GardNet Has Expanded the Range of Roofs That Can Be Greened
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Since when do green roofs need to be flat? For countless years, roof tops were the forgotten plane of the building envelope. Relegated to keeping the dry, the rooftop was often only thought of as the convenient place for HVAC elements and similar functions.
Illustrations of various roof decks. By Rahman Ali Khan.
Amenity decks began to flourish as building owners used them to attract customers and tenants. Exquisite and luxurious roofpark environments have been successfully created in the process. All of these projects require a roof deck that is relatively flat with minimal slope. This creates a feeling of 2-dimensional space, even with elaborate landscape and structures on the roof.
Fortunately, a number of designers have been looking at 3-dimensional expressions on the roofscape, especially where it manifests in the green roof. They often rely on complex mathematics and ever increasing computing power in developing the geometries of new structures including the roofscape.
Bringing vegetation to the standard flat roof has been a stalwart for the green roof industry around the world for many years. Installing the varied components of green roof assemblies on a flat roof most often don’t have to deal with gravity trying to pull the components down. Even slightly sloped roofs rely on parapets and other simple building building elements to contain the engineered growing media, drainage and plant materials required for a green roof.
The roofing industry has developed assemblies and techniques for steep roof slopes. Most homes have shingle roofs and some of these can have very significant slopes. Metal roofs and some single ply roofing can achieve very steep slopes.
New Technology for Sloped Green Roofs
Adding a green roof to these steeply sloped roofs requires a different approach. American Hydrotech, Inc., (AHI) based in Chicago, Illinois developed an assembly to address this issue while still providing the benefit of a protected membrane roof with insulation. AHI has developed a long term reputation of providing its waterproofing and Garden Roof components on projects where it is anticipated to be a “life of the structure” installation.
In the early 2000’s, the GardNet assembly was researched and developed initially for a mockup for the California Academy of Sciences. The installer and General Contractor (GC) put together a mockup in a parking lot to test the viability of the system. Another method of securement was selected for the sloped areas in the final design.
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Hydrotech’s GardNet® Assembly is specifically intended for steeply sloped Garden Roof projects. On roofs from 0 to 2:12 slope, gravity’s effects are minimal and components stay in place through internal friction. Once a roof slope exceeds 2:12, gravity will begin to act on the components and potentially cause them to shift in certain conditions. At a slope of 3:12 and over, slippage of green roof components is all but a certainty. Rain events become problematic and compound the challenges unless measures are included to stabilize the green roof.
Providing a surface erosion control component only has very limited impact on steeply sloped green roofs as it is intended to protect the top surface only.
Lacey Elementary School Garden Roof. Photo: American Hydrotech, Inc.
Hydrotech’s GardNet assembly stabilizes the entire growing media layer and the plants that are planted within the assembly. Consisting of a 3-dimensional geogrid webbing supported with stainless steel cables (tendons) and other hardware, GardNet is supported at the top of the roof slope where the roof is strongest. Hydrotech works with its consultant to determine the number and spacing of the tendons and the estimated loading at each tendon. The project designers and engineers determine how these tendons are connected to the top of the slope and to the building structure. The geowebbing is available in a variety of thicknesses and can be stacked to achieve growing media thicknesses more than 12 inches. This can create growing environments suitable for groundcovers, perennial plants and shrubs. With special considerations, even trees have been incorporated into GardNet assemblies.
While starting out as a solution to address simple, steeply sloped roofs, the architects soon realized that the GardNet assembly could address more complex, multi-dimensional roof shapes. Hydrotech worked with designers in exploring innovative layouts and shapes that take advantage of the inherent capabilities of this flexible assembly including horizontal and vertical curved roof shapes similar to the illustrations above.
Simple, single plane roof slopes are very easy to achieve with the GardNet assembly. The Lacey Elementary School Garden Roof above is an example of a simple, single plane sloped roof.
The GardNet assembly is very flexible in multiple directions and accommodates the complex geometries created by designers. Adding a third dimension to an otherwise flat roof deck can create very dramatic green roofscapes as in the H2Hotel project in Healdsburg, CA.
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Completed GardNet assembly at H2Hotel - Healdsburg, CA ready for LiteTop growing media. Photo: American Hydrotech, Inc.
The GardNet assembly is very flexible for plants and we often see sedum carpets being installed as the final planting, but other plant materials can also be used. In the Lincoln Center Hypar Pavilion, a sod lawn was installed over the GardNet assembly in this hyperbolic paraboloid roof shape that covers a restaurant space. It creates a unique space for visitors to relax in this dense urban environment without realizing they are on a rooftop over a habitable space.
At-grade views of Lincoln Center Hypar Pavilion – Turf steps supported by stone facilitate pedestrian access to upper portion of lawn panel. Perimeter glass railings provide for pedestrian safety. Photo: American Hydrotech, Inc.
GardNet has the ability to achieve unique green roof shapes envisioned by the designers. Its flexibility in layout can accommodate circular or spherical alignments too. The Bellevue Youth Theatre in Bellevue, Washington was achieved by a circular alignment of GardNet that supports a lawn planting. The GardNet was fashioned around the uniquely shaped skylights that bring light down into the subterranean theatre.
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The completed GardNet assembly at Bellevue Youth Theatre, Bellevue, Washington. Sod plantings over GardNet assembly. Photos: David Sundberg
With 60 years of proven performance, Hydrotech’s recognized by many for having the best waterproofing membrane. This is important in all applications, but critical for green roofing, which has led it into many of these projects where the designers have a particularly challenging project vision.
The Statue of Liberty Museum started as a project with several distinct purposes. The building must be absolutely watertight to protect the museum structure and contents as well as the original torch from the Statue installed in 1876. As on all GardNet projects, Hydrotech’s Monolithic Membrane 6125 was employed for this purpose. The roof is also designed to have a granite paved observation platform that is reached by a grand, tiered, exterior staircase. Flanking that staircase is a sloping GardNet green roof installation that was designed to resolve the geometry of the green roof and the staircase edge.
Left: Statue of Liberty Museum roof - view from upper observation terrace toward Statue. Photo: American Hydrotech, Inc. Right: Statue of Liberty Museum roof - view of GardNet area with initial established erosion control grass planting: Photo: American Hydrotech, Inc.
Originally planted with erosion control grasses, the ultimate planting palette includes native ornamental perennial plants that will add additional color and texture to the overall roof as well as the GardNet area.
One of the most expressive applications to date of the GardNet assembly was created at the REACH Addition at the Kennedy Center in Washington DC. In this project, Hydrotech’s staff worked closely with the architects to achieve a unique and dimensionally complex green covered roof that was difficult to express in printed documents. Covering a concrete structure protecting performance spaces below, this GardNet installation started out in a horizontal plane adjacent to the lawn panel. As the geometry moved along the glass-encased façade of the new building, the GardNet panels moved and rose in elevation. At the same time, the slope of the GardNet installation increased from 0 degrees to 90 degrees, a fully vertical installation. While the majority of the GardNet installation included Hydrotech’s LiteTop engineered growing media, Hydrotech worked with its partner – Sempergreen – to include a special hydroponic irrigation technology and mineral wool substrate to support the plantings at the extreme sloped end of the GardNet panel. These components were installed within the cells of the Gardnet geoweb before installing the plants.
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First: Kennedy Center Reach Addition - Washington, DC - View of GardNet area during installation. Second: View of GardNet area immediately after planting with custom sedum tiles. Third: View of GardNet area at vertical corner immediately after planting with custom sedum tiles. Fourth: view of completed GardNet area. Photos: American Hydrotech, Inc.
A special customized sedum tile was created by Sempergreen for this project and in the extreme sloped area, the tiles were wired to the internal GardNet components.
The Kennedy Center project demonstrates the collaboration of the designers with the manufacturers and plant material growers to solve the technical issues that are created in a very dramatic architectural expression. Numerous issues had to be considered including:
Accommodating the thermal insulation requirements in a geometrically complicated roof structure.
Managing rainwater flow from the adjacent vertical glass façade away from the slope.
Complicated supporting connections for the GardNet stainless steel tendons.
Creating a special growing media environment for the sedum to thrive.
Creating and providing an irrigation system that keeps water high in the slope where the plants need it.
The installation benefits from long term maintenance as part of the Kennedy Center campus.
Conclusion
With over 100 GardNet installations completed to date, GardNet continues to be included on ever more complex projects across the United States and Canada. With this vast experience, Hydrotech continues a history of working with designers and building owners around the world to address not only waterproofing issues, but how to achieve unique, signature green roof features in what will become iconic structures.
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Richard C. Hayden, is a GRP and ASLA-Emeritus. He has recently retired as Garden Roof and Blue Roof Department Manager, currently Advisor to American Hydrotech, Inc. (A Sika Company) based in Chicago, Illinois. Richard has a 43-year professional career beginning with 30 years as a private-sector consulting Landscape Architect. Since joining Hydrotech in 2010, Richard has been involved in many of Hydrotech’s iconic projects across the United States and has developed a wide array of Garden Roof and Blue Roof assemblies.
For more information on American Hydrotech, please visit: www.hydrotechusa.com
