A green roof is a top of a building that is at least partially covered with vegetation to make it a garden or other natural, live landscape. These rooftops can facilitate the growth of grass or deep-root native plants, and can act just as a natural green space or as a supplier of food.

Green roofs do more than just create another attractive outdoor space—the soil and vegetation on the roofs acts as an insulator, and helps the building retain heat or remain cool. In addition, a green roof helps to retain storm water, and may also contain root barriers to enable drainage and irrigation; ponds located on green roofs can also be a receptacle for treating grey water and can reduce rainwater runoff by as much as 75 percent. The oxygen produced by the plants is beneficial to the environment. In Scandinavia, green roofs have been used for centuries—and have been particularly prevalent in Iceland and Norway.

A form of green roofs is also commonly used in densely populated cities, particularly those in located in less prosperous countries, as a way to provide additional sustenance for the building’ tenants. But more people today are adopting the green roof concept not as a way to grow additional food, but to reduce heat loss and energy consumption. In the 1960s, Germans began embracing the green roof trend—statistics estimate that about 10 percent of Germany’s roofs are green. The Swiss are even bigger advocates of greening roofs—developers have been required to install them in most new constructions for nearly 10 years. In fact, the country has one of the oldest green roofs, constructed in 1914 atop a water treatment plant.

A recent study found that green roofs can help reduce temperatures in urban areas –actually lowering surface temperatures and harmful emissions. For instance, the City Hall of Chicago, which has a green roof, can reduce the temperature of its surroundings by as much as 12 degrees Fahrenheit compared to a traditional roof made of materials that absorb the sun’s heat. Green roofs can also have a tremendous impact on insulation.

When Environment Canada conducted a study on the benefits of green roofs, it found that green roofs reduced summer cooling needs by 26 percent, and helped buildings retain 26 percent more heat in the winter. Not only with these roofs help save money, can they also extend the life of your roof by as much as three times.

There are different levels of maintenance required for green roofs, and they are often labeled as intensive, semi-intensive or extensive, depending on the maintenance needs of the roof. Roof gardens, which have a sizable amount of soil to grow plants are among the most intensive green roofs, requiring irrigation and care similar to that of caring for flower beds planted in a front lawn. Pitched roofs that are simply made of sod, which are popular in Iceland and Norway, are much less labor intensive and drain easily because of the slanted design.

The largest green roof in the U.S. is currently at Ford Motor Company’s plant in Dearborn, Mich., which boasts 450,000 square feet of green roofs.