Green Roofs in Alexandria: What They Are and Where You Can Find Them
Green Roofs: What are they?
A green roof is a roof that is covered in vegetation rather than traditional roofing materials such as tile or shingles. They are often incorporated as part of a green building design plan because they offer a number of environmental, as well as aesthetic and financial, benefits. Green roofs apply to residential, commercial, and industrial buildings and are also commonly called “living roofs,” “vegetated roofs,” “greenroofs,” and “rooftop gardens.”
Green roofs, consisting of the modern system of manufactured layers and vegetation, have been installed widely in Europe since the 1960’s and are growing in popularity in the United States. In recent years, some large companies have recognized the benefits of incorporating green roofs into their buildings. In 2003, Ford Motor Company installed a 454,000 square-foot extensive green roof on at its River Rouge Plant in Dearborn, Michigan.
Where are the benefits of green roofs over conventional roofs?
Green roofs contribute to a number of environmental benefits such as:
- Absorbing and filtering rainwater: The vegetation and roof materials act as a sponge to absorb rainwater. Green roofs filter pollutants and heavy metals out of rainwater to reduce the quantity and improve the quality of stormwater run off.
- Providing temperature and sound insulation: The thick layer of vegetation provides auditory and thermal insulation to reduce outside noise and lower heating and cooling loads.
- Reducing Heat Island Effect, the phenomenon that describes why urban and suburban temperatures are warmer than nearby rural areas.
- Improving air quality by capturing filtering airborne pollutants.
- Creating a habitat for birds and butterflies.
In addition to environmental benefits, green roofs offer additional benefits such as expanding the usable square footage of the building, contributing to LEED points, and improving the aesthetic of the rooftop. Importantly, green roofs can be a sound financial investment. Green roofs have a longer lifetime than traditional roofs, resulting in sustained reductions in heating and cooling costs and increased property value. For a full list of benefits of green roofs, click here.
How much does it cost?
Costs for a green roof vary based on the building’s special requirements and the system design. The cost for a green roof in Virginia ranges between approximately $15 -$30/ft2, as compared to typical roof construction at $5-$15/ft2.
Although the initial upfront costs for green roof construction are higher than for typical roofs, the overall lifecycle costs of green roofs are actually lower. Because green roofs are protected from ultraviolet light, they last longer – with a longevity of 40 years compared to 7-13 years for a typical roof. Additional savings from reduced heating and cooling loads also contribute to making green roofs affordable over the course of their lifetime.
What is the difference between an Intensive and Extensive Green roof?
There are two types of green roofs: intensive roofs and extensive green roofs.
Intensive green roofs are widely used for commercial buildings where the roof will be used as a rooftop garden or recreational area. Intensive roofs are thicker and have deeper soil (6 inches or more) to accommodate larger plants or even trees. Intensive rooftops are also designed to hold up under park-like
pedestrian use and to support benches and tables. Upfront and maintenance costs for intensive roofs are usually higher than for extensive roofs.
Image 1: Components of an intensive green roof. Diagram from Green Roof Plan. (Image source: Green Roof Plan)
Extensive green roofs are often used on residential or multi-family buildings, and are best suited for spaces that will not get much foot traffic. The design is intended to give high performance to water use and thermal advantages while keeping the overall weight and cost of the roof low. The planting base ranges from 1.6 to 6 inches deep and the plant stock is composed of a less diverse group of vegetation with shallow roots, such as sedums or grasses. (Source: Green Roof Plan)
Where can I go to see an example of a green roof in Alexandria?
T.C. Williams High School (3330 King Street): Alexandria’s new high school has incorporated green roofs and other water and energy-saving methods.
James Duncan Branch of the Alexandria Library (2501 Commonwealth Ave): The first City building with a green roof, Duncan’s green roof was sown in early September 2005 with a variety of sedum. The 2,850 square-foot vegetated roof has been effective at filtering stormwater runoff and reducing heating loads in the summer.
Alexandria Health Department (4480 King Street): A green roof project has been installed at the Alexandria Health Department and the “Clubhouse” program of the Department of Mental Health, Mental Retardation and Substance Abuse. This new vegetated roof covers 10,765 square feet and is projected to reduce stormwater discharge by 10,000 gallons of rainfall from a 1.5” rainstorm. The vegetation also acts as a filtration mechanism to improve water quality for the water that does runoff. The hardy sedum plant species selected for the project is drought- and cold-resistant and is a low maintenance feature for building owners considering green roof applications. The vegetated roof is also projected to save in energy costs by reducing roof temperatures during summer months.
Cora Kelly School for Math, Science, and Technology (3600 Commonwealth Avenue) has a 5,000 square foot green roof and a bioretention area. The green roof reduces non-point source runoff and provides added insulation for the classrooms below. The Bioretention area provides water quality improvements to parking lot runoff. The green roof is presently not available to the public but will eventually be integrated across the curriculum as a teaching tool.