Cities are the engines of human society. They hold the majority of the global population, power entire economies, and enable human ingenuity. 

But the furious pace of urban development, combined with stagnant laws and regulations, often leaves cities overcrowded, heavily polluted, and vulnerable to environmental disasters. 

In recent years, a global movement has emerged to make cities more sustainable. Cities are creating more parks, instituting limits on vehicles and other forms of pollution, and pursuing energy efficiency measures for buildings.

One area ripe for sustainable disruption has been building rooftops. Traditional building rooftops — concrete or asphalt surfaces — not only reflect a lack of imagination in urban planning, they also generate a cascade of problems for cities.

“Roof spaces are largely wasted and every time you put a building up and you don’t use that space productively, the public loses and the building owner loses,” Steven Peck, the founder and president of Green Roofs for Healthy Cities, told Global Citizen. 

“When we just put up a traditional roof that just keeps the rain and snow out, we all lose,” he added. “Rainwater is going to run off the roof system and contribute to flooding. These roofs get hot in the summer and heat up the entire city. They’re like a desert. They have no aesthetic value and they don’t contribute to biodiversity.”

Green roofs represent a new frontier of humane and sustainable urban planning. With climate risks looming on the horizon for many cities, their implementation has become essential and many cities are requiring them for new buildings.

Here are six environmental benefits of green roofs. 

1. Cool the Air 

Cities are hotter than the rural areas that surround them. Asphalt roads and concrete buildings radiate heat after soaking up sunlight, and exhaust from vehicles and air conditioners traps additional heat. This is called the “urban heat island effect” and it can make cities several degrees hotter than neighboring areas. 

During heat waves, which are becoming more common as climate change intensifies, the urban heat island effect can have deadly consequences. In fact, hundreds of thousands of people in the US alone are expected to die in mega heat waves in the decades ahead. 

Green roofs can mitigate this phenomenon, making cities more climate resilient, and protecting people most at-risk of heat waves, according to Peck. They do this by replacing dark surfaces with bright vegetation that reflects rather than absorbs sunlight. Plants also undergo a process called evapotranspiration, when they release moisture into the atmosphere, which further cools cities down. 

For people with access to green roofs, plants can also provide shaded relief on sunny days. 

Another way cities can optimize roofs to cool the atmosphere is by either painting them white or applying a cover that can reflect sunlight. 

2. Reduce Energy and Health Care Costs 

The cooling effects of green roofs can also save people money. In the summer, green roofs cool entire buildings down, which reduces the need for air conditioning. 

Since green roofs improve insulation, they also improve heat retention in buildings during the colder months. As a result, green roofs can significantly lower energy costs for a building. Over time, they essentially pay for themselves, according to Katherine Gloede, academic manager of City College of New York’s sustainability in the urban environment program.

Buildings also account for the majority of a city's greenhouse gas emissions. By reducing energy use in buildings, cities end up releasing less carbon dioxide, methane, and other pollutants into the atmosphere. 

Sustainable infrastructure, in general, improves health outcomes in a city by reducing air pollution, lowering temperatures, and promoting exercise, Gloede said. 

“You receive a return on investment,” she said. “When we think about health care costs, by making our air and water cleaner, we reduce the burden on our health care system because people will be less sick.”

3. Prevent Flooding

Cities feature magnificent feats of engineering, but sometimes the systems that help them operate smoothly also have downsides.

During precipitation events, cities rely on drainage systems to prevent flooding, but extreme storms can overwhelm drains and pipelines, resulting in submerged streets. 

“A large part of why floodwater was such an issue during and after Hurricane Harvey in Houston was because there wasn’t anywhere for floodwater to go,” Gloede said.

Unlike traditional roofs that simply shed water, green rooftops can absorb water, which removes some of the strain on drainage systems. Plants need water to thrive and green rooftops feature a moisture absorption layer that safely holds on to water during storms. 

“Green roofs can be designed to hold more or less rainwater,” Peck said. “Some of them have layers that are like cisterns that hold extra water.”

4. Filter Water 

Pollution is another problem with drainage systems. As rain falls on a city, the water gets saturated with pollutants. These pollutants then get carried by underground pipelines to rivers and lakes, which can result in drinking water contamination.

“When it rains, we’re basically crapping in the Chesapeake Bay, or Ontario Bay, or the Mississippi,” Peck said.

Plants on green rooftops filter rainwater, removing harmful toxins, and lowering the risk of drinking water contamination. 

5. Improve Food Security 

Although it’s still a relatively new field, rooftop farming is becoming increasingly popular. Rooftop farms are harder to implement than standard low-maintenance green rooftops, but they have a lot of benefits. 

Rooftop farms can bolster a city’s food security by providing a steady supply of produce. They can also improve the nutrition rates of people in food deserts and food swamps by diversifying the diets of community members. 

“If you can create an urban garden on a rooftop that is absolutely critical for food security,” Gloede said.  “A new term that’s emerging is food swamps, when an area has a lot of fast food options, where insecurity comes from not having access to healthy foods or grocery stores and farmer’s markets where people can become exposed to them.”

Finally, rooftop farms reduce the ecological footprint of food because they don’t have to be transported cross-country or internationally, and they don’t have to be refrigerated, two steps in the food production system that generate major greenhouse gas emissions. 

6. Social Cohesion and Advocacy

Green roofs provide welcome relief for city residents who might not otherwise have access to vegetation. Being near greenery has numerous physiological benefits, from reducing stress to improving memory to promoting health. 

Green roofs also provide an incentive for people to socialize on their roofs. This has two benefits, according to Gloede. First, it allows neighbors to get to know one another. During extreme weather events this can come in handy because it allows people to assist one another. 

The increase in biodiversity can also promote a deeper appreciation for nature, which in turn can spur people to become environmental advocates. The many bee farms in New York, for example, can remind people of the plight of bees around the world and the crucial role they play in pollinating wildlife.

“There are so many benefits and it’s just a wonderful intervention that we should really be maximizing,” Gloede said.