Quickly, without checking: Do you know how hot your roof is? If you don’t know, odds are that it’s costing you valuable space and resources.
Green roofing is exactly what it sounds like: It’s the use of living plants and soil on top of commercial and multi-tenant buildings to improve insulation, better manage stormwater, and extend the life of the underlying roof. The up front costs are high, but over the medium- to long-run, green roofing is often a savvy financial decision.
The high concentration of flat, dark surfaces in cities makes for a unique environment: one that is especially prone to high temperatures. In the summer, roads and black-topped buildings absorb sunlight and radiate that heat back into the surrounding area, including into the buildings themselves in the latter case. As a result, temperatures in cities tend to be higher than in nearby rural areas in an effect known as “urban heat island”.
Urban heat island is enough to raise temperatures by several degrees in some cities, affecting residents’ health, increasing the amount of energy used for air conditioning, and hurting local wildlife. Research indicates that the effect may also have an indirect impact on global climate change via nearby water systems.
From a financial perspective, urban heat island effect results in significantly higher air conditioning costs. In 2009, the Heat Island Group estimated that the effect was costing ratepayers in Los Angeles roughly $100M per year.
Stormwater drainage is another major city planning issue. Rainwater that would normally sink into the ground has to be drained in an urban environment or it will flood streets and damage property. Worse still, in many U.S. cities, heavy rains can cause untreated sewage to drain into waterways via emergency relief valves.
“It happens anytime you get a hard rainfall,” Bob Connaughton, a Brooklyn water treatment plant engineers, told The New York Times. “Sometimes all it takes is 20 minutes of rain, and you’ve got overflows across Brooklyn.”
This sewage inevitably finds its way into human environments, resulting in illnesses and additional property damage. As a result, cities are often highly motivated to find cost-effective ways to reduce the strain on their stormwater drainage systems.
Green roofs are an effective solution to both urban heat island effect and stormwater drainage problems.
In the summer, the plants on top of a green roof cool the surrounding area through evapotranspiration. This is the process by which they pull water up through their root systems and allow it to evaporate through their leaves.
Research conducted by the National Research Council Canada found that a green roof consistently kept the underlying roof membrane below 86°F, whereas a nearby patch of exposed roof membrane reached temperatures as high as 158°F. The study concluded that green roofing is an effect means of combatting the urban heat island effect.
Another, more anecdotal experiment conducted by the City of Chicago found that the green roof above city hall was keeping the roof an average of seven degrees cooler than surrounding roofs, reducing noise pollution by approximately 40 decibels, retaining up to 75 percent of rainwater, and significantly cutting the building’s energy bills.
This ability to retain rainwater is what helps green roofs to alleviate demand on city storm drains. Instead of running off the roof and down to the street, rain that falls on green roofs is largely absorbed by the soil. Some of this water is then consumed by the plants, removing it from the equation indefinitely. The rest does eventually find its way into city drains, but at a delay that helps to spread the work that the system has to do over a longer period of time.
Another communal benefit is air quality. Many plants are effective at filtering harmful pollutants out of the air, particularly ozone. A 2008 study in Chicago found that 1675 kilograms of air pollution were removed by less than twenty hectares of green roofing over the course of a year. The study’s authors estimated that more than 2,000 tons of air pollution could be removed from the air if every rooftop in the city were covered with intensive (soil deeper than six inches) green roofing.
“There are a lot of community and environmental benefits [to green roofing] that don’t accrue to the business owner but accrue to the community as a whole,” Rohan Lilauwala, Senior Researcher at Green Roofs for Healthier Cities, told Aquicore. Green Roofs for Healthier Cities is a 501(c)(6) industry association that advocates for and promotes awareness about green roofs.
Unfortunately, while many of the benefits of green roofing are communal, the costs are generally private. Lilauwala said that, in his opinion, the most compelling benefit from a business perspective is the way that green roofs extend the life of the underlying roof membrane. A roof outfitted with soil and plants can last for 40 years or longer, whereas the average traditional roof lasts about 17 to 20 years because of the more extreme temperature fluctuations that it experiences. This alone does a lot to justify the larger initial outlay of capital needed for installing a green roof.
Green roofs also better insulate a building, reducing heating costs in the winter and cooling costs in the summer. A University of Michigan study compared the cost of a 21,000-square-foot green roof with that of a traditional roof of the same size. It found that the green roof cost an additional $129,000 (in 2006 dollars) but saved about $200,000 over the life of the building. Almost two-thirds of the savings came from reduced energy costs.
Some financial benefits are difficult to see. Lilauwala explained that, in some cases, a green roof accessible to residents can result in higher property values. There is even research to suggest that office workers and hospital patients who can see a green roof from their window are more productive and experience shorter recovery times, respectively.
Green roofing can be especially appealing when city governments work with the private sector to align environmental and business interests. Several major cities offer incentives for green roof construction because they view them as a cost-effective way of mitigating the strain on their storm drainage systems. Others have enacted stormwater management requirements that can be met by including a green roof.
DC has one of the most generous initiatives, offering $10 per square foot of green roofing. This compares favorably with Lilauwala’s estimate of the marginal cost of installing an extensive green roof: $15 to $20 above the cost of a traditional roof per square foot. An intensive (more than six inches of soil) green roof is more costly.
“DC is on the cutting edge of stormwater management,” Lilauwala said. “I think that you’ll see more than more [of this] as cities see the benefits of these programs.”
Asked about payback periods, Lilauwala pointed to General Services Administration research estimating that extensive green roofs pay for themselves over an average of 6.2 years.