Rooftop Disconnection

Rooftop Disconnection

This credit can be applied to encourage disconnection of rooftops, thus promoting overland treatment of these surfaces. The credit needs to clearly document criteria to make this disconnection successful. Credits can be applied to water quality, recharge, and quantity control requirements.

Rooftop Disconnection: Summary
Goals	Encourage the use of overland flow or infiltration areas to treat rooftop runoff.
Stormwater Management Objectives	Water Quality/ Recharge	Subtract the total disconnected area from the site's impervious cover.
	Channel Protection, Flood Control	Adjust hydrologic parameters of disconnected rooftops to reflect woods in good condition.

Example Language:

In the rooftop disconnection credit, disconnected 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, disconnections must meet the following criteria:

The rooftop cannot be a designated hotspot.
Disconnection must ensure no basement seepage.
The contributing length of rooftop to a discharge location shall be 75 feet or less.
The rooftop contributing area shall be no more than 1,000 sq. feet per disconnection.
The length of the "disconnection" shall be equal to or greater than the contributing rooftop length.
Disconnections will only be credited for residential lot sizes greater than 6000 sq. ft.
The entire vegetative "disconnection" shall be on a slope less than or equal to 3.0%.
The disconnection must drain continuously through a vegetated channel, swale, or through a filter strip to the property line or STP.
Downspouts must be at least 10 feet away from the nearest impervious surface to discourage "re-connections."
Disconnections are encouraged on relatively permeable soils (HSGs A and B) without soil testing.
In less permeable soils (HSGs C and D), the water table and permeability shall be tested by a geotechnical engineer to determine if a spreading device is needed to provide sheetflow over grass surfaces. In some cases, dry wells, french drains or other temporary underground storage devices may be needed to compensate for a poor infiltration capability.
For those rooftops draining directly to a stream buffer, one can only use either the rooftop disconnection credit or the stream buffer credit, not both.
Credit is documented the concept plan stage and then is verified with the final grading plan and as-built certification, or as required by the appropriate review agency.

The use of this credit should be accompanied with an educational program to ensure that disconnected areas are not "reconnected" by residents over time.

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

C = (A_DR/A_I)WQ_v

Where:
C = Rooftop Disconnection Credit (ac-ft)
A_DR = Disconnected Roof Area (acres)
A_I = Site Impervious Area (acres)
WQ_v = Original Water Quality Volume.

The recharge and water quality volumes would both be 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.