General Questions

What are green roofs?

Charlie says: Green roofs, also called “vegetated roof covers,” “living roofs” and “eco-roofs,” are thin layers of living plants that are installed on top of conventional roofs. Properly designed and engineered, they are stable living ecosystems that replicate many of the processes found in nature.

What are the major advantages of green roofs?

Charlie says: They provide many ecological and aesthetic benefits, including:

  • Controlling stormwater runoff, erosion and pollution
  • Improving water quality
  • Mitigating urban heat-island effects; cooling and cleaning the air
  • Conserving energy
  • More than doubling the service life of the roof
  • Reducing sound reflection and transmission
  • Creating wildlife habitat
  • Creating amenity spaces that are refuges from the urban jungle
  • Improving the aesthetic environment of the building

For an in-depth German case study on green roofs and urban stormwater control see Study of Extensive Green Roofs in Berlin, by Manfred Kohler and Marco Schmidt.

Are there proven, scientific standards for green roof design?

Charlie says: Presently, the only widely-accepted, established standards for green roof construction are those developed in Germany by the Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau. e.V. (FLL). Although it sounds like a mouthful, these standards and guidelines are comprehensive, and include industry standard tests for the weight, moisture, nutrient content, and grain-size distribution of growing media. FLL also certifies laboratories to conduct critical tests, such as the root penetration resistance of waterproofing membranes. These guidelines are available in English translation directly from the FLL or through us. For a copy, just give us a shout.

Within the past few years, the American Standard Testing Methods (ASTM) convened a Green Roof Task Force to create domestic standards. As the President and founder of Roofmeadow, one of the first American green roof advocates and a big believer in the importance of standards, I have been a very active member of this task force from the beginning.

Are all green roofs effective stormwater control replacements for open space?

Charlie says: All green roofs can replicate open space (meadow) conditions for some storm events. The generalization that green roofs can replace open space for stormwater control must be qualified. This assessment requires 1) a determination of the rainfall conditions identified in local stormwater regulations for evaluating runoff performance and 2) a determination of the type of green roof profile (often characterized in terms of moisture retention potential) that will satisfy the management objective.

For many regions of the United States, computing tools can predict the response of green roofs to different rainfall amounts and patterns. Work is underway to extend the usefulness of these tools to other climates as well. Roofmeadow can provide the following types of input to assist local organizations and governments in developing guidelines appropriate to their respective climatic conditions:

Prediction of runoff peak rate and volume reduction for specified storms. We can generate runoff versus rainfall curves for green roofs when we are provided with the rainfall distribution, potential evapotranspiration, and antecedent moisture conditions. Statistical analysis on runoff peak attenuation and runoff volume reduction.

This analysis involves simulating a particular green roof for an extended period (typically 10 years) using local rainfall records.

Conventional runoff coefficients (i.e., NRCS RCNs or rational method coefficients) can be back-calculated from these analyses. These will be effective over specific ranges of storm event sizes.

At present, our ability to predict green roof performance using these methods is limited to existing green roof assemblies for which calibration data exists. These analyses typically can be completed in about two weeks. Feel free to contact us for more information on running a Stormwater Performance Simulation for you.