Eco roofs, or green roofs, replace or cover conventional roofs with vegetation. They conserve energy by insulating the building below, which reduces the urban heat island effect, cleans the surrounding air, increases the life of the roof by a factor of two or three, looks attractive and decrease stormwater runoff. Roof gardens act like sponges, absorbing rainfall and allowing it to transpire from the leaves of the plants, or detain water for release into catchment systems. The performance of an eco roof depends on many factors, including the season of the year, seasonal rainfall patterns and the interval between rains, according to the authors of a study for the U.S. Environmental Protection Agency (see References 2, page 3-14).
Types of Eco Roofs
A green roof is multilayered, with a root barrier to contain invasive roots, a waterproof membrane, drainage and soil systems and the plants. Green roofs are either extensive or intensive. Extensive eco-roofs consist of a shallow substrate planted with drought-tolerant grasses, herbs, succulents and mosses. Once established, they are not meant to be walked on, and require little to no maintenance. Intensive roofs are more likely to be public spaces with complex substrates and varied plantings that resemble gardens, with annuals, perennials, shrubs and trees. Intensive eco-roofs are high-maintenance and sometimes open to the public. (See References 3)
Both intensive and extensive green roofs share a common set of benefits. The vegetation absorbs the storm water and releases it slowly. The result is fewer incidents of flooding, less erosion, less water that needs treatment before it is potable -- a significant decrease in the stress placed on municipal drainage systems. The water conservation function of a green roof may be its most compelling benefit. A Michigan State University study of extensive roofs found they retained 60 to 100 percent of rainwater (see References 3).
Storm runoff is a serious problem for densely populated areas. It results in pollution of surrounding rivers, lakes and oceans, and it can affect aquifers and the purity of drinking water. When soil and vegetation absorb rainwater, they release the moisture back to the atmosphere through normal evaporation and transpiration from plant leaves. Concrete and other building materials interrupt this cycle, and green roofs restore it to some degree. The Michigan State University study of different roof surfaces showed that the green roofs in the study, on average, retained around 60 percent of rainfall, whereas conventional roofs retained only about 27 percent (see References 3). The EPA's study of extensive roofs found green roofs retained an average of 50 percent of rainfall, but this ranged from less than 20 percent during colder weather to 95 percent during the summer (see References 2, page 2-1). This is similar to the City of Portland's review of its own data, as well as literature from a variety of other studies. They conclude that while the average rainfall retention of eco roofs in Portland was 56 percent, green roofs may retain anywhere from 26 to 100 percent of a rain event, depending on the roof and the region (see References 1, pages 5 and 6).
Homeowners typically invest in an extensive roof, the kind that, once planted, maintains itself. The roof is not a recreation area. The membrane insulates and covers the house, while permitting recapture of the water that falls on it. Drainage systems channel the water from the roof to rain barrels and other catchment receptacles for use to irrigate a garden or lawn, wash cars or flush toilets. Some systems purify collected rainwater for drinking. Researchers at the University of Central Florida, for example, note that much of the work on green roof efficiency doesn't include the addition of a cistern to collect the percentage of rainfall not retained (see References 4, page 5). Adding such systems can significantly affect an eco roof's average water conservation.
- Bureau of Environmental Services: City of Portland; Cost Benefit Evaluation of Ecoroofs 2008; November 2008
- U.S. Environmental Protection Agency; Green Roofs for Stormwater Runoff Control; Robert D. Berghage; et al.; February 2009
- Michigan State University: Green Roof Research Program
- University of Central Florida; Designing Cisterns for Green Roofs in Florida; Mike D. Hardin and Marty Wanielista ; May 2007