Green Rooftop

Green Rooftop Credit

The term "green rooftops" refers to a few practices that detain and treat stormwater runoff on rooftops using vegetation on the roof surface. Several different options exist, including variations on the type of vegetation used, and the specific design of the green roof. The criteria presented below are adapted from the Portland Stormwater Manual criteria for the Eco-Roof.

Green Rooftops: Summary
Goals	Encourage the use of vegetated cover on rooftops to detain and treat rooftop runoff.
Stormwater Management Objectives	Water Quality/ Recharge	Subtract the total green rooftop area from the site's impervious cover.
	Channel Protection, Flood Control	Reflect green rooftops as woods in good condition.

Example

In the green rooftop credit, green rooftops are subtracted from the total site impervious cover, and assigned a curve number for woods in good condition. In order to receive the credit, green rooftops must meet the following criteria:

The system shall include a 6" soil bed, with a silt loam texture.
The soil bed shall be underlain with a 2" gravel layer, and these two layers shall be separated by a layer of filter fabric.
An impermeable layer shall be placed between the rooftop and the gravel layer.
The roof shall have a maximum slope of 25%
The roof shall be designed to hold an additional 25 lbs/sf, beyond minimum regional design criteria
Vegetation shall be established within two growing seasons.
Vegetation should require minimal fertilization, watering and pesticides.
A 2" mulch layer shall be immediately placed above the soil layer to prevent erosion.
The vegetation and mulch layer shall be maintained at least quarterly, removing dead vegetation and eroded mulch.
If the rooftop is used as an amenity (e.g., a rooftop sitting area) as well as to detain stormwater, credit shall only be applied to pervious sections of the rooftop.
The credit shall only apply for households and businesses where owners sign a maintenance agreement.

The water quality and recharge credit can be calculated with the following equation:

C = (A_GR/A_I)WQ_vWhere:
C = Green Rooftop Credit (ac-ft)
A_GR = Green Rooftops (acres)
A_I = Site Impervious Area (acres)
WQ_v = Original Water Quality Volume (ac-ft)
The recharge and water quality volumes are then reduced by the credit, C.

Quantity credit is achieved by assigning disconnected rooftops a curve number equal to forest in good condition. The example below illustrates how this credit would be applied.