How Green Roofs Work
Green roofs essentially act as sponges on a roof, says Richard Hayden, garden roof product manager with Chicago-based American Hydrotech. “The green roof stores water in the structure of the lightweight media as well as in the drainage and retention panels,” he says. “Succulent vegetation such as sedums also can store water. During a rain event, rather than immediately flowing off the roof through the drains, the water is absorbed in the media and drainage or retention panels.”
During small rain events, water often never leaves the roof. In larger events, the rain continues to fall and saturate the assembly. At that point of saturation, excess water begins to flow off the roof and into the drains.
How Green Roofs Work
Green roofs essentially act as sponges on a roof, says Richard Hayden, garden roof product manager with Chicago-based American Hydrotech. “The green roof stores water in the structure of the lightweight media as well as in the drainage and retention panels,” he says. “Succulent vegetation such as sedums also can store water. During a rain event, rather than immediately flowing off the roof through the drains, the water is absorbed in the media and drainage or retention panels.”
During small rain events, water often never leaves the roof. In larger events, the rain continues to fall and saturate the assembly. At that point of saturation, excess water begins to flow off the roof and into the drains.
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“This takes time—sometimes hours—to occur,” says Hayden. “This time is often long enough to help downstream storm facilities catch up to the storm. This delay is a very important BMP feature. When properly acknowledged in the design process, green roofs can reduce the need for and size of downstream stormwater facilities.”
Green roofs also can reduce the total amount of water that would otherwise enter the storm sewers. “Normal evaporation of the water via the media plays a part in reducing this water volume,” says Hayden. “Plants transpire a significant amount of this water as well through the normal natural growing processes.”
The ability of green roofs to detain, reduce, and delay stormwater is getting more and more attention as a BMP, says Hayden. “There are a lot of civil and stormwater engineers that are taking notice of these benefits,” he adds.
Types of Green Roofs
An intensive green roof is used for visual aesthetics as well as stormwater benefits. Building owners often install them to save energy and provide a garden environment for the enjoyment of building occupants or visitors.
Intensive garden roofs incorporate media depths of 6 inches or more. “These deeper assemblies can allow for a broader array of plant materials including groundcovers, shrubs, and small trees,” says Hayden. “With special considerations, full-size shade trees can be planted on roofs.”
An extensive roof is a green roof dedicated primarily to stormwater management. Although it has aesthetic value, its main purpose is to minimize stormwater runoff and the urban heat island effect.
Hayden notes that the reduction in the heat island effect is the result of actions of water storage and plant growth to create a cooling effect in the immediate area.
“As one of the early identified environmental benefits of green roofs, this cooling effect has been known to create more than a 50 degree Fahrenheit differential between vegetated and non-vegetated surfaces,” he says.
Extensive garden roofs are the most common green roof type. With growing media depths between 3 and 6 inches, extensive green roofs can support a wide array of drought-resistant vegetation such as sedums and certain perennial plants.
Extensive roofs typically capture between 50% and nearly 100% of rain depending on the amount of growing medium used, the density of vegetation, the intensity of an individual rainstorm, and the frequency of local rain events, according to an EPA report, “Reducing Urban Heat Islands: Compendium of Strategies.” Roof slopes and types of growing media are other influencing factors.
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American Hydrotech’s Lawn Garden Roofs are assemblies designed specifically for supporting turf grasses for lawn areas, with media depths ranging from 8 to 12 inches, says Hayden.
“The features in Garden Roofs that are designed to support thriving plant materials also make them ideal for stormwater management,” he says. “LiteTop is Hydrotech-engineered growing media. It has large amounts of pore spaces that create tremendous water storage capacities within the media.
“Where standard top soils may have 5 to 10% open pore space, LiteTop-engineered growing media often have 40 to 50% or more open pore space,” he adds. “In addition, synthetic drainage/retention panels are incorporated into the assemblies. These panels store excess water for plant use when the media dries out. These panels also perform an important aeration function by adding air in the lower portions of the assembly, which reinforces plant viability. When these growing media are combined with synthetic drainage and retention panels, tremendous amounts of stormwater can be stored.”
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“This takes time—sometimes hours—to occur,” says Hayden. “This time is often long enough to help downstream storm facilities catch up to the storm. This delay is a very important BMP feature. When properly acknowledged in the design process, green roofs can reduce the need for and size of downstream stormwater facilities.”
Green roofs also can reduce the total amount of water that would otherwise enter the storm sewers. “Normal evaporation of the water via the media plays a part in reducing this water volume,” says Hayden. “Plants transpire a significant amount of this water as well through the normal natural growing processes.”
The ability of green roofs to detain, reduce, and delay stormwater is getting more and more attention as a BMP, says Hayden. “There are a lot of civil and stormwater engineers that are taking notice of these benefits,” he adds.
Types of Green Roofs
An intensive green roof is used for visual aesthetics as well as stormwater benefits. Building owners often install them to save energy and provide a garden environment for the enjoyment of building occupants or visitors.
Intensive garden roofs incorporate media depths of 6 inches or more. “These deeper assemblies can allow for a broader array of plant materials including groundcovers, shrubs, and small trees,” says Hayden. “With special considerations, full-size shade trees can be planted on roofs.”
An extensive roof is a green roof dedicated primarily to stormwater management. Although it has aesthetic value, its main purpose is to minimize stormwater runoff and the urban heat island effect.
Hayden notes that the reduction in the heat island effect is the result of actions of water storage and plant growth to create a cooling effect in the immediate area.
“As one of the early identified environmental benefits of green roofs, this cooling effect has been known to create more than a 50 degree Fahrenheit differential between vegetated and non-vegetated surfaces,” he says.
Extensive garden roofs are the most common green roof type. With growing media depths between 3 and 6 inches, extensive green roofs can support a wide array of drought-resistant vegetation such as sedums and certain perennial plants.
Extensive roofs typically capture between 50% and nearly 100% of rain depending on the amount of growing medium used, the density of vegetation, the intensity of an individual rainstorm, and the frequency of local rain events, according to an EPA report, “Reducing Urban Heat Islands: Compendium of Strategies.” Roof slopes and types of growing media are other influencing factors.
American Hydrotech’s Lawn Garden Roofs are assemblies designed specifically for supporting turf grasses for lawn areas, with media depths ranging from 8 to 12 inches, says Hayden.
“The features in Garden Roofs that are designed to support thriving plant materials also make them ideal for stormwater management,” he says. “LiteTop is Hydrotech-engineered growing media. It has large amounts of pore spaces that create tremendous water storage capacities within the media.
“Where standard top soils may have 5 to 10% open pore space, LiteTop-engineered growing media often have 40 to 50% or more open pore space,” he adds. “In addition, synthetic drainage/retention panels are incorporated into the assemblies. These panels store excess water for plant use when the media dries out. These panels also perform an important aeration function by adding air in the lower portions of the assembly, which reinforces plant viability. When these growing media are combined with synthetic drainage and retention panels, tremendous amounts of stormwater can be stored.”
