This article was originally featured on Undark.
Sweat was dripping down Luis Cassiano’s face. It was 2012, and Rio de Janeiro’s hottest day to date: At nearly 110 degrees Fahrenheit, the seaside city had just barely beaten its previous record set in 1984.
Cassiano and his mother, then 82, had lived in the same narrow four-story house since they moved to Parque Arará, a favela in northern Rio, some 20 years earlier. Like many other homes in the working-class community — one of more than 1,000 favelas in the Brazilian city of over 6.77 million — its roof is made of asbestos tiles. But homes in his community are now often roofed with corrugated steel sheets, a material frequently used for its low cost. It’s also a conductor of extreme heat.
While the temperatures outside made his roof hot enough to cook an egg — Cassiano said he once tried and succeeded — inside felt worse. “I only came home to sleep,” said Cassiano. “I had to escape.”
Parque Arará mirrors many other low-income urban communities, which tend to lack greenery and are more likely to face extreme heat than their wealthier or more rural counterparts. Such areas are often termed “heat islands” since they present pockets of high temperatures — sometimes as much as 20 degrees hotter than surrounding areas.
That weather takes a toll on human health. Heat waves are associated with increased rates of dehydration, heat stroke, and death; they can exacerbate chronic health conditions, including respiratory disorders; and they impact brain function. Such health problems will likely increase as heat waves become more frequent and severe with climate change. According to a 2021 study published in Nature Climate Change, more than a third of the world’s heat-related deaths between 1991 and 2018 could be attributed to a warming planet.
The extreme heat worried Cassiano. And as a long-time favela resident, he knew he couldn’t depend on Brazil’s government to create better living conditions for his neighbors, the majority of whom are Black. So, he decided to do it himself.
While speaking with a friend working in sustainable development in Germany, Cassiano learned about green roofs: an architectural design feature in which rooftops are covered in vegetation to reduce temperatures both inside and outdoors. The European country started to seriously explore the technology in the 1960s, and by 2019, had expanded its green roofs to an estimated 30,000 acres, more than doubling in a decade.
“Why can’t favelas do that too?” he recalled thinking.
Scientific research suggests green infrastructure can offer urban residents a wide range of benefits: In addition to cooling ambient temperatures, they can reduce stormwater runoff, curb noise pollution, improve building energy efficiency, and ease anxiety.
More than 10 years since that hot day in 2012 — and several heat records later — Cassiano heads Teto Verde Favela, a nonprofit he started to educate residents about how they can build their own green roofs. Favela construction comes with its own set of technical peculiarities and public policy problems, and Cassiano enlisted the help of local scientists to research best practices and materials. But covering the roofs of an entire neighborhood requires time and — even with cost-reducing measures — a big budget.
His work has been steady, but slow. He is still far from converting every roof in his community of some 20,000 people. And with the effects of climate change arriving quickly, time may not be on their side. Still, Cassiano sees Teto Verde Favela as a template for others in similar situations around the world.
“I started to imagine the whole favela with green roofs,” he said. “And not just this favela, but others, too.”
Green roofs have been around for thousands of years, but it wasn’t until the 1960s and 70s that the modern-day version really took off, thanks to new irrigation technology and protection against leaks developed in Germany.
The technology cools local temperatures in two ways. First, vegetation absorbs less heat than other roofing materials. Second, plant roots absorb water that is then released as vapor through the leaves — a process known as evapotranspiration that offers similar cooling effects to how sweat cools human skin.
Green roofs can also help prevent flooding by reducing runoff. A conventional roof might let 100 percent of rain run off, allowing water to pour into streets, but a green roof, depending on its structure and slope, “can reduce this runoff generation rate to anywhere from 25 to 60 percent,” Lucas Camargo da Silva Tassinari, a civil engineer who researches the effectiveness of green roofs, wrote in an email to Undark.
Such interventions could be helpful in Brazil, where flooding is an ongoing issue, and temperatures are rising. A 2015 study showed that land surface temperatures in the city’s heat islands had increased 3 degrees over the previous decade. But greenery appears to help: Researchers from the Federal Rural University of Rio de Janeiro, or UFRJ, found a 36 degree difference in land surface temperatures between the city’s warmest neighborhoods and nearby vegetated areas.
In Parque Arará, Cassiano said the temperature regularly rises well above what is registered as the city’s official temperature, often measured in less dense areas closer to the ocean. He decided his community’s first green roof prototype would be built on his own home. As he researched the best way to get started, Cassiano came across Bruno Rezende, a civil engineer who was looking at green roofs as part of his doctoral thesis at UFRJ. When he told him about his idea, Rezende came to Parque Arará right away.
There isn’t necessarily a one-size-fits-all approach to green roofs. A designer must take into account each location’s specific climate and building type in order for the project to not only be effective, but also structurally sound.
The problem is that green roofs can be quite heavy. They require a number of layers, each serving its own unique purpose, such as providing insulation or allowing for drainage. But Parque Arará, like all of Rio’s favelas, wasn’t built to code. Homes went up out of necessity, without engineers or architects, and are made with everything from wood scraps and daub, to bricks, cinder blocks, asbestos tiles, and sheet metal. And that informal construction couldn’t necessarily hold the weight of all the layers a green roof would require.
After looking at Cassiano’s roof, Rezende’s first suggestion was to cover it with rolls of bidim, a lightweight nonwoven geotextile made of polyester from recycled drink bottles. Inside those rolls of bidim, leftover from a recent construction project, they placed several types of plants: basket plants, inchplants, creeping inchplants, and spiderworts. They set the rolls in the grooves of the asbestos roof, and then created an irrigation system that dripped water down.
With a cheap way to install lightweight green roofs, Rezende brought Cassiano to meet his advisers and present what they had found. The university agreed that the project showed such promise that it would provide materials for the next step, Cassiano said.
Once the plants on Cassiano’s roof had time to grow, Rezende and André Mantovani, a biologist and ecologist at Rio’s Botanical Gardens, returned to see what effect it had on Cassiano’s home. With several sensors placed under the roofs, the researchers compared the temperature inside his house to that of a neighbor’s for several days. (The researchers intended the study to last longer, but the favela’s unreliable energy system kept cutting power to their sensors.)
Despite the study’s limitations, the results were encouraging. During the period that researchers recorded temperatures, Cassiano’s roof was roughly 86 degrees. His neighbor’s, on the other hand, fluctuated between 86 and 122 degrees. At one point, the roofs of the two homes differed by nearly 40 degrees.
For Cassiano, the numbers confirmed what he suspected: If he wanted to make a difference, he needed to put green roofs on as many homes as possible.
“When we talk about green roofs, we think about one house. But that’s not enough,” said Marcelo Kozmhinsky, an agronomic engineer in Recife who specializes in sustainable landscaping. “When you start to imagine a street, a block, a neighborhood, and a city or a community as a whole with several green roofs, then you have something. Because it’s about the collective. It benefits everyone.”
But thinking on a larger scale comes with a host of new challenges. In order for a green roof to be safe, a structure has to be able to support it, and studying the capacity of individual buildings takes time. And even with low-cost materials such as bidim, installing green roofs on hundreds or thousands of homes requires significant funds.
“The biggest obstacle is the cost,” said Bia Rafaelli, an architect based in São Paulo who has worked with communities like Cassiano’s to teach them about sustainable building options. “To make this all viable on a large scale,” installing green roofs on all the favelas, she said, “there would need to be sponsorship from companies or help from the government.”
While some municipalities in Brazil have legislation requiring green roofs on new construction when possible, Rio de Janeiro does not. A bill that would create a similar law to those in other cities has been at a standstill in Rio’s city council since May 2021.
Rio does, however, incentivize builders to install green roofs and other sustainable options — like solar panels and permeable paving. But such efforts don’t typically benefit residents of the favelas, where most building is done informally, without construction companies looking to legislation for guidelines and benefits.
In addition to red tape and other bureaucratic hurdles, any project related to the favelas also faces longstanding racism. According to a 2021 study conducted by Instituto Locomotiva, Data Favela, and Central Única das Favelas, 67 percent of the population in favelas across Brazil is Black. That’s disproportionately higher than the country’s general population, which is 55 percent Black.
“Public policy doesn’t reach” favelas, said Diosmar Filho, a geographer and senior researcher at the research association Iyaleta, where he heads studies on inequality and climate change. The working-class communities, he said, are heat islands because of environmental racism — the disproportionate impact of environmental hazards on people of color — which has left much of Brazil’s Black population with inadequate housing and health care, both of which are aggravated by the effects of climate change.
Such trends aren’t isolated to Brazil. A 2020 study published in the journal Landscape and Urban Planning found that White neighborhoods in South African cities had disproportionately higher access to urban green infrastructure, including parks and green roofs — which the authors dubbed a “green Apartheid.” In a 2019 study, researchers at the University of Michigan used a spatial analysis to determine that green roofs were predominantly located in the city’s downtown, which they noted was more White and affluent than the rest of the city. (The study had limited data, however, and only analyzed 10 green roofs.)
Without support from the government or other authorities, Filho said, Black people often turn to each other for help. “It’s always the Black population that’s producing quality of life for the Black population,” he said, referring to people like Cassiano and projects like Teto Verde Favela.
“The actions of Teto Verde would be a great point of reference for urban housing policy for the reduction of impacts of climate change,” said Filho. But when municipalities deny people of color the right to safe housing and ways to push back against climate change, he added, “that’s when it becomes a case of environmental racism.”
Back in Rio, Cassiano continues to collaborate with research scientists and students at UFRJ. Together, they test new materials and methods to improve on the initial green roof prototype first installed on his home more than 10 years ago. To adapt for favela construction, his primary focus has been to reduce cost and reduce weight.
Instead of using an asphalt blanket as a layer of waterproof screening, Cassiano uses a vinyl sheet sandwiched between two layers of bidim. This means the cost of roofs installed by Teto Verde Favela is roughly 5 Brazilian reais, or $1, per square foot; conventional green roofs, though difficult to estimate in cost, can run as much as 53 Brazilian reais ($11) for the same amount of space. His roofs also started out hydroponic, meaning no soil was used, in order to decrease their weight.
Cassiano’s mother, now 93, loves caring for the plants on their roof. It not only helps lower the temperature in their home on hot days and retains rainwater to help prevent flooding in a downpour, but Cassiano said it also gives their mental health a much-needed boost.
“Now I couldn’t live here in this house without this green roof,” said Cassiano. “It makes me so happy when I see birds, when I see butterflies, when I see a flower or a fruit,” he added.
“It’s so much more than I ever imagined.”
Jill Langlois is an independent journalist based in São Paulo, Brazil. Her work has appeared in The New York Times, The Guardian, National Geographic, and TIME, among others.