Stormwater Quality Design Manual (May 2007) - City of Sacramento ...

Runoff Reduction Fact SheetDisconnected Roof DrainsHow do Disconnected Roof Drains work?Disconnected Roof Drains effectively disconnect the rooftop from the local storm system, which helpsreduce runoff and provides incidental pollutant removal as the water travels over and through thevegetation and soil. In this approach, roof runoff is directed to spread over a vegetated area (theconveyance zone), or into underground infiltration devices, if approved by the local permitting agency.Greater surface area and contact time within the conveyance zone promote greater runoff treatmentefficiencies.Other Names: Disconnected downspouts, disconnected roof leadersPlanning and Siting Considerations• Consult a geotechnical engineer about site suitability and other design considerations.• Assess soil permeability to determine if infiltration option is viable for the type of system desired.Consult an engineer if needed, particular in areas adjacent to building foundations.• Design buildings to take advantage of vegetated areas. Direct roof flow away from pavedsurfaces.• Design site with a minimum of 2% positive slope away from building foundations.• Maximize the length, and minimize the slope, of the conveyance zone. The land surrounding thedownspout/emitter should be graded to spread and convey storm water (minimum 2 feet wide)and prevent concentration of flows.• Integrate the disconnected roof drain system into the site landscaping plan.• Consider using dry wells or dispersal trenches where the conveyance zone slope exceeds 25% andlocal permitting agency allows. Such devices must typically be located a minimum of 20 feetfrom any buildings, but verify with local permitting agency.• Use of dispersal trenches and dry wells maybe restricted on commercial and industrial projectsdepending on pollutant potential; check with local permitting agency.VariationsFour types of disconnected roof drain systems are discussed in this fact sheet: 1)splash block, 2) pop-updrainage emitter, 3) dispersal trench and 4) drywell. Check with the local permitting agency to determineif all types are allowed and if they have any local specifications or details to add to, or replace, thoseshown here.Splash BlockSplash Blocks reduce the velocity and impact of water exiting the roofdownspout and direct water to a pervious conveyance zone. Storm watertraveling across the conveyance zone is filtered and infiltrated. Where theslope of the conveyance zone is greater than 8%, a gravel level spreaderis required at the end of the splash block. A gravel spreader is a pocket ofgravel that collects water and encourages sheet flow. The spreader maybe covered with geotextile, soil and vegetation to fit with sitelandscaping.For single family residential sites, the stormwater must flow acrossappropriate vegetation throughout the entire conveyance zone (fromthe downspout to the sidewalk). For commercial and multi-family sites,minimum travel distances across vegetation are specified based oncontributing roof area (see Table DRD-1).Source: Alemeda Countywide CleanWater ProgramStormwater Quality Design Manual for the Sacramento and South Placer RegionsDRD-2 May 2007