Top 3 Reasons Why Cities are Mandating Green Roofs

RCS Manhattan Green roofs
October 9, 2022 at 6:00 a.m.

By Cass Jacoby.  

Green roofs are coming to a city near you! But why are they suddenly gaining so much traction? 

With green roof ordinances being passed for new buildings across the U.S., you may be asking yourself, “why?” Sure, green roofs are a pretty way to fill unused space on a roof, but what is it about them that is making cities demand that new projects incorporate them? Far from just a way to add some more greenery to a city, these kinds of roofs actually provide ample benefits to the city itself, like capturing stormwater, cooling the environment, reducing pollutants and supporting biodiversity. It isn’t hard to see why so many cities are adopting the ordinances. There is a significant amount of data demonstrating additional benefits of green roofs, but here are the top reasons why green roofs are more than a growing trend. 

They reduce urban heat island effect 

The urban heat island effect is a result of urbanization, where heat is amplified due to all of the asphalt and concrete that retains and absorbs the heat. This heat often is worse in neighborhoods with fewer green spaces, and according to a research article disproportionately affects older adults, low-income communities and some communities of color.  Thus, a green roof, which uses natural vegetation to reduce the roof’s temperature by absorbing energy from the sun is a logical solution to help cool cities. 

They are an effective solution too!  According to research from climate scientists at NASA’s Goddard Institute for Space Studies (GISS) in New York, green roofs can harness the cooling power of plants to lower the temperatures of cities. Christian Braneon, a climate scientist and co-author of the NASA study, tells Planetizen that cities can use green roofs as one tool to fight climate change. “In the context of more frequent heatwaves and more extreme heat, it’s important to understand how these urban design interventions can be effective.” 

They clean the air 

Green roofs can do more than just reduce the urban heat island effect, they also effectively work to suck carbon out of the air which can help reduce the impacts of climate change. A green roof has the potential of reducing the amount of CO2 that’s released into the atmosphere.  

“Adding vegetation is a way to filter that air and essentially sequester a lot of those pollutants,” Horticulturalist Jennifer Bousselot tells Denver7. “As our population becomes more urbanized, we have to start looking at rooftops as spaces to help make our cities more livable." 
Garden roofs reduce greenhouse emissions, filter toxins from the air and produce oxygen.  

They help with stormwater runoff retention 

Living Roofs reports that green roofs absorb up to 75% of rainfall, thus reducing the runoff dramatically, and lowering the risk of flooding. Depending on the roof garden design, a green roof can typically reduce storm water run-off by 50 to 90%. Additionally, the peak flow volume is greatly reduced and the peak flow period is delayed by as much as four hours, minimizing the impact on existing sewer systems. 

“The more water they have flowing into their massive stormwater infrastructure, the more costly it is,"  Green Roofs for Healthy Cities Steven Peck told Denver7. "So they are incentivizing paying almost 100% of the cost of green roofs on existing buildings throughout New York City.”  

By retaining rainwater, green roofs reduce stormwater runoff and keep it from adding to the overtaxed sewer system, preventing floods, overflows and other water-related issues. 


Green roofs help cities to solve some of their most pressing issues, from the urban heat island effect to stormwater infrastructure issues. With the many benefits that come with green roofs, they are clearly not only good for the environment, but good for the cities as a whole. Everyone benefits from the benefits of a green roof, which is a huge win for both the city and its citizens. It is no wonder that so many cities now offer incentives or even mandate green roofs be included in new construction. 

Have a question? AskARoofer.   

Find your local roofing contractor in the RoofersCoffeeShop® Contractor Directory.  

About Cass  

Cass works as a reporter/writer for RoofersCoffeeShop, AskARoofer and MetalCoffeeShop. When she isn’t writing about roofs, she is putting her Master degree to work writing about movies and dancing with her plants.    

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