When one first hears the phrase "green roof", one might imagine daring homeowners rebelling against the cookie-cutter colors of their tract homes, and painting their rooftops green. A more environmentally-minded individual might instead imagine shingles, shakes, and other roofing materials made out of recycled plastics — with a variety of natural dyes built right into their surfaces, and never splashed with oil-based paints, of any color. But the phrase usually refers to any roof that has been largely covered with vegetation (intentionally!).
There are many advantages to green roofs versus bare ones: Green roofs absorb or deflect all of the sunshine that would otherwise strike and heat up conventional roofing material. This reduces the heat transmitted through the roof, and also helps to draw heat from the top floor or attic of the building during the hot summer months, and in turn lessen its cooling costs. Collectively, buildings topped with vegetation help ameliorate urban heat islands by reducing rooftop temperatures, which can exceed 175 degrees Fahrenheit for a black roof. By providing natural insulation to the top of the structure, green roofs also help ease the heating requirements of a building.
Green roofs reduce storm water runoff, and thus help to reduce evaporation of rainwater, as well as the amount of water that settles into the soil close to the building's foundation, which can accelerate decay of the concrete or wood used in the foundation. As urban and suburban sprawl continues, an increasing number of areas can no longer accommodate the volume of water that runs off of roofs, roads, and other impervious surfaces. Green roofs hold rainwater and thawed snow, like a sponge, allowing the excess water to dissipate naturally, at a slower pace, and reducing the burden on aging storm sewer systems.
Rooftops covered with living plant material help to absorb carbon dioxide and airborne pollutants, and also generate more oxygen and healthy negative ions. This can help compensate for the loss of natural groundcover caused by the clearing of the land for a building, depending upon the vegetation used on the roof versus what would naturally be growing on the site. In addition to reducing air pollution, these naturally green roofs help block and absorb urban sounds, and thus diminish noise pollution.
The vegetation on a roof protects the roof membrane itself from degradation from ultraviolet rays, air pollution, acid rain, and wind-borne dust. This in turn lessens the frequency that the roofing materials must be replaced, and thus reduces the need for new materials to be manufactured, and the refuse materials to be discarded — usually into landfills.
The advantages of green roofs are not just ecological, but psychological as well. A properly landscaped roof garden can provide aesthetic delight to any visitor — particularly if it can be easily accessed by people otherwise confined to an urban environment, such as on top of an inner-city hospital or the parking garage of a large corporate complex. Humans are not the only creatures to benefit from green roofs, which can offer sanctuary for local creatures of all kinds, resting places for migrating birds and butterflies, and even food and water for these animals if the flora and water-catching basins in the rooftop garden are well-chosen.
These benefits may appear attractive in theory, but they are also being manifested in actual practice. For instance, homeowners who switch to green roofing typically see a 50% reduction in their air conditioning costs, as the plant materials absorb the heat from the sun, and allow it to dissipate back into the atmosphere.
Green roofing has much to offer to the owners of homes and commercial buildings. Everyone should consider this environmentally sound option, if only to scandalize any narrow-minded and eco-clueless neighbors.