Rooftops

Roofs make up a significant portion of the total imperviousness on a site, and can contribute large volumes of stormwater runoff. There may be pollutants on rooftops, especially in industrial areas that are washed off and delivered to water resources every time it rains. When they are covered with dark shingles or similar heat absorbing materials, they also elevate the temperature of the first flush of stormwater runoff.

Tips for reducing runoff from rooftops

Planning/Regulatory Techniques
Reducing Imperviousness
Reducing Impacts

Planning/Regulatory Techniques

Review your zoning and subdivision requirements for lot coverage definition and requirements. Some regulations define lot coverage as the percentage of the lot covered by structures. If this is true of your definition, the size of building footprints (aka roofs) is determined here. NEMO recommends that your lot coverage definition includes all impervious surfaces ("impervious surfaces" must also appear in your definitions!) such as structures, roads, sidewalks, parking areas, recreational facilities (pools, tennis courts) and compacted earth or pipe.

Reducing Imperviousness

Allow and encourage the use of green roof technology wherever it is feasible. This can be especially effective with commercial and industrial sites. Green roofs have been shown to retain 62% of precipitation on average. Newer designs are available that do not require excessive structural support, and some modular designs are available that allow for easy removal of planted trays, if roof repairs are necessary.

Reducing Impacts

Instead of letting drainpipes direct runoff water onto driveways and then into streets, collect the water in rain barrels (see below) or other containment for reuse on-site. In addition to reducing the volume of stormwater leaving the site, this can also help to reduce your water consumption.
Direct remaining downspouts to rain gardens if possible, or at least to a lawn area. This will help to recharge the local groundwater, and reduce the stormwater runoff that leaves the site.
Heat island effects can be reduced by using light colored materials or green roof installation as discussed above under reducing imperviousness.

Rain Barrels for Storing Roof Runoff

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