If you’ve ever been downtown in a big city on a summer day, then you’ve probably experienced the “urban heat island effect.” “Urban forests” overgrown with asphalt and concrete soak up sunlight throughout the day and then release it slowly at night, resulting in hotter urban temperatures.
One study at NASA reported that man-made surfaces (rooftops, parking lots, buildings) can be 68° to 104° F hotter than a vegetated area after basking in the sun. Leaves and grasses, on the other hand, actually cool the air through a process called evapotranspiration. The EPA estimates that the annual air temperature of a city with 1 million people or more can be 1.8° to 5.4°F warmer than its surroundings, and in the evening the difference can be as high as 22°F. These conditions lead to increasing air conditioning costs, heat-related illness and mortality, and air pollution.
The good news is that there is a solution to this problem that offers benefits beyond cooling off a city. Earthen or “green” roofs can reduce the urban heat island effect by removing heat from the air. By absorbing heat and increasing insulation, they lower a building’s energy use and costs by reducing energy needed for heating and cooling. They also alleviate the strain on drain and sewer systems by reducing immediate water runoff, and clean the air by removing pollution and sequestering carbon. Not to mention they offer green space for city dwellers and many species, as well as a place to grow local produce.
A green roof is essentially a vegetative layer grown on a rooftop. The two most common types of green roofs, extensive and intensive, have to do with the amount of soil installed on the roof.
“Extensive” green roofs are more suitable for residential buildings and typically cost anywhere from $8 to $20 per square foot to install. They often have one to six inches of soil. These roofs require very little maintenance (intermittent watering and weeding), cost less, and can withstand most types of weather because the plants are small, hardy, and drought-resistant.
“Intensive” green roofs are more likely to be found on large, flat-roofed buildings that can support the intense weight. They typically have a soil depth of six inches to two feet (or more), which increases the roof’s weight. This style can support a wider variety of plants, including shrubs, flowers, and even trees. They will typically cost anywhere from $15 to $25 per square foot to install, but require more maintenance due to the required care of a wider variety of plants.
Green roofs are growing in popularity and are found in all shapes and sizes around the world. The spectrum ranges from farmhouses with goats grazing on rooftop grass to fertile gardens set atop urban skyscrapers. Given their adaptability to several different kinds of roofs, schools, businesses, and even governments are able to install them.
The Calhoun School in New York City, for example, created a semi-intensive green roof which reduces storm water runoff by up to 40 percent throughout the year. It also provides a space for students to learn about gardening, astronomy, geometry, poetry, and meteorology. The school has effectively turned unused space into a learning center that enriches the entire school curriculum.
The City of Chicago turned its City Hall into a green roof space in 2001, which now hosts 100 species of plants. It provides habitat for insects and birds, as well as an eye-pleasing view to the buildings that surround it. So far, studies have concluded that the new green roof has decreased ambient air temperature by 78°F when compared to the previous black tar roof.
Installing a green roof is one more solution to climate problems that has a long list of great benefits. It can save money, make homes and cities more beautiful, and provide a habitat for wildlife while simultaneously transforming our urban landscapes and cleaning up our shared environment for future generations. Green roof installation, along with retrofitting buildings, is also an industry that could create jobs as part of a green recovery program focused on moving toward a low-carbon economy.