Green roofing is the use of vegetation as the weathering surface for a roof. Its many benefits include:

• Enhanced insulation and more moderate rooftop temperatures, which reduce cooling and heating requirements, saving energy and money
• Filtration and detention of stormwater, reducing pollution and the exorbitant cost of new and expanded infrastructure as paved areas increase
• Absorption of dust and airborne pollutants (like a house-plant, but outdoors)
• Reduced ambient air temperatures, reducing urban heat-island effects and helping to keep surrounding areas more comfortable
• Extended life of roof membranes, which are protected from ultraviolet radiation, extreme temperatures, and mechanical damage. (Plant species, soil depth, and root-resistant layers are carefully matched to ensure the roof membrane is not damaged by the roots themselves.)
• Lightweight extensive green roof systems can be designed with dead loads comparable to standard low-slope roofing ballast. Structural reinforcement may not be necessary, and installed cost can be comparable to conventional high quality roofing options.

A building's shell protects us, and the rest of the building, from the elements. Roofing materials, in turn, comprise 12 to 15% of construction and demolition waste partly because the roof is a harsh environment. On a typical low-slope black-surfaced roof, summer roof surface temperatures commonly vary by more than 80° Fahrenheit on a daily basis. Wild temperature variation shortens the life of a roof, and rooftop temperatures up to 170° F also increase summer cooling costs. But what if the roof itself was protected from the elements, and maintained a much more stable temperature?

There are three ways to reduce swings in rooftop temperature, save energy, and extend the useful life of roofing: reflect and re-emit incoming radiation, shade the roof deck, or use the evaporation of water (i.e. latent heat). Cool roofs have excellent reflectivity and emissivity, helping reduce cooling costs by up to 40%. Photovoltaic panels not only generate clean electricity, but most mounting systems also help shade the roof deck. Vegetated cool roofs combine all three methods; the large surface area of soil and plants helps to re-radiate heat, to shade and insulate the roof membrane, and transpiration provides cooling. The net result is a 25 to 80 degree decrease in peak roof temperature, along with up to 75% reduction in cooling energy demand, according to a 2002 Canadian Research Council study (reference below.)

Green roofs provide many benefits beyond waste reduction through extended roof life. The soil and vegetation and soil in many common extensive designs can detain up to 75% of a 1-inch rainfall event and will filter the remainder. Such onsite stormwater management helps reduce demand on stormwater infrastructure, saving resources and money for the entire community. Green roofs provide urban wildlife micro-habitat. Roofs are not a replacement for wildlands, but a vegetated roof accommodates birds, beneficial insects, and native plants far better than tar and gravel. Cooler roof temperatures reduce the urban heat island effect, helping to reduce the cooling load for surrounding buildings.

There are two classes of green roofs:

Extensive green roofs have layer of soil media that is relatively thin (2 to 6 inches of soil), and lightweight (10 to 50 lbs per square foot for the entire system when saturated with water). Because the weight of an extensive green roof is comparable to other roofing - river rock ballast weighs roughly 12 lbs per square foot, for example - an extensive green roof can cost as little as $8 per square foot. Standard extensive systems are most commonly used on slopes up to 30°, but steeper slopes (up to fully vertical green “walls”) can be accommodated with minor modifications to hold soil in place. Extensive green roofs are not suitable for food cultivation, but provide all of the stormwater, wildlife habitat, waste reduction, and energy conservation benefits described above. Lightweight, relatively easy-to-install, durable, and cost-effective green roof options are available for virtually any building type or location.

Intensive green roofs are designed to accommodate trees and gardens. Soil can be as deep as is needed to accommodate the desired tree or plant species, but deeper, denser soil dramatically increases dead load, requiring a stronger and more expensive structure, greater maintenance, and either terracing or a relatively flat roof.