A green roof refers to a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. There are two types of green roof: intensive and extensive. Intensive green roofs are essentially elevated parks. They can sustain shrubs, trees, walkways and benches with their complex structural support, irrigation, drainage and root protection layers. Extensive green roofs support hearty native ground cover that requires little maintenance. They usually exist solely for their environmental benefits and don’t function as accessible rooftop gardens.
A green roof has several benefits for a building. It not only provides the owners of buildings
with a proven return on investment, but also represents opportunities for significant social,economic and environmental benefits, particularly in cities. The economic benefits include:
- Increased roof life span dramatically
- Increased real estate value
- Increased rentable space
- Reduction in energy cost by an average of 25-75 percent
- Reduced building stormwater damage
- Stormwater fee or tax reduction
- Possible federal, state and local tax incentives
Other than the economic benefits, the environmental benefits are also significant:
- Reduced need for heating and cooling
- Reduced heat island affect in cities
- Reduced annual storm water runoff, by an average of 50-75 percent
- Creation of a natural habitat
- Filtering pollutants and carbon dioxide out of the air
- Filtering pollutants and heavy metals out of rainwater
- Helping to insulate a building for sound
- Increased agricultural space
- Retaining rainwater (water is stored by the substrate and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation)
- Moderating the temperature of the water
If installed correctly many living roofs can also contribute to points toward LEED certification. Despite their growth in popularity elsewhere, green roofs are largely unknown in Oklahoma, which is there I am writing from. In 2010, the University of Oklahoma initiated the state’s first experimental university vegetative roof system at the rooftop of National Weather Center Complex Building.
The green roof comprises 1,280 square feet and consists of 160 planted green roof trays. The trays contain an engineered substrate consisting of lightweight sand, expanded clay and organic material. The substrate weighs about a third of the weight of regular topsoil. The roof is planted with sedums – exotic succulents that are highly adaptive to a green roof’s growing conditions. As part of the experiment, native dry-land prairie grasses and flowers will be seeded over the sedum for comparison. Primary investigation areas for the green roof include plant performance, changes in the radiation balance, cooling efficiency for various climate conditions and impacts on building day lighting.
Besides its goal to contribute to the green roof research in the state, the project can also demonstrate broader economic benefits for the public as well. This will make Oklahoma more competitive for urban development, cost of living and overall quality of life. For example, covering 90 percent of the rooftops of Chicago with green roofs could save the city 750 megawatts of peak flow electric consumption annually. Rising temperatures created by the urban heat effect can be reduced three to four degrees.
A symposium was held in Oklahoma recently to increase interest in green roofs in the state. The symposium included presentations on green roofs and tours to three different types of green roofs in Oklahoma City. Slowly but surely, the concept is starting to take hold. For example, one green roof was included in a rooftop garden as part of a renovation of a decades-old building. The green roof was a success, and the rooftop has been rented out for everything from Greater Oklahoma City Chamber events to weddings to parties. The rooftop has provided the community an affordable alternative for weddings. With 20 percent of the rooftop covered with vegetation, the green roof has reduced energy costs and developed its own ecosystem filled with butterflies and bees.
To learn more about the green roof project in Oklahoma, visit http://www.environment.ok.gov/sustainability/GreenRoof.html
*Photo courtesy of Urban Design Tools: Low Impact Development