Article from Waterproof! Magazine's Summer 2011 Issue discusses the challenges of sloped roofs and potential solutions.

1. Sloped Green Roofs
The Gallie Craig Coffee House, build atop a cliff
overlooking the stormy North Atlantic, features a
undulating green roof that blends seamlessly with the
native prairie grasses surrounding it.
Photo Courtesy ZinCo USA/Nicholas Smith
G
reen roofs, developed in Europe plant mentioned above eliminated doesn’t pool, and leaks are reduced.
in the 1970’s, are becoming the need for a $10 million stormwater But as the pitch increases, so do the
increasingly common in North handling system. Many major cities, challenges. Adequate moisture for plant
America. In the early 2000’s, a number including Chicago, New York, Toronto, growth becomes an issue, as rainfall
of major green roof projects were and Portland, Ore., offer financial runs off the roof instead of staying in the
completed—including Chicago’s City incentives for this reason alone. growth medium.
Hall, Salt Lake City’s five-acre Conference Finally, studies in Europe show that “Practically speaking, soil acts like a
Center and Ford’s 500,000 sq. ft. factory workers that can view a nearby green roof sponge,” explains Amber Ponce, business
roof in Dearborn, Michigan—which put are happier, more productive, absent less development manager at LiveRoof LLC.
the technology in the media spotlight. often, and stay with the company longer. “If a moist sponge is angled upward, water
Since then, nearly a hundred million The building owners, in turn, are able to will run out of it. The same is true of soil;
square feet of green roofs have been charge more for the space. the greater the angle, the less capable
installed in virtually every possible For flat and low-pitch roofs, most the soil is of retaining water. For this
climate zone across the U.S. of the issues involved with designing reason, pitched green roofs will retain
The upfront cost of installing a and installing green roofs have been less stormwater, and they will dry out
green roof is at least double that of resolved. Vegetated roofs can be adapted faster, which means that they will have
other options, but the advantages are to a stunning variety of buildings using an increased reliance on supplemental
significant. The soil and plants on a off-the-shelf components. waterings than flat green roofs.”
green roof protect the membrane from Over the last decade, professionals Plant positioning becomes more
UV decay, temperature fluctuations, and have proven that even sloped roofs can
other destructive elements, so they’ll last be green, thanks to a few innovative
50 years instead of only 15 or 20. solutions.
The growing medium is excellent
insulation, so it leads to an increase in The Challenge
energy efficiency, and sound absorption. In designing a green roof, a very
Green roofs also decrease storm slight slope—less than a 1:12 pitch—is
runoff. The Ford truck manufacturing advantageous. Drainage is better, water
Photo Courtesy ZinCo USA/Nicholas Smith
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2. Photos Courtesy ZinCo USA/Nicholas Smith
important, as south-facing slopes receive According to ZinCo, a green roof roofs, safety is critical and harnessing
far more daylight (and heat) than those supplier, a root-resistant waterproofing may be required.”
on the north side of the roof. Similarly, barrier is absolutely essential. That’s The International Green Roof
plants on the ridgeline receive less water because installing an independent root Association (IGRA) has developed
than those on the eaves, so species barrier is often difficult or impossible guidelines to help determine how much
must be chosen and placed carefully. since they’re so slippery. Construction, additional engineering is needed on
Vegetation must also be fast growing, so maintenance, and roof access is more sloped surfaces.
it will cover the roof within a few months difficult. IGRA states that “traditional green
of installation. “Almost always, construction must roof components can generally be safely
The biggest challenge is dealing start at the low end of the slope and work and effectively used on roof slopes up
with the enormous sheer forces. Soil its way up to account for compression of to 10 degrees [about 2:12].” Beyond
retention systems become mandatory, the green roof components,” says Ponce. that, installation will be significantly
Eaves require structural support and soil “Contractors must use care to plan for different from a traditional green roof
retention boards. In some cases, plant access in such a manner that minimizes and requires a different approach.
anchors have to be installed to keep trees or eliminates walking on the plants Dennis Yanez, national marketing
and bushes in place. during installation. As will other pitched manager at American Hydrotech, Inc.
Summer 2011 21

3. Photos Courtesy LiveRoof LLC/Amber Ponce
To retrofit a sloped green roof onto the aging community center in North London, Ontario, Canada, designers turned
to Liveroof LLC. Installed on a 4:12 pitch (left) the system consists of interlocking trays. Modules are delivered to the
jobsite fully grown with mature plants (right) ensuring all of the benefits of a green roof are available from Day One.
says “Anything over 2:12 pitch is going plants will receive less water, especially foliage shades the soil, less moistures is
to require special engineering. Anything along the ridgeline. lost to evaporation. At least a half-dozen
over a 3:12 pitch will require specialized Solution 1: Retain more water. companies offer pre-planted trays, sacks,
systems and products.” This can be addressed by simply using or mats. The LiveRoof Hybrid System
Although establishing a pitched a deeper soil mix on the roof, or at least comes with full-grown, mature plants to
green roof can be more challenging on the ridges. If the weight exceeds the create a dense and seamless vegetative
than installing a flat roof, dozens of maximum roof load, perhaps a more surface the day the system is installed.
sloped green roofs have been installed robust water retention layer (e.g. one Solution 4: Irrigate. A third
successfully across North America. They with a deeper dimple) would solve the solution—perhaps the only one in dry
just require a little more planning. problem. climates—is to install a drip irrigation
Solution 2: Select hardier plants. system that distributes more moisture
Solutions It may be possible to resolve the water along the upper edges of the roof.
Solutions for all of these challenges issue by planting vegetation that can
are readily available. Below are some tolerate drier conditions on roof peaks, Soil Retention: This is perhaps the
options for dealing with the two while reserving species that need more biggest challenge for sloped green
biggest issues. The solution chosen will moisture for positions along the eaves. roof construction, as sheer forces will
likely be determined by the roof pitch. Solution 3: Use vegetated mats. try to pull the membranes, substrate,
Mature plants with a fully established root and plants off the roof. Fortunately,
Water: In a sloped environment, water system will be able to better access the manufacturers have created a variety of
will flow downhill relatively quickly, so water that’s available. Also, because their products to address it.
Soils on steep green roofs are easily stabilized with ZinCo USA’s Georaster, even on slopes as steep as 5:12 (30°) such
as these pictured below on the Vancouver Convention Center
Photos Courtesy ZinCo USA/Nicholas Smith
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4. Photo Courtesy American Hydrotech/Dennis Yanez
Lincoln Center, the world’s leading performing arts center, is home of The Metropolitan Opera, The New York City Ballet,
The New York Philharmonic Orchestra, and the Julliard School. Since 2004, it’s also home to one of the nation’s most
innovative green roofs. Designers wanted a lush, tilting lawn that curved up on two sides, and could withstand public
access. To keep the vegetation and growing media from sliding off the roof, they used GardNet from American Hydrotech,
a product originally developed for the U.S. Military during the first Gulf War in 1991. The new green roof lawn,
affectionately nicknamed “Green on the Tavern” is currently open to the public on the North Plaza of the Lincoln Center.
Solution 1: For low to moderate Ponce explains, “The low side parapet the soil in place.
pitches. For pitches between 2:12 and must be built to have the structural ZinCo’s Floraset has been used to
3:12 (10-15%), jute or burlap over the integrity to resist whatever forces exist. create sloped green roofs as steep as 25o
growing medium may be adequate. Depending on the steepness and the (5:12). The product is basically a rigid,
Colbond’s Enkamat can also be used length of the run, a structural parapet dimpled EPS foam board with each
beneath the soil to stabilize these may not be sufficient to contain the force dimple or tooth measuring a full three
moderate slopes. Made of a tangled, of the green roof. For these roofs, it may inches tall. It’s best for shallow extensive
nonwoven mass of polypropylene fibers be appropriate to incorporate ‘stops’ green roofs where the EPS “teeth” are
up to two centimeters thick, this 3-D mat or buttresses in the design in order to nearly the same height as the growing
is quickly infiltrated by roots, and locks prevent all of the load from being exerted medium.
soil into place. against the eave. Care must be exercised Some pre-grown plant mats and
Solution 2: For moderate pitches. in the design and the construction of grids, such as LiveRoof, Green Grid,
Beyond a 3:12 pitch, specialized systems these stops to minimize penetrations in and Green Living Panels made by Green
are needed. The sheer forces are just too the waterproofing system.” Living Technologies (GLT), are suitable
extreme. Various systems can be used to hold for moderate pitches as well. These
The 15,000 sq. ft. sloping green roof on the MacAllen building in south Boston, Mass., posed serious installation
challenges. When the 12-story condominium was completed in 2007, it could claim the steepest sloping green roof in
the United States. Once again, Gardnet was used to stabilize the soil. The innovative roof contributed to the building
earning LEED-Gold status.
Photo Courtesy American Hydrotech/Dennis Yanez
Summer 2011 23

5. systems are plastic or metal “trays” that Perhaps the most well-know is tons. But because of the unique shape,
are prefilled with growing media, and GardNet, sold by American Hydrotech. the weight on one side of the roof
often pre-grown as well. The trays serve It’s made of perforated polyethylene counteracts the weight on the other. No
as excellent soil retention, and they solve strips, fastened together to form an offset fixed anchors were needed, despite the
water retention issues as well. With the grid. It’s available in standard depths of fact that the roof pitch reaches 43o at the
GLT panels, water capacity is adjustable 3 to 12 inches. Each section, measuring 8 eaves.
via GLT water retention mats. feet by 20 feet, is secured with a network Presto’s Geoweb, the “original
If the roof is fairly steep, the trays of steel cables, which are fastened to cellular soil stabilization system”
must be secured to the roof deck, but they the roof ’s framework. It has been used designed more than 30 years ago for
have some advantages no other system on a number of high profile projects, road construction, has also been used on
offers. It’s easier to do maintenance, for including the convention center in green roofs.
instance, when the trays can simply be Vancouver, Canada, the Lincoln Center
lifted out of place. The vegetation can in New York City, and the MacAllen Conclusion
easily be moved or rearranged. Building in Boston, Mass. The Boston “Part of the decision-making process
LiveRoof trays were used to create project has sections of its green roof will depend upon cost and availability,”
a sloped green roof on the London approaching 45 degrees (12:12). says Robie Anson, a New York City-
Community Centre in Ontario, Canada. ZinCo’s Georaster is another soil based green roof enthusiast. “The
The 1,750 sq. ft. project has a 22o slope. retention system with outstanding availability of proprietary components
Solution 3: For Steep Slopes: proven results. Made from rigid high- will, to an extent, influence the use of
When roof pitches exceed 6:12 (26o), density polyethylene (HDPE), it comes nonproprietary components.”
engineering becomes extreme. Sheer as a bundle of 20”x20” tiles, which Whether it’s a gentle 2:12 pitch in
forces are so great that eave and parapet are snapped together and structurally the suburbs or an extreme 12:12 pitch
reinforcement is no longer adequate. attached to the roof. The five-inch- spilling down the side of a downtown
Installers must use some other system high tiles are then filled or covered skyscraper, experts are available to make
to hold the growth media and plants in with growing medium. Like GardNet, the dream a reality.
place that transfers the load onto the Georaster has been used successfully on “The bottom line is that with careful
structural roof deck. slopes up to 45o. planning it is possible to successfully
At least four companies market Roofmeadow’s slope stabilization install a pitched green roof,” Anson says
soil-retention products specifically for system is another option. This product “It is crucial, however, that builders make
steeply sloped green roofs. They work by was used on the barrel roof of the Testa considerate design choices and source
dividing the roof area into smaller cells, Produce building in Chicago, Ill. The appropriate materials.” n
usually about a foot square, which are 4-inch thick growing medium covers
structurally anchored onto the roof deck. 7,500 sq. ft and weighs more than 90
The 7,500 sq. ft. barrel roof on this building reaches a maximum pitch of 43o, yet required no fixed anchors, thanks to
an innovative soil stabilization system from Roofmeadow. Owned by Testa Produce and located on the outskirts of
Chicago, the soil was covered with pre-grown reinforced sedum mats. Once a brownfield site, the building is now
LEED Platinum making it the first industrial building to achieve that status. The facility also includes a wind turbine, a
solar hot water system, recycled concrete, LED lighting, and a 30,000 sq. ft. flat green roof.
Photos Courtesy: Roofmeadow.com
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