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Through a variety of physical, biological and chemical treatment processes that filter pollutants and reduce the volume of precipitation runoff, green roofs reduce the amount of pollution delivered to the local drainage system and, ultimately, to receiving waters.

One pollutant that green roofs help control, for example, is nitrogen.  While nitrogen gas occurs naturally as a major component of the atmosphere, nitrogen compounds from automobile exhaust, agricultural fertilizers and industrial activities can create a significant pollution problem. 

Airborne nitrogen compounds can fall to the ground in dust, raindrops, or simply due to gravity.  When these compounds are carried away with stormwater runoff, they contribute to eutrophication problems in surface water.   

Green roofs, however, can help make nitrogen pollution less of a water quality problem.The following figure shows how the breakdown of nitrogen occurs in the soil. 

The end result of this process is the NO3 (nitrate) form of N that plants can take up as they grow. Green roofs contribute to improved water quality not only by retaining and filtering the rainwater through the soil and root uptake zone, but also through:·     

The vegetation, which slows down the water through friction and root absorption. ·     

The foliage in particular, which collects dust, transpires moisture and provides shade.  ·     

The binding of potential pollutants to clay and organic matter in the roof top soil matrix (Dramstad, et al, 1996). 

In addition, the temperature of the water not retained on the rooftop is moderated before draining downstream. Results of the limited number of studies pertaining to the water quality benefits of green roofs are promising. Typical study results indicate that most of the heavy metals and other nutrients present in stormwater are bound in the soil substrate instead of discharged into streams or rivers. 

It was estimated that over 95% of cadmium, copper and lead and 16% of zinc can be taken out of rainwater by green roofs and that nitrogen levels can also be diminished (The London Ecology Unit, 1993). Water Quantity

A major objective of LID site design is to minimize, detain and retain the post-development runoff uniformly throughout a site so as to mimic the site’s predevelopment hydrologic functions. This is achieved by infiltrating and temporarily storing runoff water using one or more Integrated Management Practices (IMPs) that help maintain the predevelopment peak discharge rate and timing. 

The storage provided by green roofs helps to reduce the volume of runoff that would otherwise need to be controlled elsewhere in order to replicate natural watershed conditions and attenuate peak flows. Water quantity control by green roofs is well documented.

In recent years its use has increased due to its potential for addressing urban flooding and in reducing hydraulic loads on combined storm sewer systems. The quantity of rainfall retained or detained by a green roof can vary, but for small rainfall events little or no runoff will occur and the majority of the precipitation will return to the atmosphere through evapotranspiration.

For storms of greater intensity and duration a vegetated roof can significantly delay and reduce the runoff peak flow that would otherwise occur using conventional roof design. This helps to reduce the risk of flash flooding and the frequency of combined sewage overflow events. As with natural soil/plant systems, green rooftops reduce runoff problems by a variety of means, including:

  • The storage of water in the substrate
  • Absorbing water in the root zone 
  • Capturing and holding precipitation in the plant foliage where it is returned to the atmosphere through transpiration and evaporation

Slowing the velocity of direct runoff as it infiltrates through layers of vegetated cover

 Greater grass & plant diversity provides better plant uptake and increased friction, creating less erosion and more water retained on the green roof surface.  The characteristics of the soil substrate have a major influence on the effectiveness of a green roof as a whole.

The soil layer traps sediments, leaves and other particles, thereby treating the runoff before reaching an outlet. The water retention capacity of the soil is dependent upon both the properties of the soil substrate and the vegetative cover.

For example: ·       

A 1-inch deep moss and sedum layer over a 2-inch gravel bed retains about 58% of the water. ·       

A 2.5-inch deep sedum and grass layer retains about 67% of the water. ·       

A 4-inch layer of grass and herbaceous vegetation retains about 71% of the water.·       

A major 2-inch rainstorm, generating about 1.25 gallons of water per square foot, on a 2.5-inch thick extensive green roof would retain approximately 0.50 gallons of water per square foot, or 40% of it.

Besides the depth and composition of both the soil substrate and vegetation, environmental factors that affect stormwater management efficiency of green roofs include air and roof temperature, sun and wind. Rooftop runoff interception rates may vary between 15 and 90 %. 

Average runoff absorption rates may vary between 50 and 60 % due to differences in the soil matrix and roof top cover conditions.  Absorption rates may also vary considerably, by as much as 50 %, between summer and winter cover conditions due to temperature, wind, evapotranspiration rates and plant uptake through both the root zone and plant foliage. 

Studies in both North America and Europe have provided ample evidence as to the effectiveness of green roof tops in controlling stormwater. These studies have determined that the greatest cost benefit for the control of stormwater is provided by the first inch of soil and vegetation cover.

  • In Toronto, Canada, where the average rainfall event is 1.6 inches, a three-month long summer study showed that a green roof with a 2.8 inch deep vegetation layer produced no runoff, while the soil surface at grade, without planting, produced 42% runoff and a gravel surface produced 68% runoff.  
  • A study in Berlin, Germany showed that green roofs can absorb 75% of the precipitation that falls on them, which translates into an immediate discharge reduction to 25% of normal levels.
  • In Portland, Oregon a garage roof top planted with a mixed layer of sedum and grass retained up to 90% of all the rain that fell on it, becoming less effective only during continuous and heavy rainfall.  The effectiveness of the roof top could have been improved upon if additional storage had been provided through the use of additional detention devices such as modified roof drains and water tanks.


Additional Ecosystem and Environmental BenefitsGreen roofs, beyond their use for stormwater management, provide a number of ecological amenities. They can help to preserve habitat and biodiversity and provide an oasis of life in an otherwise sterile urban environment. Even in densely populated areas, birds, bees, butterflies and other insects can be attracted to green roofs and gardens up to 20 stories high (The London Ecology Unit, 1993). Some of the ecological and environmental amenities of green roofs are summarized as follows:

  • Green rooftops can provide a micro 'stepping stone' habitat for birds and insects, connecting natural isolated habitat pockets with each other, or provide an 'island' habitat above those at ground level.
  • Green roofs can be specifically designed to resemble endangered ecosystems or habitats, i.e., prairie grasslands or desert xeriscapes.
  • A green roof designed for minimal maintenance is very protected and can provide habitat to both plants easily damaged by walking and/or to ground nesting birds.
  • Green roofs can improve air quality through increased evapotranspiration and the filtering and shading effect of their foliage, thus helping to ameliorate the urban heat island effect, especially in sparsely vegetated areas.
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