Green Roofs Save Energy and Reduce Stormwater Runoff

D.C. Approaches a Million Square Feet of Green Roofing

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Posted: Wednesday, December 15, 2010 12:21 pm | Updated: 11:12 am, Thu May 15, 2014.

With interest in conservation growing, green roofs are becoming increasingly popular. From reducing heating and cooling costs to helping manage and filter stormwater, vegetated roofs make economic as well as environmental sense, municipal officials say.

There are currently just over 100 green roofs installed in Washington, D.C. alone, with an additional 25 under construction and many more in the concept/planning stage.

Storm water retention in urban areas with combined storm and sanitary sewers is often the primary motivation behind the installation of green roofs, according to the Washington, D.C. District Department of the Environment (DDOE). Green roofs help reduce the volume of storm water flowing into the combined sewer system, which will reduce the number and volume of combined sewer overflows.

According to the International Green Roof Association (IGRA), green roof profiles fall into three categories:

1. Extensive (less than 6 inches of growing medium with low-grading sedum plants). According to the IGRA, the maintenance requirements are low; irrigation is not needed; the system build up height is 60-200 millimeters; the weight is 13-30 pounds per square foot; they are low-cost; and they are used as an ecological protection layer.

2. Semi-intensive (4-½ inches to 7-½ inches of a growing medium with a greater diversity of vegetation). According to the IGRA, this type of green roof requires periodic maintenance; periodic irrigation; has a system build up height of 120-250 millimeters; weighs 25-40 pounds per square foot; is mid-priced; and is designed.

3. Intensive (more than 6 inches of vegetation with the highest level of plant diversity potential and may include plants as large as trees). The IGRA reports that this type of green roof requires a lot of maintenance; regular irrigation; weighs 35-100 pounds per square foot; has a high cost; and is a park-like garden.

Russell Clark, environmental protection specialist for the DDOE Watershed Protection Division, said "If the structural analysis tells you that you can do four inches of growing medium and plants, then the best you can do on that roof is an extensive system. The more structural capacity you have, the thicker the growing medium can be and the more diverse plant types you can choose from; but the cost is going to go up as well," he said.

"However, you can just imagine the correlation with the benefits, because, you can support more types of wildlife; you'll have more benefits in the way of producing oxygen, cleaning the air, and cooling the air around the roof. So, all of the benefits that you receive from a green roof rise as you get toward the thicker growing mediums and the more diverse palates of plants," Clark said.

The green roofs installed in the nation's capital span the commercial and residential spectrum, but the vast majority of the square footage falls on larger government and commercial buildings. Green roofs have been installed on buildings in Washington in all of the following categories:

• Federal and local municipal buildings

• Private and public secondary schools

• Libraries

• Fire stations

• Embassies

• College and university buildings

• Professional associations' headquarter buildings

• Non-governmental organization headquarter buildings

• Stadiums

• Zoo buildings

• Hotels

• Medical centers

• Utility company buildings

• Commercial office buildings

• Private apartment and condominium buildings

• Private residences

The earliest green roof installation in the District of Columbia, according to DDOE records, was in 2004; however the majority of green roofs have been installed in the last two years.

The DDOE reports that when properly designed, installed and maintained in the form of weeding and watering during the establishment period of the roof, the vegetation has performed well and the storm water benefits have been realized.

The District of Columbia is approaching 1 million square feet of installed green roofs, according to the DDOE. This will translate into a total approximate annual storm water retention volume of 15 million gallons. This number is based on the extensive roof profile, which is the dominant type installed in Washington, D.C.

According to the DDOE, the benefits that green roofs provide include:

• Filtration of rain and some of its pollutants by the plants and the dirt and gravel that hold the plants.

• Production of oxygen by the plants, which helps to clean the air.

• The reduction of a building's heating and cooling costs, due to the fact that a green roof acts as a form of insulation.

• A reduction of the heat island effect, in which buildings warm up so much that they heat the surroundings.

• Extending the life of roofs and reducing roof replacement costs.

The DDOE reports that disadvantages include:

• Additional maintenance requirements, particularly in the first two years after the roof is installed.

• Problems with improper installations, which could be more challenging to address than improper installations of conventional roofs.

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