Green roofs, developed in Europe in the 1970’s, are becoming increasingly common in North America.  In the early 2000’s, a number of  major green roof projects were completed—including Chicago’s City Hall, Salt Lake City’s five-acre Conference Center and Ford’s 500,000 sq. ft. factory roof in Dearborn, Michigan—which put the technology in the media spotlight.

Since then, nearly a hundred million square feet of green roofs have been installed in virtually every possible climate zone across the U.S.

The upfront cost of installing a green roof is at least double that of other options, but the advantages are significant.   The soil and plants on a green roof protect the membrane from UV decay, temperature fluctuations, and other destructive elements, so they’ll last 50 years instead of only 15 or 20.

The growing medium is excellent insulation, so it leads to an increase in energy efficiency, and sound absorption.

Green roofs also decrease storm runoff.  The Ford truck manufacturing plant mentioned above eliminated the need for a $10 million stormwater handling system.   Many major cities, including Chicago, New York, Toronto, and Portland, Ore., offer financial incentives for this reason alone.

Finally, studies in Europe show that workers that can view a nearby green roof are happier, more productive, absent less often, and stay with the company longer. The building owners, in turn, are able to charge more for the space.

For flat and low-pitch roofs, most of the issues involved with designing and installing green roofs have been resolved.  Vegetated roofs can be adapted to a stunning variety of buildings using off-the-shelf components.

Over the last decade, professionals have proven that even sloped roofs can be green, thanks to a few innovative solutions.

The Challenge

In designing a green roof, a very slight slope—less than a 1:12 pitch—is advantageous. Drainage is better, water doesn’t pool, and leaks are reduced.  But as the pitch increases, so do the challenges.  Adequate moisture for plant growth becomes an issue, as rainfall runs off the roof instead of staying in the growth medium.

“Practically speaking, soil acts like a sponge,” explains Amber Ponce, business development manager at LiveRoof LLC.  “If a moist sponge is angled upward, water will run out of it.  The same is true of soil; the greater the angle, the less capable the soil is of retaining water.  For this reason, pitched green roofs will retain less stormwater, and they will dry out faster, which means that they will have an increased reliance on supplemental waterings than flat green roofs.”

Plant positioning becomes more important, as south-facing slopes receive far more daylight (and heat)  than those on the north side of the roof. Similarly, plants on the ridgeline receive less water than those on the eaves, so species must be chosen and placed carefully.  Vegetation must also be fast growing, so it will cover the roof within a few months of installation.

The biggest challenge is dealing with the enormous sheer forces.  Soil retention systems become mandatory, Eaves require structural support and soil retention boards.  In some cases, plant anchors have to be installed to keep trees and bushes in place.

According to ZinCo, a green roof supplier, a root-resistant waterproofing barrier is absolutely essential.  That’s because installing an independent root barrier is often difficult or impossible since they’re so slippery.  Construction, maintenance, and roof access is more difficult.

“Almost always, construction must start at the low end of the slope and work its way up to account for compression of the green roof components,” says Ponce.  “Contractors must use care to plan for access in such a manner that minimizes or eliminates walking on the plants during installation.  As will other pitched roofs, safety is critical and harnessing may be required.”

The International Green Roof Association (IGRA) has developed guidelines to help determine how much additional engineering is needed on sloped surfaces.

IGRA states that “traditional green roof components can generally be safely and effectively used on roof slopes up to 10 degrees [about 2:12].”  Beyond that, installation will be significantly different from a traditional green roof and requires a different approach.

Dennis Yanez, national marketing manager at American Hydrotech, Inc. says “Anything over 2:12 pitch is going to require special engineering.  Anything over a 3:12 pitch will require specialized systems and products.”

Although establishing a pitched green roof can be more challenging than installing a flat roof, dozens of sloped green roofs have been installed successfully across North America.  They just require a little more planning.

Solutions

Solutions for all of these challenges are readily available.  Below are some options for dealing with the two biggest issues.  The solution chosen will likely be determined by the roof pitch.

Water: In a sloped environment, water will flow downhill relatively quickly, so plants will receive less water, especially along the ridgeline.

Solution 1: Retain more water.  This can be addressed by simply using a deeper soil mix on the roof, or at least on the ridges.  If the weight exceeds the maximum roof load, perhaps a more robust water retention layer (e.g. one with a deeper dimple) would solve the problem.

Solution 2: Select hardier plants. It may be possible to resolve the water issue by planting vegetation that can tolerate drier conditions on roof peaks, while reserving species that need more moisture for positions along the eaves.

Solution 3: Use vegetated mats. Mature plants with a fully established root system will be able to better access the water that’s available.  Also, because their foliage shades the soil, less moistures is lost to evaporation.  At least a half-dozen companies offer pre-planted trays, sacks, or mats. The LiveRoof Hybrid System comes with full-grown, mature plants to create a dense and seamless vegetative surface the day the system is installed.

Solution 4: Irrigate.  A third solution—perhaps the only one in dry climates—is to install a drip irrigation system that distributes more moisture  along the upper edges of the roof.

Soil Retention: This is perhaps the biggest challenge for sloped green roof construction, as sheer forces will try to pull the membranes, substrate, and plants off the roof.   Fortunately, manufacturers have created a variety of products to address it.

Solution 1: For low to moderate pitches. For pitches between 2:12 and 3:12 (10-15%), jute or burlap over the growing medium may be adequate.  Colbond’s Enkamat can also be used beneath the soil to stabilize these moderate slopes.  Made of a tangled, nonwoven mass of polypropylene fibers up to two centimeters thick, this 3-D mat is quickly infiltrated by roots, and locks soil into place.

Solution 2: For moderate pitches.  Beyond a 3:12 pitch, specialized systems are needed.  The sheer forces are just too extreme.

Ponce explains, “The low side parapet must be built to have the structural integrity to resist whatever forces exist.  Depending on the steepness and the length of the run, a structural parapet may not be sufficient to contain the force of the green roof.  For these roofs, it may be appropriate to incorporate ‘stops’ or buttresses in the design in order to prevent all of the load from being exerted against the eave.   Care must be exercised in the design and the construction of these stops to minimize penetrations in the waterproofing system.”

Various systems can be used to hold the soil in place.

ZinCo’s Floraset has been used to create sloped green roofs as steep as 25o (5:12).  The product is basically a rigid, dimpled EPS foam board with each dimple or tooth measuring a full three inches tall.  It’s best for shallow extensive green roofs where the EPS “teeth” are nearly the same height as the growing medium.

Some pre-grown plant mats and grids, such as LiveRoof, Green Grid, and Green Living Panels made by Green Living Technologies (GLT), are suitable for moderate pitches as well.  These systems are plastic or metal “trays” that are prefilled with growing media, and often pre-grown as well.  The trays serve as excellent soil retention, and they solve water retention issues as well.  With the GLT panels, water capacity is adjustable via GLT water retention mats.

If the roof is fairly steep, the trays must be secured to the roof deck, but they have some advantages no other system offers.  It’s easier to do maintenance, for instance, when the trays can simply be lifted out of place.  The vegetation can easily be moved or rearranged.

LiveRoof trays were used to create a sloped green roof on the London Community Centre in Ontario, Canada.  The 1,750 sq. ft. project has a 22o slope.

Solution 3: For Steep Slopes:  When roof pitches exceed 6:12 (26o),  engineering becomes extreme.  Sheer forces are so great that eave and parapet reinforcement is no longer adequate.  Installers must use some other system to hold the growth media and plants in place that transfers the load onto the structural roof deck.

At least four companies market soil-retention products specifically for steeply sloped green roofs.  They work by dividing the roof area into smaller cells, usually about a foot square, which are structurally anchored onto the roof deck.

Perhaps the most well-know is GardNet, sold by American Hydrotech. It’s made of perforated polyethylene strips, fastened together to form an offset grid.  It’s available in standard depths of 3 to 12 inches. Each section, measuring 8 feet by 20 feet, is secured with a network of steel cables, which are fastened to the  roof’s framework.  It has been used on a number of high profile projects, including the convention center in Vancouver, Canada, the Lincoln Center in New York City, and the MacAllen Building in Boston, Mass.  The Boston project has sections of its green roof approaching 45 degrees (12:12).

ZinCo’s Georaster is another soil retention system with outstanding proven results. Made from rigid high-density polyethylene (HDPE), it comes as a bundle of 20”x20” tiles, which are snapped together and structurally attached to the roof.  The five-inch-high tiles are then filled or covered with growing medium.  Like GardNet, Georaster has been used successfully on slopes up to   45o.

Roofmeadow’s slope stabilization system is another option.  This product was used on the barrel roof of the Testa Produce building in Chicago, Ill.  The 4-inch thick growing medium covers 7,500 sq. ft and weighs more than 90 tons.  But because of the unique shape, the weight on one side of the roof counteracts the weight on the other.  No fixed anchors were needed, despite the fact that the roof pitch reaches 43o at the eaves.

Presto’s Geoweb, the “original cellular soil stabilization system” designed more than 30 years ago for road construction, has also been used on green roofs.

Conclusion

“Part of the decision-making process will depend upon cost and availability,” says Robie Anson, a New York City-based green roof enthusiast.  “The availability of proprietary components will, to an extent, influence the use of nonproprietary components.”

Whether it’s a gentle 2:12 pitch in the suburbs or an extreme 12:12 pitch spilling down the side of a downtown skyscraper, experts are available to make the dream a reality.

“The bottom line is that with careful planning it is possible to successfully install a pitched green roof,” Anson says “It is crucial, however, that builders make considerate design choices and source appropriate materials.” n