Originally, the term “green roof” referred to one that was covered in vegetation, like a sod roof. With the growing contemporary interest in environmentally friendly products—like recycled paper, cars that run on vegetable oil, and disposable dishware made from potatoes—the word “green” has been attached to anything that conserves energy. In the roofing industry, therefore, “green roofing” now refers to a roof that utilizes green technology.
There are three main technologies employed in roofing to be more energy efficient. The first is the living roof, the original green roof which is covered in plants. This technology is actually quite old, extending back to medieval Scandinavia where sod roofs were common. The living roof uses multiple membranes for the purposes of root deflection, depth, and structural support. It meets all the standards of a good roof if installed properly, having an obvious thickness and ability to weather the elements since it’s made of an element. It has fantastic temperature ratings since the plants absorb the sunlight and photosynthesize it, thereby reducing the overall temperature of the building. In fact, studies have shown that a cluster of living roofs in one area can decrease the temperature of the whole area by several degrees. The disadvantage comes in the criteria of the ease of installation. Because the soil and vegetation on a living roof is so heavy, a building usually has to be designed with extra structural support from the beginning. This makes it nearly impossible to replace a normal roof with a living one. But as far as energy efficiency goes, the living roof is an excellent candidate, absorbing heat and lessening stormwater runoff. The theory behind it is that it replaces the vegetation that was destroyed in constructing the building, and therefore gets touted as the most eco-friendly roofing option.
The second green roofing technology is photovoltaic cells, better known as solar panels. This could extend from a single black panel to a whole solar thermal collecting dish mounted on the roof. Once again, the temperature ratings and reflectivity are excellent since the cells not only absorb the sun’s radiation, but turn it into energy for the structure. Durability is not a strong point in many cases, since solar panels have a tendency of being fragile. Also, installation is a difficult point, because the photovoltaic cells are so expensive—which could be seen as an investment if you’re making your own energy. But this simply is not a realistic option for everyone.
The third technology is the cool roof. The whole idea behind the cool roof is to reduce the roof’s—and thereby the structure’s—temperature. This is done usually by means of great reflectivity, usually utilizing a white material, and insulation. Durability, ease of installation, and lifespan all depend on the material you choose, of course, but the cool roof excels in temperature ratings and reflectivity by definition.
All of these roofing technologies are more environmentally friendly than the standard choices and all of them serve to reduce the heat island effect (a whole area’s raised overall temperature from excessive roof heating and dark-surface heat absorption). The cool roof is the most common green choice, given its less drastic change and cost. If you ever have to put in a new roof, you might want to give some thought to the environment and how “green” your material really is.