessories were easy enough to select based upon research of available industry literature. None of our research, however, identified a clear precedent for stabilizing the garden roof assembly on the 4:12 gable roof forms that make up more than half of the 40,000-square-foot roof area. After talking to several industry experts, we made a decision to include the sloped roof soil stabilization as a performance-based requirement to be determined by the successful bidder by simply stating that there needed to be stabilization that was proven in similar conditions and was acceptable to the roof manufacturer. This strategy resulted in the identification of an appropriate solution during the construction phase.
Finally, in keeping with the project’s goals of reflecting, preserving and educating the public about the regional environment, the landscape architects developed a list of native plant species for the roof. In the local piedmont region, there are many granite outcrops that are part of a much larger subterranean granite deposit. These geologic conditions are home to a unique micro-ecosystem that includes a pallet of native plants that flourish in less than 4 inches of soil deposits in crevices and depressions in the granite. Regional environmentalists and landscape architects have speculated that these plants could easily be adapted to green roof applications. There is much regional excitement about this concept, and this project seemed an ideal opportunity for testing these plants given its environmental mission and partnership with the University of Georgia whose offerings include strong environmental science, landscape architecture and agricultural programs.
Drafts of the plans, details and specifications were provided to several interested parties, including a roof manufacturer and a design-build coordinator, to be sure that we had a biddable set of documents for either delivery method. However, there was still some confusion regarding the two part system and single-source warranty requirements following award of the project, resulting in unintended post-bid cost negotiations. Perhaps as green roof systems become more commonplace the delivery systems will become more clear and predictable.
CONSTRUCTIONThere were two significant developments that came out of the construction process. First, American Hydrotech won the roofing contract in conjunction with a local roofer and landscaping company. They proposed a new product for stabilization of the growing media on the 4:12 pitched roof areas. This product, generically referred to as a cellular confinement system, is composed of perforated polyethylene strips joined in accordion fashion to form a dimensional mesh.
The second significant change was the realization that the native plant varieties, in the quantities required, were not commercially available from local nurseries. Contract grown stock proved to pose a scheduling challenge by the time serious inquiries were made. Consequently, six species of sedums from a “standard” East Coast green roof palate were substituted on the main roof. However, a small roof over the loading dock area is dedicated to the originally specified natives. Our hope is that this approximately 800 square feet of test plot application will help to prove the hypothesized success of the native plants in this application for future projects.
CONCLUSIONThe growing awareness of the ecological impacts of development and the associated growth in the green building industry is prompting renewed interest in green roofs. Due to their lighter weight and reduced costs, extensive green roofs are poised to become much more common in the U.S. market. Lord, Aeck & Sargent has incorporated the lessons learned creating the GEHC and two additional extensive green roof projects currently under construction for the Southface Energy Institute and the National Park Service. We look forward to sharing our experience with these projects upon completion.
acknowledgement: The Gwinnett GEHC Case Study was adapted from “A Green Roof for the Next Generation: The Gwinnett Environmental and Heritage Center,” written by Meg Needle and Jim Nicolow and published in the 2006 Conference Proceedings of the Fourth Annual Greening Rooftops for Sustainable Communities Conference, Awards and Trade Show.
Sidebargwinnett environmental &
heritage center credits
owner: gwinnett county, ga.
architect: lord, aeck & sargent
landscape architect: the jaeger company
general contractor: juneau construction company
roofing trade contractor: metro waterproofing
landscape trade contractor: prolandscapes
roof deck - tectum, inc., 2-inch tectum i
roof sheathing - georgia pacific, densdeck
roof membrane - american hydrotech, 6125-ev with hydroflex
root barrier - american hydrotech, 10 mil root stop
insulation - dow corning, extruded polystyrene
drainage mat - american hydrotech, hydrodrain
cellular confinement system - american hydrotech, gardnet
growing media - american hydrotech, litetop