Sourced from the Colorado Real Estate Journal
Denver has joined North American cities San Francisco and Toronto in enacting a regulatory mandate for green roofs. Green roof incentive programs have been used successfully for years in cities such as Chicago, New York and Washington, D.C., but the mandate is the most efficient way to get Denver to become a more resilient and sustainable city. Questions remain on the effects of the Denver Green Roof Initiative, and this article addresses some of common areas of concern for green roofs while highlighting some of the benefits. The benefits of the green roof initiative were quantified in a study by The Green Infrastructure Foundation and Green Roofs for Healthy Cities and reported that Denver will receive $1.85 billion in savings by 2058.
Higher costs. Green roofs have higher installation and initial maintenance costs than conventional roofs. However, over the roof’s full life cycle, studies have shown that green roofs can offset these higher costs with a return on investment of six to eight years. Part of this offset is due to the fact that green roofs’ operational membrane has a 40- to 50-year lifespan, as compared to 15 to 20 years for conventional roofs. Green roofs have a comparatively long lifetime because the waterproofing membrane is protected from damaging elements like ultraviolet radiation and foot traffic.
Green roofs further offset costs by reducing the energy needed for heating and cooling. The effect is most pronounced in the summer, during which evapotranspiration from the green roof keeps the rooftop temperature close to ambient temperature compared with typical rooftops, which can be 90 degrees Fahrenheit hotter. The cumulative effect of cooling rooftops is what helps mitigate the urban heat island effect.
There also are significant cost savings associated with storm-water management. Green roofs have been shown to detain the first inch of storm water, and slow down peak flow to city sewer systems. When the Environmental Protection Agency’s 20,000-square-foot green roof in Denver was built, the agency was allowed to eliminate a storm-water detention vault, and then use the additional space as salable parking spaces. Based on average costs and these savings, the green roof paid for itself the day it was installed, according to a report from the U.S. General Services Administration.