Green roofs, also
known as vegetated roof covers, eco-roofs or nature roofs, are
multi-beneficial structural components that help to mitigate the effects
of urbanization on water quality by filtering, absorbing or detaining
rainfall. They are constructed of a lightweight soil media,
underlain by a drainage layer, and a high quality impermeable membrane
that protects the building structure. The soil is planted with a
specialized mix of plants that can thrive in the harsh, dry, high
temperature conditions of the roof and tolerate short periods of
inundation from storm events.1
Historically,
engineered green roofs originated in northern Europe, where sod roofs
and walls have been utilized as construction materials for hundreds of
years. The development of contemporary approaches to green roof
technology began in the urban areas of Germany over 30 years ago.
Because of ongoing water quality degradation and a limited existing
infrastructure for the control of stormwater in these areas, few
alternatives were available for improved stormwater management designs.
Environmental and economic considerations helped spur the development of
green roof systems that could provide the necessary stormwater treatment
on-site. Roofscapes, Inc. have developed a numerical
saturate-unsaturated flow simulation model that investigates the
variables influencing the effectiveness of green roofs.2
Chicago City Hall Urban Heat Island Initiative
project
(Source: Roofscapes, Inc.)
Green roofs
provide stormwater management benefits by:
Utilizing the
biological, physical, and chemical processes found in the plant
and soil complex to prevent airborne pollutants from entering the
storm drain system.
Reducing
the runoff volume and peak discharge rate by holding back and
slowing down the water that would otherwise flow quickly into the
storm drain system.
Green roofs are not only aesthetically pleasing, but they also:
Reduce city “heat island” effect
Reduce CO2 impact
Reduce summer air conditioning cost
Reduce winter heat demand
Potentially lengthen roof life 2 to 3
times
Treat nitrogen pollution in rain
Negate acid rain effect
Help reduce volume and peak rates of
stormwater
The hydrologic
processes that can be influenced by design choices and aid in the
management of stormwater include:3
Interception of
rainfall by foliage, and subsequent evaporation.
