As extreme summer heat continues to affect cities across the United States and Europe, green roofs are being promoted as a way to cool buildings and make urban areas more resilient. However, new research suggests that simply installing a green roof is not enough to guarantee long term benefits.
Researchers studied 46 green roofs across the southeastern United States to examine how well they continued functioning years after installation. Nearly half were no longer operating as intended. Nine had been abandoned, while 12 had been completely removed.
Green roofs are designed with layers of waterproofing, drainage and growing material that allow vegetation to grow on top of buildings. Plants can absorb sunlight, cool the surrounding air through evapotranspiration and provide additional insulation, potentially reducing energy demand.
They can also capture rainfall, reducing the amount of water entering storm drains and helping lower flood risks. Green roofs can provide habitat for birds, butterflies and other insects, making them an increasingly popular form of urban green infrastructure.
Several cities offer incentives to encourage their construction. Philadelphia, for example, provides businesses with a tax credit covering up to 50% of green roof construction costs.
However, researchers found that long-term maintenance is a major challenge. Plants may require watering, fertilisation and replacement, while irrigation systems, drainage layers and invasive vegetation need regular attention. Maintenance costs can range from a few cents to more than $3 per square foot annually.
The study also identified an unexpected factor linked to green roof failure: changes in building ownership. Researchers found that every time ownership changed, the green roof either ended up abandoned or was removed. A new owner may lack information about the roof or may not have the budget or willingness to continue maintaining it.
Researchers say cities should therefore look beyond construction incentives and develop long term maintenance plans, clearly assign ownership responsibilities and provide funding for continued care.
The findings suggest that green roofs can be an effective tool against urban heat, but their environmental benefits depend on keeping the vegetation and infrastructure functioning for years after installation.