Storm Drainage and Special Waste Systems
Storm Drainage and Special Waste Systems tests roof drainage hydraulics, secondary drainage, and the separate piping systems that must never meet the sanitary drain.
Roof drainage
- Calculate storm flow from the horizontally projected roof area, including overhangs, and the local design rainfall intensity: GPM = 0.0104 x rainfall (in/hr) x area (ft2).
- Size roof drains, leaders, and horizontal storm pipe from the code tables using that GPM, remembering that vertical leaders and horizontal drains have different capacities at the same size.
- Where a roof is enclosed by parapets, provide secondary or overflow drainage so a blocked primary drain does not pond the roof to failure.
- Storm drains slope to the discharge, stay separate from the sanitary system, and may not receive sewage or industrial waste.
Special wastes
- Indirect waste receptors receive a pipe terminating with an air gap or air break, never a direct connection.
- Grease interceptors are required for commercial kitchens, sized on fixture capacity and the code's detention criteria, fitted with a flow-control fitting, and accessible for pumping.
- Acid and chemical wastes go to neutralizing or dilution systems in corrosion-resistant pipe; dental, laboratory, and photographic wastes are typical.
- Condensate, pool backwash, and relief valve discharge go to approved indirect waste, not to a floor drain connection below the flood level rim.
Worked example
A 5,000 ft2 roof in a region with a 4 in/hr design storm: 0.0104 x 4 x 5,000 = 208 gpm. Two roof drains each sized for at least 104 gpm from the table, with leaders sized for that flow, plus a secondary system sized for the code's required fraction of the storm.
Traps
Using 3 in/hr when the jurisdiction adopts 4, ignoring the leader-versus-horizontal capacity difference, connecting a leader to a sanitary drain, and omitting the grease interceptor flow control.