Types of Green Roofs
There are 2 primary types of green roofs; 1) Extensive and 2) Intensive. An extensive green roof generally has a more shallow growing depth and is therefore suitable for smaller vegetation and/or plants/shrubs. Extensive green roofs, due to their more shallow soil, are lighter than intensive roofs, may require less maintenance and are generally cheaper. Intensive green roofs have more soil depth and therefore can accommodate larger plants, shrubs and even trees. Intensive roofs, due to the deeper soil and larger plant life, require a much higher structural loading capability than an extensive roof does and may require periodic maintenance and trimming. Green roofs offer several benefits ranging from aesthetic, environmental and practical benefits.
Urban Heat Island Effect
One of the benefits of green roofs is in helping to reduce the impacts of something known as Urban Heat Island (UHI) Effect. The UHI effect basically identifies that the temperature in developed urban areas can be warmer than the surround rural areas due to the lack of vegetation, and resulting evapotranspiration, as well as the increased amount of concrete and asphalt that absorb the suns energy and release it as heat. There are many other factors and details associated with Heat Island Effect but I just wanted to touch on the very basics. According to the EPA "the annual mean air temperature of a city with 1 million people or more can be 1.8-5.4°F (1-3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C). Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality."
By installing green roofs on buildings instead of asphalt roofs the heat island effect can be reduced as the vegetation absorbs the suns energy and reduces the heat that would normally be radiated from traditional roof surface. Green roofs also promote evapotranspiration in which plants release moisture into the air which helps to cool the surrounding area.
Storm Water Management
As more and more buildings and roads are built within urban areas there is less permeable (i.e. soil etc.) surface to absorb water during rainfall events which can result in a number of concerns in managing the storm water in urban areas. A green roof will absorb much of the rainfall that would have landed on a standard roof and then become storm water. According to the Green Roofs For Healthy Cities organization; "In summer, depending on the plants and depth of growing medium, green roofs retain 70-90% of the precipitation that falls on them; in winter they retain between 25-40%. For example, a grass roof with a 4-20 cm (1.6 - 7.9 inches) layer of growing medium can hold 10-15 cm (3.9 - 5.9 inches) of water."
Another benefit is that a green roof itself actually acts as an insulator which helps control temperature. During the summer and winter a green roof will actually help to insulate the top of a building and control the interior temperature. This results in less energy required to heat/cool the building.
Oxygen is obviously very important to all of us. Since plants absorb (breath in) carbon dioxide and produce (exhale) oxygen a green roof provides additional oxygen generation and carbon dioxide absorption to urban areas where much of the plant-life has been removed. According to Green Roofs for Healthy Cities 1.5 m2 (16.15 ft2) of uncut grass, produces enough oxygen per year to supply 1 human with their yearly oxygen intake requirement. In addition to producing oxygen, plants actually filter air as it passes through them so they can also help to improve air quality.
Last but not least, Green Roofs are beautiful. If you haven't already come to that conclusion from some of the pictures in this article I invite you to search the web for some more pictures of green roofs.