Description
A greenroof is a vegetated cover on a roof membrane. It is usually followed by storage in a cistern (or other similar device) for the filtrate that is harvested for irrigation of the plants on the roof or for other beneficial purposes. A greenroof and cistern system is classified as a retention and harvesting BMP. Effectiveness is the annual volume of roof runoff that is captured, retained, and used on site. The term harvesting is preferred to avoid confusion with wastewater reuse.
A cistern is sized for a specific amount of filtrate and receives no other stormwater. The retained water is used to irrigate the roof since experience in Florida has shown that irrigation must be provided to maintain the plants. A backup source of water for irrigation is necessary during the dry season. Excess filtrate and excess runoff can be discharged to other stormwater treatment systems, infiltrated into the ground, or used for irrigation or other non-potable purposes.
The greenroof and cistern system functions to attenuate, evaporate, and thus lower the volume of discharge and pollutant load coming from the roof surface. Greenroof systems have been shown to assist in stormwater management by:
- Attenuating hydrographs
- Neutralizing acid rain
- Reducing volume of discharge
- Reducing the annual mass of pollutants discharged
They are frequently used on commercial or public buildings but have also been successfully used on residences.
If no cistern is used, the greenroof will provide some storage and eventual evapotranspiration. The BMPFast software also provides estimates of annual removal effectiveness when no cistern is used.
Greenroof Classifications
Green roofs are classified in two ways:
- By Access to the Finished Roof
- Passive – access limited to maintenance personnel
- Active – accessible to the public or building occupants
- By Root Zone Depth
- Extensive – root zone less than 6 inches deep
- Intensive – root zone equal to or greater than 6 inches deep
Extensive green roofs typically have passive use.
Intensive green roofs support larger plants and are typically active designs.\
Extensive Green Roof Section
Intensive Green Roof Section
Wind Mitigation
As an option to the use of wind netting, a parapet usually 30 inches or more in height is used to minimize wind damage during high wind conditions. The use of wind mitigation procedures does not affect the annual removal effectiveness of a green roof.
Input Data
There are two (2) input parameters:
- Rainfall Station
- Cistern Storage (inches over the greenroof area)
The rainfall station selected must reflect the geographic area of the project or the closest available location. There is a drop-down menu containing 18 locations. Each location supplies rainfall data used for simulation over at least 25 years.
The greenroof area must be the same as the catchment area. In this case it was one acre or 43,560 SF. The cistern size is 25,000 gallons or 0.9206 inches over 43,560 SF. The project is located in the Jacksonville area. The annual retention is 63%.
There is an option to use no cistern by entering zero as the cistern storage. For this example, the capture is 40% as shown in the software screen capture below.
