Green roofs have been prominent for many years in Europe, and are rapidly gaining popularity in North America. The term “green roof” is used to describe a roof that is substantially covered by vegetation. This covering provides many environmental and economic benefits characteristic of sustainable building principles. Green roofs help combat the urban “heat island” effect and detain stormwater. The shading and evapotranspirative cooling value of the vegetative cover provided by green roofs can reduce building energy consumption and protect the roof surface, thereby extending its useful life. Many environmentally responsible building owners have capitalized on the benefits of green roofs and have begun to enhance the urban landscape.

Conventional built-in-place green roof systems have certain inherent limitations, such as the complicated engineering and logistics associated with their installation. Additionally, built-in-place green roofs may be heavy and therefore limited to roofs with adequate structural capacity. Recent developments in modular green roof technology promise to address these limitations and reduce project costs by simplifying the green roof system, while maintaining the environmental benefits. There are a growing number of projects utilizing innovative, modular green roof systems to meet sustainability objectives.

Building owners in Europe understand why green roofs are smart business. In Germany, for example, 7 percent of all newly constructed flat roofs are green. As a result, Germany now has 140,000,000 square feet of green roofs.1 Green roofs in Germany typically come with warranties of up to 30 years,2 testifying to their durability and reducing capital costs associated with roof repair.

Adoption of high performance building systems and features in North America is a rapidly growing trend. The Department of Defense and General Services Administration (GSA) mandate that all new government buildings follow green building protocols developed by the U.S. Green Building Council (USGBC). Green roofs directly integrate both the economical and environmental principles of sustainability that organizations such as the USGBC and GSA are actively pursuing.

Major cities have also recognized the benefits of green roofs and have created economic incentives to promote their continued use. The city of Tokyo for instance, mandates that new construction with a roof area greater than 10,000 square feet include cultivation on at least 20 percent of that area.1 Chicago, Seattle, Portland, Ore., and the state of Maryland have established requirements for incorporating green building standards into new construction projects as well.

Benefits of Greening Your Rooftop

A GreenGrid™ modular green roof system installed on a Chicago area rooftop in less than one week.

A green roof, while beautiful, provides a building owner with a number of important benefits that can decrease the operating costs and environmental burden created by a building. The cooling and shading properties of a green roof can significantly limit the heat flow into a building, thus lowering the load placed on air conditioning equipment and reducing energy consumption. Similarly, due to cooling and shading, green roofs lower rooftop temperatures from 130 degrees Fahrenheit or greater to approximately 75 F during a summer day. Less heat is radiated to the surrounding environment, helping to reduce the urban heat island effect. Additionally, scientific studies have empirically shown that green roofs are highly effective at managing stormwater discharge from a roof, thereby protecting sensitive watersheds and potentially reducing costs of drainage basins and other stormwater management structures.

On a qualitative level, green roofs can reduce the levels of indoor sound by as much as 40 decibels, making for a healthier work or living environment. Green roofs installed on schools or universities also have the ability to offer students a unique, living laboratory in which various research projects and education can be conducted. Another noteworthy benefit of green roofs is improved air quality. Smog forming chemicals such as oxides of nitrogen and sulfur, as well as carbon dioxide are taken up directly by plants, lowering their concentrations in the air.

Green roofs contribute to a building’s energy efficiency by shading the roof, and by providing evaporative cooling via transpiration. The combined effect substantially reduces summer time roof temperatures as compared to conventional roofs and lowers energy demands in the building areas directly below a green roof. A study conducted by the National Research Council Canada found that an extensive (4-inch soil depth) green roof installed on a low slope roof facility reduced average daily energy demand for space conditioning by 75 percent.3 The effect was most noticeable during the summer months.

The reduced cooling demand associated with green roofs is due to their ability to maintain surface temperatures of approximately 75 F during a typical 95 F day, while asphalt roofs can reach temperatures of 150 F or higher. The lower roof surface temperatures translate into lower surrounding area temperatures, reduced energy demand, and extended roof life.

Green roofs are a sustainable building technology that has immediate visibility in the community and can be universally employed to combat heat islands and improve air quality in urban areas. They moderate the urban warming effects that exist due to pavement, structures, and dark surfaces that absorb heat during the day and release it at night. Environment Canada, Canada’s natural resource protection agency, found that greening only 6 percent of a city’s buildings would reduce summer urban temperatures by 1 to 2 degrees Celsius. Resulting summertime energy usage would be reduced by 5 percent to 10 percent, due to reductions in the demand for cooling. With only a 1 C decrease in temperatures, energy savings would reach an estimated $650,000 per year, while approximately 2 tons of greenhouse gas emissions and 30 tons of other pollutants would be prevented or removed from the air, both measured as CO2.4

The stormwater benefits of a green roof are significant. Green roofs can detain and use 50 to 75 percent of a typical 1-inch (2.5cm) rainfall event. When combined with water reuse systems, excess stormwater runoff can be eliminated. For larger storm events, green roofs moderate the amount and rate of excess runoff from the roof. The dampening effect whereby excess runoff is reduced and delayed minimizes impacts to sewer systems, preventing subsequent releases of untreated water to surface water bodies.

Green roofs are able to reduce the noise entering a building due to the insulating buffer provided by the plants and the soil media. Even in a system with only 4 inches of soil sound entering a building can be reduced by up to 40 decibels as previously mentioned. For comparison, a 30-decibel reduction is equivalent to the difference between the sound of a lawnmower (90 dB) and that of a normal conversation (60 dB).

The potential educational and research benefits of green roofs are noteworthy. Because the amount of detailed data on the effects of green roofs in North America is less than that found in Europe, there are many opportunities for university level research. Primary and secondary schools could also use an accessible green roof for horticulture experiments, biodiversity studies, or habitat creation.

Conventional Green Roof Technologies

Most conventional green roof systems utilize a series of components that must be installed in layers on a roof surface. These have been referred to as “built-in-place” green roofs in this article. Their primary drawbacks include complicated installation logistics, excess weight, and lengthy installation schedules.

The number and types of materials that must be mobilized to the roof complicate the installation of built-in-place systems. These materials need to be placed sequentially and plants must be installed at the correct times and tended to ensure growth. Construction time frames are typically measured in months and roof access must be carefully planned, especially in urban areas. Unless larger, more expensive nursery stock is used, the green roof installation may require one to two growing seasons before becoming “green”.

The layers of components used in a built-in-place green roof system tend to be heavy (approximately 40 to 100 lbs/ft2). Retrofitting an existing roof may not be feasible if the structural capacity of the roof is not adequate. Supplemental irrigation systems can be problematic and must be designed for site-specific requirements. Repair and maintenance of the underlying roof can be cumbersome and costly since peeling back or replacing large sections of green roof may be required. For the reasons discussed above, built-in-place green roofs can be more complicated to install and remove than conventional roofs, discouraging their construction in the United States.

As a result, modular green roof technologies are rapidly gaining popularity. Modular systems retain all the benefits of the green roof while addressing the limitations of a built-in-place system.

The Modular Green Roof Solution

The most notable benefits of using a modular approach to green roof construction are simplicity and flexibility. Pre-planted modules are commercially available in various sizes that are easily installed in a wide array of configurations. Plants are established and mature in the module prior to placement on the roof. The plantings are chosen to be wind and drought resistant and modules can be rotated seasonally if desired. Overall, plant survivorship is improved and the owner is provided with a green roof that has a very high degree of customization.

The pre-planted modules are mobilized on racks or palletized for transportation and brought to the roof intact via elevators, hoists, or cranes. The modules are then placed according to the design; although, the layout of the roof can be changed anytime during installation (or afterwards) to avoid construction delays. The installation of the modular green roof can be completed in a matter of days or weeks and modules can be installed at nearly any time during the year because plants are pre-established. This design flexibility and installation efficiency help to further lower the cost of a modular green roof. It also allows an owner to modify the layout or add other modular features such as walkways, benches, etc. at any time in the future.

Using modular technology also eliminates difficulties associated with accessing a green roof’s waterproofing deck. If the deck needs to be maintained or repaired, only the modules immediately above the affected area need to be moved. The plantings themselves will not be disturbed. Additionally, because green roofs protect the underlying deck from thermal shock and other damage, the longevity of the waterproofing membrane is extended. Depending on the waterproofing membrane and green roof products selected, an extended warranty may be available for the roof membrane.

As states and communities push for increased energy efficiency, cleaner air and water, and more sustainable building technologies, building owners stand to make a dramatic contribution to the success of these initiatives. In urban areas and areas of concentrated development, even small improvements in energy efficiency or decreases in air and water pollution, widely applied, can have an immediate and significant impact on quality of life in the built environment.

Increasing energy efficiency, minimizing watershed impacts, lowering urban temperatures and improving air quality are significant challenges. Green roofs offer a wide range of environmental benefits that make good business sense. Credits and rebates offered by some states and utilities provide even more incentive by reducing capital costs. Research suggests that green roofs may be one of the fastest and most cost effective means of improving environmental quality and contributing to a large-scale transformation in the urban environment.

FOOTNOTES

GreenGrid and ABC Supply Co., Inc., are trademarks of American Builders & Contractors Supply Co., Inc. GreenGrid System is a proprietary technology of ABC Supply. U.S. and International patents pending. Weston is the licensee of the GreenGrid System.

1. “Plant-Covered Roofs Ease Urban Heat” by Donald Dawson, National Geographic News, Nov. 15, 2002.

2. “Green roofs in temperate climates and in the hot-humid tropics – far beyond the aesthetics” Kohler, et. al. Environmental Management and Health, Vol. 13 No. 4, 2002, pp. 382-391.

3. “A National Research Council Canada study evaluates green roof systems’ thermal performances” by Karen Liu, Ph.D., Professional Roofing Magazine online.

4. “NRC Launches Green Roof Demonstration Project” by Brian Burton, Landscaping and Groundskeeping Journal online.

Side bar: Why Green Roofs are Smart Business

Cost Effective

Lowers roof surface maintenance and replacement costs
Dramatically increases roof life by providing a protective layer over the waterproofing surface
Protects roof membrane from UV, thermal shock, and physical damage
Reduces mechanical equipment requirements through energy efficiency
Potential incentives and tax credits exist for installation
Enhances property value

Environmentally Responsible

Consistent with U.S. Green Building Council’s LEED system
Combats urban heat island effect
Provides natural habitat
Some systems use recycled materials for construction
Produces oxygen
Adds green space
Absorbs sound rather than reflecting it

Optimizes Stormwater Management

Retains 50 to 95 precent of rainfall
Filters atmospheric dust/particulates from excess runoff
May minimize or replace need for detention basins

Improves Air Quality

Removes harmful gasses in air – relieves smog
Absorbs carbon dioxide and replaces oxygen in the atmosphere
Foliage slows air flow to allow deposition of dust and particulates
Creates cooling effect on downwind areas

Energy Efficient

Absorbs less solar heat
Improves energy efficiency of building area directly below it
Lowers overall heating and cooling requirements
Lowers energy consumption, meaning smaller mechanical equipment and lower utility bills