Study: Green roofs worth the cost for urban residents

A new study from Reed College, in collaboration with the University of Illinois and Portland State University, explores the benefits of green roofs and how much Portland residents are willing to pay to increase the number of green roofs across the city.

“Countries around the world are investing significant public resources to reduce the impact of stormwater runoff,” explains Amy Ando, professor of agricultural and consumer economics at the University of Illinois and a co-author on the study. “Green roofs are part of that solution because they capture some of the rain that would otherwise end up in sewage systems. Knowing the benefits from investing in green roofs is important for implementing sound public policies.”

The study investigates how much people would pay for benefits that include reduced sewer overflow events (CSOs), reduced urban heat island effect, and increased presence of pollinators like bees and butterflies. Like many cities, extreme rainfall events in Portland can quickly overwhelm old sewer systems and lead to flooding, which impacts water quality as well as transportation and properties.

“Although CSO events have declined dramatically in Portland after a major system upgrade ($1.4 billion “Big Pipe Project”), they still happen,” says Noelwah Netusil, professor of economics at Reed College and lead author on the paper. “Our findings show that survey respondents place the largest value on reducing CSO events further and are willing to support additional funding for this.”

Plant-covered roofs have become a regular sight in Portland, Oregon. The city is a leader in incorporating green infrastructure for stormwater management, including free street trees, rebates for small residential housing footprints, and green roofs.

Since 2018, the city of Portland has required new buildings in the central city with a footprint over 20,000 square feet to have an ecoroof covering 100 percent of the area (minus a few exceptions like solar panels and evacuation routes) to further protect the city from flooding. Most green roofs are concentrated in the city center, covering 1.4 million square feet.

Survey respondents indicated how much they would pay for a green roof to result in a scale of benefits. For example, it would cost more to decrease sewer overflows three times per year rather than two, lower the air temperature by over one degree rather than half a degree, and bring in a significant amount of bees, birds, and butterflies rather than keep pollinators at the same level.

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