Water Resources Program  |  Green Roof -- Benefits -- More Information

Green Roof
Plant Types
& Benefits
& Care
Green Roof Monitoring Monitoring Results

Storm water Quality: Once a green roof is completely vegetated, the benefits that will result are multifold. In fact, there is not one single benefit, but a variety of positive outcomes. These include improvements to storm water quality; decreases in storm water quantity flowing away from developed areas, particularly during large storms; energy efficiency; reduction of the urban heat island effect; provision of habitat, and photosynthesis, which helps to remove pollutantion from the air. Intensive green roofs (active roof garden) may provide garden plots for food production and outdoor recreation and contemplation areas, particularly in urban areas where outdoor space is limited. The County's extensive roof provides the following benefits: Rainfall on conventional roofs flow directly into streams or storm water retention facilities. This rain increasingly contains phosphates, nitrates, mercury, and other byproducts from the burning of fossil fuels in our homes, automobiles, and electricity power plants. These byproducts are pollutants. With the exception of heavy metals, such as mercury, the plants on the roof filter out these pollutants and use them as plant nutrients. As a result, the water flowing away from the roof is cleaner than the polluted rainfall that lands on the roof.

Storm water Quantity: During storm bursts, rainfall flows quickly away from roofs an often back into the natural environment. This is the case for the CountyOfficeBuilding. The roof drains from the building lead to underground storm sewers and directly into Shenk's Branch, which flows parallel to McIntire Road toward the 250 By-pass. Streams that flow away from urbanized areas often show the signs of flashiness. Flashiness is what occurs when a large storm events fall on an urban area. With no facilities to capture or slow the stormwater, streams often become scoured out and eroded, destroying the stream bank and the riparian areas directly adjacent to the stream. The greenroof is capable of retaining the first inch of rainfall during a storm. This sponge-like action decreases the volume of the flow from urban areas. In addition, the greenroof slowly releases this water, allowing it to return to the natural system at a measured pace, more in keeping with natural systems. Additionally, since the water has flowed through the greenroof, it will contain fewer pollutants than if it had fallen on a conventional roof. Rainfall in excess of on inch will begin to flow over the top of the soil medium and down the drains. For up to 48 hours after a storm, water slowly trickles away from the roof.

Energy Efficiency: Because the roof shades the building from the extreme heat of the sun, the building uses less energy to cool the interior during the summer. Energy efficiency increases of up to 30% can be achieved during certain times of the day. In general, the roof provides an average yearly energy savings of 10%. However, the cost of energy is increased by electricity providers during summer months due to large demand. This large demand has led to rolling blackouts in some parts of the country in the past several years. Green roofs allow buildings to demand less energy and make buildings more adaptable in the event of electricity supply disruption. In addition, a combination of high efficiency insulation and the soil medium, work to retain heat inside the building during winter months. County staff is currently collecting data on stormwater quantity and energy efficiency.

Urban Heat Island Effect: Large area of paved surfaces with masonry and other construction materials work to trap heat during the warmest periods of the summer, increasing the overall temperature by up to 10 degrees. If you have ever walked from a wooded area across a treeless parking lot on a warm sunny day, you have experienced the urban heat island effect. Large cities with little open space, such as Chicago, tend to be much warmer than areas outside the city where tree cover and open space work to mitigate the heat of the sun. The greenroof, when fully covered, will act much like a lawn or naturalized open space. As a result, the temperature specific to the roof will maintain a temperature more in keeping with natural systems. This, in conjunction with open space and ample tree plantings work to keep an area cool. By contrast, areas with paving and conventional roofs that are exposed to the sun trap heat and release it slowly. The effect is an overall increase in the temperature in the area and a slowly release this heat energy into the evening, prolonging the most uncomfortable period of the day. By extension, the urban heat island effect also causes air conditioning units to use more energy to keep the interior of buildings cool.

Provision of Habitat: Because the roof introduces a green area that was once a harsh environment, the roof reclaims naturalized habitat that was lost with the construction of the building. This new habitat provides areas for birds and other wildlife. So far, an unusual number of ladybugs have been seen on the roof. Ladybugs are very beneficial insects that feed on aphids and other pests, providing the same function as agricultural pesticides. Additionally, birds on the roof have broken off portions of the sedums, which propagate easily in the soil medium. This activity has led to the random cultivation of additional plants on the roof.

Filter Airborne Pollution: The plants on the roof act somewhat like a filter through the process of photosynthesis. As the plants grown, they utlize water, sunlight, and carbon dioxide to produce oxygen. This combination of a natural system with a human-made structure has positive implications for balancing our development footprint within the growing constraints of our natural environment. Though not quantified with any study, planning staff estimates that the County's green roof provides the air filtering and oxygen capacity of several trees.

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