One of the things I noticed when I was home from university over the summer was that my town's library had a new green roof. I've also spotted green roofs around my school, and they're lovely to see. They serve a purpose other than being pretty, though! Green roofs have advantages that other roofs don't, the most prominent of which is that they are much more environmentally friendly than regular roofs.
The most common kind of green roof is a rooftop garden. These can consist of regular plants, or the entire roof may be covered with moss. When these roofs are made, the existing roof is first coated with a layer of rubber, then drainage layers, and then a small amount of soil for the plants to grow in. Moss is an especially good plant for green roofs, because it requires only an inch of soil to grow properly. Plants with deeper roots grown on intensive (thicker) green roofs, which require more maintenance. Something like moss, which has shallower roots, would be grown on an extensive (thinner) green roof, which requires less maintenance and is lighter. Another less common type of green roof is a rooftop pond, and these green roofs are used to treat greywater (wastewater from activities such as laundry that can be recycled on site).
Why are green roofs more environmentally friendly than regular roofs? One reason is that most materials used to make traditional roofing are often toxic, and when it rains, these toxic chemicals produce a lot of runoff that gets into our water supply. With green roofs, even if there is runoff, it will not include toxic chemicals. The plants on green roofs also take in carbon dioxide and other pollutants, and convert those chemicals into oxygen, which is better for the atmosphere. Green roofs are also very good insulators, and help keep the temperature of the building from changing too much in extreme weather. In addition to all of these environmental benefits, they also increase the roof's life span, since traditional roofing materials are prone to breakage and decay after a while.
Overall, green roofs are a very good investment in the future, and although they are relatively expensive now, they should become more feasible and commonplace. With the amount of pollution increasing in the atmosphere, we must do all we can to reduce those levels and improve the environment.