MythBusters

MYTH 5:  All green roofs are effective replacements for open space

All green roofs can replicate open space (meadow) conditions for some storm events.  Shallow green roofs will be effective in controlling small rainfall events, while deep assemblies may be required to control large storms.     

The generalization that green roofs can replace open space must be qualified.  This assessment requires 1) a determination of the rainfall conditions that are relevant for evaluating runoff performance of roofs, and 2) a determination of the type of green roof (often defined in terms of moisture retention potential) that will satisfy the management objective. 

For some regions of the United States, computing tools can predict the response of green roofs to rainfall.  Work is underway to extend the usefulness of these tools to other climates as well.  Roofscapes, Inc. is able to 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.  Roofscapes, Inc. 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.

For more information on stormwater management click here.