TEXTILES.ORG
By Dhani Narejo
Introduction
The forecast for the North American roofing market is an anticipated worth of $18 billion in 2014(1) with the most rapid gains expected in alternative roofing technologies, including green roofs. This is good news for the geosynthetics industry. While traditional roofs do not use any geosynthetics materials, green roofs use many.
Figure 1 illustrates many functions of geosynthetics in green roofs; 
Figure 2 presents examples of materials used for each function. 
Not all functions in Figure 1 are called upon in all green roof projects.
The actual “build-up” of any given green roof depends on the project-specific objectives as well as on the specific system utilized. Water storage, temperature control, vegetation, aesthetics, land-use, and city ordinances are some of the considerations that go into selecting the design cross section for a particular green roof.
Waterproof membrane
The most important component of any green roof is a high-quality waterproofing membrane. While geomembranes in waste containment projects are known to have a few leaks per acre, the waterproofing membranes found in green roofs are required to be leak-free. In fact, zero leakage is an intrinsic part of most warranties found within the green roof market.
Waterproofing may consist of a liquid-applied membrane, a single-ply sheet membrane, or a build-up system consisting of several layers. The membranes are in one of the three categories: thermoset, thermoplastic, or modified bitumen(2). Liquid-applied membranes, which also include polyurethane and polymeric materials, are most appropriately adhered to the roofing deck. A liquid-applied membrane results in an excellent seal because the material easily spreads in difficult-to-reach areas. Sheet membranes are generally adhered to decks, but are also installed over thermal insulation.
Zero leakage is essential for green roof construction. This is achieved through stringent quality control, attention to detail, and a water impermeability or leak-location test within 24–48 hours after installation. In Germany, where green roof construction has been effectively improved during the last three decades, it is rare to find a membrane leak. When an installation is not of the German standards, a leak-location test can find even pinhole size defects.
Buildup layers
Membranes that are not root-resistant, which include most bituminous-based membranes, also require a root barrier.
Standard tests for root-resistance have been developed in Europe. Root barriers are usually thermoplastic sheets. In some waterproofing systems, these are embedded into the still-liquid surface of the membrane. In other instances, root-barrier sheets are loose laid on top of the finished waterproofing. Thermoplastic membranes, as well as many polyurethane liquid-applied systems, are inherently root resistant and do not require supplemental root barriers.
Green roofs offer little thermal insulation. However, they offer resistance to heat flow into and out of buildings because of their ability to absorb heat, cool roof surfaces by evaporation, and stabilize temperatures at the freezing point. These effects are seasonal in nature and variable in effect.
Building codes and insulation requirements necessitate the use of one or more additional insulation layers in most parts of North America. High-density extruded polystyrene is the material of choice for insulation when it is installed above the waterproofing membrane.
For roofing systems where the insulation is installed under the waterproofing membrane, polyisocyanurate insulation is also common. Insulation material comes in panels in varying thicknesses and is placed on the roof as one or more layers. It is also used to construct lightweight mounds or gradation changes on flat roof surfaces.
The next category of materials in green roofs (refer to Figure 1) is the water management system. Depending on the slope of the green roof, this system can vary greatly.
The essential functions required are drainage, filtration, aeration, and moisture retention. Drainage is the timely disposal of excess water not held within the engineered growth media. Filtration is the retention of the growth media while allowing movement of excess water into the drainage layer. Aeration pertains to the supply of air to the plant root zone. A continuous supply of water to the root zone in the absence of irrigation and precipitation is referred to as moisture retention.
Drainage and aeration functions are often achieved with a polymeric core known as a geonet. A cuspated drainage core is preferred when significant moisture retention is desired in addition to drainage and aeration. A nonwoven needlepunched geotextile is typically used as a filter. The same layer, or another type of nonwoven needlepunched geotextile, is sometimes used also as a moisture retention layer. The geotextile is bonded to the core and the resulting material is installed as a single geocomposite. On flat roofs, drainage/moisture/aeration core and filter/moisture retention geotextiles can be installed separately.
Engineered growth media varies in thickness from 3in. to 24in., depending on the type of green roof. The nature of the growth media is as important to the success of the green roof as the membrane. For roof slopes exceeding a few degrees, a reinforcement material is desired that will prevent downward sliding of the media.
There are many approaches to stabilizing green roof slopes. These include:
- geogrids, with or without cleats
- rigid slope panels and cribbing frames
- battens attached to the slope
- cellular confinement systems
Cellular confinement offers flexibility that can be incorporated into any shape of the roof. Polymeric or stainless steel tendons, which pass through cell walls, are anchored at the top of the slope through a structural connection with the building.
(Photos in the box above show such a system installed on the roof of Lincoln Center in New York City.)
Depending on the type of climate, growth media, and vegetation, an erosion control layer can be installed at the top of the growth media to prevent media loss from wind or rain.
Market growth
If the first Green Roofs for Healthy Cities (GRHC) conference in Chicago in 2003 is taken as the reference point, the North American green roof market was still in its infancy up until 2005.
At that time, Charlie Miller and this author wrote a paper where Figure 3 was presented as a market projection for extensive green roofs(3). 
Only two or three years of data was available at that time and many projects, especially large ones, had too great an influence on the projection and were therefore excluded.
The growth trajectories were to take the shape in this figure based on optimistic, most likely, and pessimistic scenarios. As is often the case with such projections, the expectation was that the actual numbers will fall somewhere between the optimistic and pessimistic projections.
GRHC is an industry association working to promote the development of green roofs in North America. The association has more than 800 corporate and individual members and is a presenting organization for the eighth Cities Alive! conference in Vancouver, B.C., Canada, Nov. 30–Dec. 2 this year. Attendance by more than 1,000 delegates from all over the world is expected.
Every year the association sends a questionnaire to its members to assess the volume of green roofs in North America. This is, by its nature, not a precise exercise because some organizations are not willing to reveal their total shares of the market, thus, not all projects get reported. It is estimated that the actual numbers are about 30% higher than the questionnaire results.
Nevertheless, the numbers published by GRHC are plotted in Figure 4 on the same graph published in Figure 3 as projections in 2005. 
he actual market data, even without considering the under-reporting, are higher than even the optimistic projections from 2005. From 2004 to 2007, the market grew at approximately the same rate as predicted by the optimistic projections. Beginning in 2008, the growth rate is seen to slow down to the most likely projection rate as a result of the recession and housing crisis in the U.S.
The actual size of the green roof market in North America will likely be around 7 million sf in 2010. Assuming that three different geosynthetic materials are used on average, the volume of geosynthetic materials becomes 21 million sf in 2010 alone. This is effortless growth for the geosynthetics industry because the materials used are often the same materials currently available for other applications such as landfills, erosion control, and highways.
The drivers of growth
The factors that influence the growth of the green roof market can be placed into four categories: regulatory, environmental, aesthetic, and economic.
The specific contribution of each factor is hard to quantify. Based on the European experience, regulations are expected to have the highest impact followed by, in the opinion of this author, economic factors. It is often true that each green roof project has some influence of all four factors. With increasing environmental awareness, more and more people are “driven to do good.” This drive, when combined with some help from governing bodies in the coming years in the form of grants and incentives, is often enough to convince a developer to go green.
The regulatory support typically takes the shape of ordinances, bylaws, zoning decisions, and tax incentives. A good example of the regulatory support is the recent passing of Green Roof By Law by the city of Toronto, mandating the use of green roofs based on the size of the development. For example, a building with a gross floor area of 10,000m2, is required to have a green roof on 4,000m2. All new buildings in downtown Toronto must have a green roof
Tax incentives in Chicago have sustained green roof growth for nearly 10 years. In Portland, Ore., all new city-owned buildings are required to have green roof that covers 70% of the roof area. The city offers developers floor area bonuses when they implement a green roof. The cities with the most active green roof market are the ones that have clear regulatory support for green roofs.
Environmental factors are often the drivers of regulatory support. The number one environmental factor helping with the green roof market growth is the increased concern over the combined sewer overflow problem found in nearly every North American city. This problem is a result of centuries-old shared stormwater and sewage systems found in these cities.
Increased development decreases pervious surfaces, which increases the volume of runoff into these overtaxed systems. Green roofs can reduce stormwater runoff from a building by 30%–80%, depending on the system installed and the rain event considered. Some manufacturers that are consistently on the cutting edge of the market have unique tools to accurately estimate the amount of stormwater management a specific assembly will require.
When the ability of a green roof to manage stormwater is combined with porous pavements, stormwater planters, or rain gardens, it is possible to reduce the amount of stormwater runoff to nearly zero during common storm events. Many green roofs are part of the Leadership in Energy & Environmental Design (LEED) green building rating system developed by the U.S. Green Building Council.
Several aspects of green roof design, including stormwater considerations, heat island effect, water efficiency, energy efficiency, vegetable gardens, regional materials, and innovative design, help earn LEED credits.
Environmental factors are addressed by green roofs but their impact is hard to quantify. These include heat island effect, biodiversity, insulation, reduced energy consumption, and improved air quality.
Large industrial and urban areas with asphalt and concrete roofs can be aesthetically unpleasing. An unpleasant view has a negative impact on property values. Conversely, green areas, with trees, grass, and green roofs have higher property values. A green community generally values life more than areas covered entirely by lifeless concrete and asphalt. Green roofs are quickly becoming part of the restoration of large urban areas.
Large industrial and urban areas with asphalt and concrete roofs can be aesthetically unpleasing. An unpleasant view has a negative impact on property values. Conversely, green areas, with trees, grass, and green roofs have higher property values. A green community generally values life more than areas covered entirely by lifeless concrete and asphalt. Green roofs are quickly becoming part of the restoration of large urban areas.
Challenges and obstacles
Obstacles to green roof projects vary in scope and impact. And, of course, there are always naysayers.
Will it really stay green?
Yes, if properly planned, installed, and maintained.
What about maintenance?
Every living thing requires some level of maintenance. Some types of roofs are low maintenance, but there is no such thing as “no maintenance.”
Green roofs are just a big hype!
In Europe, many cities have mandated green roofs for more than 30 years. In the days of the Vikings and Babylonians, earth-covered structures were used for some of the same reasons they are used today.
Green roofs leak!
Not if the proper waterproofing membrane is used. The membrane is not visible but it is the most vital part of a green roof.
They cost too much!
The costs are going down and many cities are offering grants or incentive programs to offset the costs to the consumer.
They catch fire!
This is highly unlikely and there are now industry standards that directly confront this potential issue. But, again, if properly designed and planned this should never be an issue.
Is this building strong enough for a green roof?
Most buildings have some excess structural capacity and can be reinforced to accommodate a green roof. In new construction, the inclusion of a green roof and its benefits normally outweigh the increased structural costs.
And then one hears about a true failure of a green roof project. The popularity and rapid growth of green roofs construction may itself prove a big hurdle in the long-term success of green roofs. There is a profusion in green roof options, methods, materials, designs, and expertise.
Then there’s the enterprising do-it-yourselfer who says, “What could be better than me building my own green roof on my own garage? After all, I’ve had a beautiful garden for the past 10 years and I am a local expert on plants.”
That’s a disaster waiting to happen!
Anyone planning to install a green roof should consult not only with a structural engineer, but also an organization or company with experience in the field. Some companies in the North American market have been installing green roofs for more than 12 years. These are the people you should talk to if you want it done right. Many experienced companies and proven systems have perfect track records of zero problematic installations.
Permits, zoning requirements, and fire codes can also be issues. In some areas, there is actually a reactionary movement that will strive to establish further limits on building green, but this is changing as the benefits of green roofs are realized.
Another challenge is the current economy and general condition of the housing market. Two years, from mid-2008 to mid-2010, may be more a story of lost opportunities than green roof successes.
Many large green roof projects have come from municipalities, school boards, auto companies, and hospitals. And the starting cost of a green roof can be close to $30 per square foot. The roof may pay off with savings in energy efficiency and the life of the roof in the long term, but initial investment can be significant. This can be enough to kill a perfect project if funds are hard to come by.
Weight is a design issue, especially for existing buildings. How to uplift an ugly building with a green roof, when the structure was not designed to take the additional weight? Green roofs can add 30–120lbs per square foot. Before thinking of selecting the plants for the roof, invest in upgrading the structure of the building. This does add to the cost.
Green roofs can require special care, particularly if the growth media and the plants are not selected properly. Make maintenance and care part of the design requirement and planning from the start.
Summary
Green roofs are thriving, as are the many geosynthetic materials that are ideally suited for this relatively new market.
From nonwoven needlepunched geotextile filters to cellular confinement systems, geosynthetic materials are making it possible to design for the drainage, filtration, separation, and reinforcement functions that are so clearly associated with geosynthetics.
Dhani Narejo, Ph.D., P.E., is the owner and senior engineer at Caro Engineering LLC in Conroe, Texas, and is a member of the Editorial Advisory Commitee for Geosynthetics magazine.
His previous article on this subject, “(Geo)Synthetics for green roofs,” appeared in the January/February 2005 issue of GFR (now Geosynthetics).
This article is a follow-up to paper (3) that is cited on page 43 and that was presented at Geo-Frontiers 2005 in Austin, Texas.
For the Lincoln Center project, a geotechnical stability analysis was completed by Caro Engineering. Materials, including cellular confinement system, by American Hydrotech.
Acknowledgements
The author gratefully recognizes review comments and suggestions from Charlie Miller of Greenscapes Inc. and Nathan Griswold of American Hydrotech Inc.
(1) Newswire, Green Roof Market Update, March 4, 2010
(2) Osmundson, T., 1999, Roof Gardens, History, Design, Construction. New York: W. W. Norton and Co.
(3) Miller, C., and Narejo, D., 2005, State of the Green Roof Industry in the United States, Geotechnical Special Publication Number 130-142 & GRI-18, Conference Proceedings of ASCE Geo-Institute Conference Geo-Frontiers 2005, Austin, Texas, January 23-26.