The Role of Green Roofs in Future American Urban Forests
“Work began Tuesday on reinforcing the 20,000-square-foot roof of Chicago’s 11 story City Hall to prepare it for the planting of two oak trees and more than 21,000 plants and shrubs this spring as part of a five-city pilot project… The project is aimed at controlling heat by restoring vegetation that once helped keep temperatures and air pollution at tolerable levels before overdevelopment.”
-Washington Post December 4, 1999 (Claiborne, 1999)
Introduction
Rooftops are one of a city’s greatest untapped resources – acres and acres of empty space just waiting to be used. Imagine driving through a city’s industrial parks and looking out over a sea of green instead of the tar, asphalt, and gravel that now exists. Imagine looking out of your downtown office window and seeing a meadow of indigenous wildflowers instead of HVAC systems and roof vents. This vision is not as far-fetched as it may sound, and the benefits of achieving this goal will be far-reaching. Green roofs are becoming fairly common in parts of Europe, where sprawl is no longer possible, or desired, and where higher population densities have made environmental conservation that much more important, roof greening has been adopted as much out of necessity as from a wish to beautify the urban landscape. With direct financial investment and regulations requiring green roofs by over seventy local governments, the market for green roof infrastructure in Germany has grown by an average of 10-15% annually over the past decade. In 1999, there were 50 million square meters of green roof infrastructure installed in Germany. This number represents over 10% of the flat roofed buildings in Germany (City Farmer, 2000).
Green roofs - variously known as eco-roofs, extensive roof gardens (to distinguish them from the more familiar intensive roof gardens) – typically cover the entire roof of a building with a continuous thin growing medium that supports low vegetation. Depending on the climate and the amount of rainfall, this may include a variety of hardy grasses, wildflowers, mosses, and sedums in a soil layer as thin as 3" (8cm). In concept, green roofs are lightweight modern versions of the sod roofs that are a centuries-old tradition in Scandinavia. Because of their lightweight green roofs require little additional load-bearing capacity from a building’s structural systems; in many cases they may be installed on existing buildings with no structural modification. Unlike conventional roof gardens, they do not require flat roofs and can be installed on roofs with slopes of up to thirty degrees, if provided with a raised grid structure to hold the growing medium in place. They are not intended to be walked upon and generally do not feature pedestrian areas. In contrast to conventional roof gardens planted with garden-variety plants that require irrigation systems, fertilization, and frequent maintenance, green roofs typically require little or no irrigation (at least in temperate climates) or fertilizer.
While the requirements of a green roof are relatively modest, the environmental benefits are considerable. These include improving the building’s thermal insulation and reducing the urban heat island effect. The vegetation on green roofs filters city air, absorbing carbon dioxide and helping to reduce air pollution. Green roofs create habitat for birds, butterflies, and other beneficial insects. Finally, they have the capacity to absorb a percentage of the precipitation that falls on them, thereby reducing runoff from impervious surfaces and alleviating potential flooding problems and combined sewer overflows (CSO’s) that plague many cities. Increasing the use of green roofs would also have several positive economic benefits for communities.
The Built Environment
Plants and Building Insulation
The role of insulation and air/vapor barriers is to slow down the rate of heat transfer between the inside and outside of a building, which is a function of the difference between the inside and outside temperatures. Insulation mitigates the impact of this temperature differential. In the winter, insulation slows down the rate of heat transfer to the outside and in the summer it slows down the rate of heat transfer to the inside. The greening of both horizontal and vertical surfaces of buildings has long been used as a technique for insulating buildings through exterior temperature regulation. By covering the building with vegetation, an air layer trapped within the plant mass prevents the summer heat from reaching the building’s envelope, and in the winter the internal heat is prevented from escaping. A plant layer can also act as a buffer that keeps wind from moving along a building’s surface. Wind can decreases the energy efficiency of a building by as much as 50% if the surface is left unprotected to the wind (Peck & Callaghan, 1999).
With a green roof, the insulation value is in both the plants and the growing medium. It is unclear which of the two has the most benefit since much depends on the depth of the growing medium and types of plants chosen. An extensive application is much more effective as an insulator than an intensive one. A layer of mixed grass performs better than a layer of limited-species grass, which in turn is better than a layer of low-growing sedum. An 8” layer of substrate plus an 8-16” layer of thick grass has a combined insulation value of R-20. Under a green roof, indoor temperatures were found to be at least 5-7°F (3-4°C) lower than hot outdoor temperatures between 77°-86°F (25-30°C) (Peck & Callaghan, 1999). In winter, a blanket of snow and fallen plant matter will also increase the building’s insulation. Although the benefit of roof greening as an insulator has been proven, the specific R-values fluctuate depending on the amount of moisture in, and on, the growing medium (during the winter, after continuous rainfall, etc.). This fluctuation occurs to such an extent that German researchers have, as yet, been unable to provide standard and approved insulation ratings for green roof systems. This has not, however, stopped them from promoting the insulation benefits of green roofs.
Green roofs can also play a role in pre-cooling the make-up air that is required by most mechanical ventilation systems. As the outdoor air temperature in the summer is often warmer than the exhausted, internal air it is replacing, the air needs to be pre-cooled before it is allowed into the building. A green roof and strategic planting of specific vegetation to shade the intake valves will lower the air temperature at roof level, thereby reducing the make-up air temperature and demands on air-conditioning equipment, resulting in net energy savings.
Soil, plants, and the trapped layer of air between the plants and the building surface can also be used to insulate for sound. Sound waves produced by machinery, traffic and airplanes can be absorbed, reflected and deflected by this layer. The substrate tends to block lower frequencies while the plants block higher frequencies. Tests have shown that a 5" layer of substrate can reduce sound by 40 dB; 8" can reduce sound by 46 dB (with some reductions as high as 50 dB) (Peck & Callaghan, 1999).
Building Envelope Protection and Life Extension
Green roofs have been proven to protect the roofing membrane against ultra-violet (UV) radiation, extreme temperature fluctuations and puncture or physical damage from recreation or maintenance. For example, a London department store installed a roof membrane under a planting in 1938 and 50 years later the membrane was still in excellent condition. This is in a climate where most flat roofs have an average life span of between 10-15 years (Kuhn, 1996).
Much of the sun’s energy falling upon a typical concrete, asphalt or hard surface is re-radiated as heat. Temperatures can swing from minus 4°-176°F (20-80°C) over the course of a day. On a hot summer day, the temperature of a black roof can easily be 90°F (32°C) hotter than the ambient air, rising to as much as 180°F (82°C) (Claiborne, 1999). Under the cover of a green roof, the variation under the cover is only 18°F (10°C), or less (Miller, 1998). Vegetated roof covers are less effective in winter when the covers freeze. However, in the winter the temperatures under the covers rarely drop below freezing. Covers with thickness greater than 12 in (30 cm) do not experience temperature excursions below 32°F (0°C), even when air temperatures drop below -4°F (-20°C) (Miller, 1998).
Using a layer of vegetation to intercept the sunlight can reduce this heat. Of the sun’s light energy that falls on a tree leaf, 2% is used in photosynthesis, 48% is passed through the leaf and stored in the plant’s water system, 30% is transformed into heat used in transpiration and only 20% is reflected (Peck & Callaghan, 1999). Since a large amount of incident radiation on a plant canopy is used for evapotranspiration, plants on horizontal surfaces are able to regulate wild temperature swings. On a warm sunny day, their absorption of energy lowers the temperature of the shaded surface and regulates humidity while at night and in the winter, they give off energy/heat. This can reduce the amount of sun-energy falling on a hot summer day by up to 90% (Peck & Callaghan, 1999).
A reduction in daily temperature variations to an 18-54°F (10° to 30°C) range ensures less expansion and contraction stress on the roof membrane, which in turn reduces cracking and aging (Kuhn, 1995). The longer life span decreases the need for re-roofing and the amount of waste material bound for landfill, both of which are direct cost savings for the building owner. Reducing building waste also helps to conserve municipal landfill capacity.
Moderation of the Urban Heat Island Effect and Air Quality
The “Urban Heat Island Effect” is a macroclimate caused by the difference in temperatures between a city and the surrounding countryside. This difference is mainly due to the expanse of hard and reflective surfaces in urban areas, which absorb incoming solar radiation and re-radiate it as sensible heat. In the surrounding areas, there is a higher proportion of “greened” surface area, which is able to absorb and transform this radiation into biomass and latent heat. Re-radiated heat, waste heat generated by industry, vehicles, mechanical equipment, and increased levels of air pollution, have combined to raise urban temperature levels up to 12°F (7°C) warmer than their surroundings on warm summer evenings. And if estimates are correct, global warming will exacerbate the Urban Heat Island Effect by raising summer temperatures an additional 9°F (5°C).
Higher urban temperatures increase the instability of the atmosphere, which in turn can increase the chance of rainfall and severe thunderstorms. The city of Cologne, Germany, for example, receives 27% more rainfall than surrounding areas (Peck & Callaghan, 1999). In cities already plagued by overextended stormwater systems and combined sewage overflows, the problems caused by severe rainfall are likely to worsen with global climate change.
When the concrete, stone, glass, and asphalt surfaces of roads, parking lots and buildings are heated during the summer months, vertical thermal air movements are created and the dust and dirt particles found on the ground and in the air are carried and spread. On a hot summer day, a typical insulated gravel-covered roof in middle Europe tends to heat up by 77°F (25°C), to between 140-176°F (60-80°C). This temperature increase means that a vertical column of moving air is created over each roof which, for 1,075 f2 (100m2) of roof surface area, can be moving upwards at 0.5 m/sec. Studies have shown that there is no vertical thermal air movement over grass surfaces. These surfaces will not heat up to more than 77°F (25°C) (Peck & Callaghan, 1999). By changing wind energy into kinetic and heat energy, planted surfaces can also have a significant impact on local wind patterns – thereby reducing the detrimental effects of wind on a building.
Green roofs reduce the amount of energy available for heating, which decreases the tendency towards thermal air movement and also filters the air moving across it. Airborne particulates tend to get trapped in the leaves, branches and stem surface areas of the plants and when it rains, they get washed into the soil/substrate below. Plants also absorb gaseous pollutants through photosynthesis and sequester them in their leaves.
Plant leaves and needles precipitate significant amounts of particulates from the air. One researcher estimates that a street lined with healthy trees can reduce airborne dust particles by as much as 7,000 particles per liter of air (USEPA, 1992). Studies in Germany have shown that treed urban streets have only 10-15% of the total dust particles found on similar streets without trees. In Frankfurt, Germany for example, a street without trees had an air pollution count of 10,000-20,000 dirt particles per liter of air while a treed street in the same neighborhood had an air pollution count of only 3,000 dirt particles per liter of air (Peck & Callaghan, 1999). In addition, some nitrogen oxides (NO and NO2) and airborne ammonia (NH3) can be taken up by foliage, with the nitrogen going to plant use. Plants can also utilize some sulfur dioxide (SO2) and ozone (O3) (USEPA, 1992). Most of the pollution reductions ability of plants is related to the soil, since pollutants are either washed to the ground from the leaf surfaces or fall directly as the result of having collided with plant structures or entering wind eddies caused by vegetation (USEPA, 1992).
Using similar figures, it is assumed that a grass roof with 2,000 m2 of unmowed grass (100 m2 of leaf surface per m2 of roof) could take 4,000 kg of dirt out of the air (2 kg per m2 of roof). However, this estimate is probably high since the lower portion of the grass layer is too dense to be in direct contact with moving air. Even if the figures were cut to one-tenth of what a forest could remove, the grass roof would still take out a significant amount, 0.2 kg of particles per m2 every year (Peck & Callaghan, 1999). This air cleansing quality of green roofs has direct benefits for people who suffer from asthma and other breathing ailments, and directly decreases summer smog and other forms of air pollution. The widespread use of these technologies would also extend the life of all urban infrastructures that are susceptible to damage from air pollution.
Stormwater management
Surface imperviousness represents the imprint of land development on the landscape. It is composed of two primary components – rooftops and transport systems. The relationship between imperviousness and runoff can be demonstrated by comparing the runoff from a storm that produced 1 inch of rain falling on a meadow vs. a parking lot (or rooftop). The total runoff from a one-acre meadow would fill a standard size office to a depth of about two feet (218 cubic feet). The runoff from the parking lot or rooftop would fill both the office and the two offices next to it. The peak discharge, velocity and time of concentration of stormwater runoff also exhibit a striking increase after a meadow is replaced by an impervious surface (Schueler, 1994).
If the marketing strategies for green roof systems used by European firms are any indication, the most significant tangible benefit of green roofs is their ability to retain stormwater. Engineering for urban development has traditionally focused on moving rainwater and melting snow away from buildings and roads as fast as possible. Since much of the surface area in a city is either paved or covered with buildings, precipitation which otherwise would have infiltrated into the ground or been intercepted by vegetation, is diverted from the cities through constructed stormwater systems. For example, according to Toronto and Region Conservation Authority studies, approximately 25% of land area in new subdivisions in Canada are paved and non-porous. The non-porous landscape of urban areas and stormwater engineering dedicated to diverting the highest amount of water possible from the urban area has created a number of problems (Peck & Callaghan, 1999).
- As water runs off of impermeable surfaces, it picks up particulates, pesticides, oil, grease, heavy metals, etc. from roads, lawns, roofs and pavement. In many cities stormwater is the number one cause of water pollution in local rivers.
- As a safety and cost savings measure, many stormwater systems run parallel to a city’s sewage system, overflowing into the sewage system if they cannot handle the volume of water during heavy rainfall or spring runoff. During a storm event, diluted raw sewage is discharged into the local streams and rivers, resulting in beach closures and other negative impacts. Combined sewage overflows is usually a problem in older urban areas.
-
- Expedient removal of runoff from impervious surfaces has led to a drop in local water tables and the base flow of streams and rivers, with up to 95% of natural precipitation being immediately discharged into major bodies of receiving water rather than being absorbed by the ground.
- Runoff’s increased water temperatures, particularly during the summer, negatively impacts aquatic plants and animals and encourages algae blooms.
- Erosion problems due to the turbidity of stormwater and the sheer volume of runoff after a storm require on going investment in infrastructure.
In highly developed urban watersheds, the control of storms larger that the 24-hour, 2-year return-frequency storm may be impractical. However, an evaluation will frequently show that most chronic problems are associated with small storms. In the Philadelphia area for instance, 90% of all rainfall is contributed by storms with 24-hour volumes of 2 in (51mm), or less. Storms of this magnitude occur on average 3 or 4 times annually (PACD, 1998). For storms larger than the 2-year return-frequency storm, the saturated surface infiltration rate becomes increasingly important in controlling peaks. The surface infiltration rate, alone, can be exploited to achieve control of runoff peak rates for the 10-year return-frequency, and larger storms (Miller, 1998).
One approach to solving some of these issues involves the enlargement or expansion of stormwater infrastructure, which can be a costly and disruptive. Many cities are commissioning studies of environmentally friendly, cost-effective alternatives to “end of pipe” solutions that involve building large, temporary storage facilities. Other natural alternatives include the disconnecting of downspouts, increased use of swales adjacent to parking lots, constructed wetlands, rain barrels, cisterns, retention ponds, and required use of porous pavement. These solutions still require some level of inspection and maintenance, and in some cases the cost of additional surface area at grade. In highly impervious and intensively sewered cities, vegetated roof covers offer one of the few practical “at-source” remedies for controlling runoff. Green roofs can provide viable alternatives where, in older urban areas, there is often a lack of suitable land at grade to properly address alternative stormwater management approaches. In recognition of this fact, vegetated roof covers have been included in the Pennsylvania Handbook for Best Management Practices in Developing Areas (PACD, 1998).
By mimicking natural hydrological processes, vegetated roof covers can achieve runoff characteristics that approximate open space conditions. The design of vegetated roof covers achieve specific hydraulic performance objectives through the appropriate selection of three components: 1) subsurface drainage systems, 2) growth media, and 3) vegetation. A wide range of hydrological responses can be achieved by varying saturated infiltration capacity, media thickness, field capacity, porosity, under-drain transmissivity, relief drain spacing (Miller, Pyke, 1999)
Plants are essential elements in the functioning of vegetated roof covers. They intercept and delay rainfall runoff by: 1) capturing and holding precipitation in the plant foliage, 2) absorbing water in the root zone, and 3) slowing the velocity of direct runoff, as rivulets are forced to follow serpentine paths through dense vegetation.
Nuisance flooding and combined sewer overflows are the legacy of past development practices of many older American cities. If sufficiently implemented in an urban area, green roof systems can help to improve stormwater management. Studies in Berlin show that green roofs can absorb 75% of precipitation that falls on them, which translates into an immediate discharge reduction to 25% of normal levels. In general, summer retention rates vary between 70-100% and winter retention between 40-50%, depending on factors such as substrate and vegetation depths, temperature, sun and wind. Runoff that does occur is also stretched out over several hours, thereby helping to reduce the risk of flash flooding and the frequency of combined sewage overflow events. Most of the stormwater is stored by substrate and then taken up by plants, through which it is returned to the atmosphere through evapotranspiration (Peck & Callaghan, 1999).
A grass covered green roof with an 8-16” thick layer of substrate can hold between 4-6” of water. In Toronto and Philadelphia, where the average rainfall event is 1.5”, a green roof could certainly become a viable stormwater management option. A three month long summer study in Toronto showed that an extensive roof with a 3" (8cm) deep vegetation layer produced no runoff, while the soil surface at grade, without planting, produced 42% runoff and a gravel surface produced 68% (Peck & Callaghan, 1999). On a test plot in Oregon, a mixed layer of sedum and grass on only 2” (5cm) of soil retained 90% of all the rain that fell on it, becoming less effective only during continuous and heavy rainfall (Thompson, 1998).
Green roofs not only retain much of the precipitation that falls on them, they also moderate the temperature of the water and act as natural filters for any of the water that happens to runoff. Heavy metals and nutrients carried by the rain end up being bound in the substrate instead of being discharged. Studies show that as much as 95% of cadmium, copper and lead, and 16% of zinc have been taken out of rainwater by green roof systems (Grozeva, 1997).
Preservation of Habitat and Biodiversity
Habitat can be defined as the specific surroundings within which an organism, species or community lives. The surroundings include physical factors such as temperature, moisture, and light together with biological factors such as the presence of food or predator organisms. With ongoing sprawl, buildings, lawns, and pavement are replacing natural habitats such as meadows and wetlands. This causes plants, animals, and insects to adapt, find other locations to live, become extirpated or, in some cases, extinct. Green roofs can be designed as acceptable alternative habitats although they should never be considered as substitutes for natural habitat or as a justification to destroy natural habitat at grade.
Roofs create their own specific microclimate, quite different from surrounding conditions, both around the building and at grade. Depending on the height, orientation and the location of surrounding buildings, the roof is subject to extreme temperature swings (hot during the day and cool at night), with constant exposure to sunlight and wind. This results in a desert-like climate suitable only to specific types of plants (Appendix 1). Although this can be tempered by additional irrigation and greater soil depth, a green roof is closer to an arid or alpine environment than it is to the surrounding environment at grade. This means that designers and installers must have a specialized knowledge of the flora and fauna best suited to these conditions.
In Europe, two types of green roof habitats have been defined and implemented as part of a larger system of wildlife corridors in urban areas. One is a stepping-stone habitat, which connects natural isolated habitat pockets with each other. It is important to remember that this connection can be by air only (nesting and migrating birds, insects, air-borne seeds, etc.) since the height difference prevents most animals and plants from reaching the roofs. The other is an "island" habitat, which remains isolated from habitat at grade. This type of habitat would be home to specific plant varieties whose seeds are not spread by air or over short distances.
Green roofs can also be specifically designed to mimic endangered ecosystems/habitats, including the prairie grasslands of the Midwest United States, and the Great Lakes Region in Canada. In Germany, 20% of all endangered plants are arid/semi-arid grassland plants, conditions specific to an extensive green roof installation. Dryness, heat, frost, and lack of oxygen are rooftop conditions that are very similar to the dry grassland ecosystem in the central U.S and Canada, which have been seriously degraded by fertilizing, irrigation, and other forms of human interference.
Extensive green roofs, because of their lack of human intervention, are more protected and can become home to sensitive plants that can be easily damaged by foot traffic and to bird species that only nest on the ground. Since the soil, on an inaccessible green roof, is also less likely to be disturbed, it becomes a safer habitat for insects as well. The deeper the soil the more insect diversity the green roof will support.
The animals and invertebrates found on a green roof tend to be highly mobile, not only because they have to be able to reach the roof in the first place, but because the varying and intense temperature and moisture levels force them to move from one location to the next. Studies show that butterflies will visit gardens as high as 20 stories high, bees have been found on the 23rd floor; and birds fly up to the 19th floor (City Farmer, 2000).
Economic Benefits
The nature and scale of economic benefits that can be achieved with the implementation of green roof technologies vary by project and jurisdiction, and are shared among building owners, operators, and the general public.
Typical economic benefits and opportunities for building owners that implement green roofs include:
- Increase in the R-values of the roof of the building, resulting in energy cost savings related to space heating and cooling and leading to reductions in greenhouse gas emissions.
- Protection of the roof membrane, which results in a longer material life span, decreased maintenance and associated savings in replacement costs.
- Increase in stormwater management may offset these costs elsewhere in a development by, for example, reducing the need for stormwater management ponds or reducing fees where lot level stormwater user fees apply. Residents with green roofs may receive discounted hook-up fees for connecting to stormwater systems.
- Increase in property values - American and British studies show that good tree cover increase the value of a home by 6-15%. Green roofs offer very similar visual and environmental benefits. Urban beautification will also have an impact on tourism and the way residents and visitors view the city.
- Provision of outdoor amenity space and aesthetic appeal can directly increase the value and marketability of a property.
- Provisions of a business-related function, such as the cooling of water used in industrial processes. The cleansing of wastewater and noise reduction benefits can help offset the additional costs for buildings where wastewater and noise control are issues (Peck & Callaghan, 1999).
Standard cost savings realized through the greening of a roof are often immediate in terms of reduced space heating and cooling costs, but payback periods are typically medium to long-term. Cost savings are, however, difficult to estimate accurately and vary considerably between projects. Installation of a green roof requires an up-front capital investment, especially in retrofit situations. However, this initial expense can be returned through long-term cost savings. If the concept is included at the beginning of the design phase for a new building, a green roof can be installed at little or no extra capital cost. For example, the green roof component of the new administrative building for the Chancellor of Germany was only 0.1% of its total cost (Peck & Callaghan, 1999).
The lifecycle cost would be moderately increased by the maintenance of the garden, but would be decreased by the extended durability and minimized maintenance of the building envelope, as well as the savings in energy costs.
Additional data shows that if the extra load bearing capacity, the railings, the root protection and the greening layers are included from the very beginning, they can end up costing less than 0.5% of the total building cost. A green roof becomes even more viable where the price of land, or the lack of available adjacent land, prevents the creation of garden or green space at grade. Marketing studies have shown that people place a high value on green space. By providing green space, developers, building owners and companies are often more effective at attracting and retaining buyers and tenants and keeping qualified and motivated staff.
The following points summarize some of the major economic benefits for the community at large:
- Job creation in design, growing, manufacturing, installation and ongoing maintenance.
- Increased livability of cities, including overall worker productivity and creativity.
- Various air quality improvements that have a direct impact on human health and well being.
- The ability to retain and treat stormwater runoff, which if sufficient, can help decrease capital and operational expenditures on related urban infrastructure.
- Reductions in operation expenses of publicly owned buildings such as schools, hospitals and offices.
- The benefits of passive and active experiences with nature and vegetation may decrease the need for health care services (Peck & Callaghan, 1999).
Job Creation
The job creation potential of implementing green roof technologies is significant, as has been demonstrated in Europe. The recent growth of the green roof industry in Europe has been remarkable, with an average annual growth in the German green roof industry of between 10-15% over the past decade. With 50 million square meters of green roof infrastructure installed in Germany, in 1999 (City Farmer, 2000), the impact on the market and job opportunities has been experienced by many sectors. Green roof installations can create and enhance the numerous job markets from suppliers and manufacturers of roof membranes, lightweight soils and amendments, and specialized green roof plants, to green roof designers and maintenance companies.
Barriers to Green Roof Technology Diffusion
All new technologies face barriers to market entry such as lack of pilot projects, uncertainties over costs and benefits, and unfamiliarity among users and clients. Even though green roof technology is proven and well established in Europe, barriers to market entry in the U.S. have prevented their widespread diffusion. One of the greatest barriers to increased use of green roofs in U.S. is the lack of knowledge and awareness in policy makers, how-to professionals, researchers, and the general public.
The development of a "green roof industry" in Europe is largely the result of legislation passed in Germany since 1989 requiring new developments to install green roof systems. Based on a 1996 survey by the Zentralverband Gartenbau (ZVG) (Miller, 1998), more than 80 cities in Germany offer incentives to building owners to install green roofs. Thirteen German cities in the ZVG survey provide indirect subsidies in the form of reduced stormwater usage fees for buildings with green roofs and/or by allowing developers to utilize green roofs as mitigation for the development of open space. Twenty-nine German cities in the ZVG survey provide a direct monetary subsidy to developers that install green roofs. Twenty-seven cities in the ZVG survey have established zoning districts that require vegetated covers on flat roofs. In 1998, the city of Mannheim passed a by-law that requires developers to install green roofs on most new and renovated industrial, retail, commercial, and some rooftop parking lots that are greater than 20 m2, are flat or have less than a 10° slope. The by-law provides developers with increased height and density allowances to compensate for the added costs of green roofs. In the U.S., there are virtually no government incentives to support green roof technology diffusion, despite their many proven public benefits.
More information needs to be assembled about the full range of "traditional" and "public" costs and benefits involved with green roofs. The lack of detailed information about benefits is exacerbated by the lack of information about associated costs. Unless green roofs are part of a new project at its initial design stage, they are much more difficult to sell to potential clients. The lack of "full-cost accounting" of externalities, such as the cost of air and stormwater pollution in the market place, is seen as a barrier since market forces alone will not drive the widespread implementation of these technologies that generate important benefits in each of these areas. There also seems to be an unwillingness of decision-makers to enter long-term investments that often yield the greatest degree of long-term public benefit.
A final category of barriers to further adoption of green roof technologies in the United States relates to technical issues and the associated uncertainties that result. Funding for research, unless sponsored by industry stakeholders, has been difficult to access. The multi-disciplinary nature of the subject has often prevented application to specific funding sources. Due to lack of knowledge in the marketplace, designers must constantly "reinvent the wheel" by sourcing off-the-shelf products and making assumptions about load bearing capacity and the compatibility between different layers of material, plants and water. Because the products and materials that are available were not specifically designed for rooftop applications, it is difficult to provide the warranty or guarantee that institutional and commercial clients often request.
Many of these barriers represent fairly standard challenges facing the adoption of new technologies. Fortunately, much of the technical, policy and market-based information on green roofs is available in Europe and can be adapted to the United States. Many of these issues will also greatly benefit from a joint Environmental Protection Agency, Dept. of Energy and City of Chicago project, beginning this spring, which will implement a $1 million green roof demonstration and laboratory on top of Chicago’s City Hall building. The roof will allow research to be conducted on the suitability of different plants, specifically species native to the Chicago 'rooftop' climate. The City will also be working with the Lawrence Berkeley National Laboratory to better understand the potential of green roofs in improving air quality by reducing the urban heat island effect (City Farmer, 2000).
Conclusion
Green roof technologies offer a wide range of social, environmental and economic benefits for building owners, building residents and the general public. These include improving the building’s thermal insulation and reducing the urban heat island effect. The vegetation on green roofs filters city air, absorbing carbon dioxide and helping to reduce air pollution. Green roofs create habitat for birds, butterflies, and other beneficial insects. Finally, they have the capacity to absorb a percentage of the precipitation that falls on them, thereby reducing runoff from impervious surfaces and alleviating potential flooding problems and combined sewer overflows (CSO’s) that plague many cities. Increasing the use of green roofs would also have several positive economic benefits for communities.
These technologies are useful for both urban and suburban applications where they simultaneously address many of the most pressing environmental problems facing these areas. Some of their benefits are well proven and result from all projects; others are project specific by nature. Once established, the green roof will have a noticeable impact on the heat gain and loss of the building beneath it, as well as the humidity, air quality and reflected heat in the surrounding neighborhood. In conjunction with other green installations, the green roof will also play a role in altering the climate of the city as a whole. Although still a relatively new technology in US cities, the proven role of green roofs in Europe indicate the important role that they will, doubtlessly, play in future urban forests of the United States.
And this future is not as far off as it may seem. Green roofs are becoming increasingly more common throughout the U.S. and Canada. Examples can be found in Portland, Oregon, Chicago, Illinois, San Jose, California, Toronto, Ontario, Victoria, British Columbia, and even in Arlington Virginia, where the new Walter Reed Community Center will have a green roof extensive enough to handle 100% or the roof’s normal stormwater runoff. As populations grow and urban in-fill consumes the final remnants of green space in developed areas, alpine and grassland ecosystems supported by green roofs will become important segments of future urban forests, throughout the United States.
