Sanitary Drainage and Storm Drainage Systems
Sanitary Drainage and Storm Drainage Systems
This domain keeps the two drainage worlds apart - sewage and rainwater - and tests the components that carry each: building sewers, manholes, ejectors, rain leaders, roof and area drains, and subsoil drainage.
Sanitary system
The building drain ends outside the foundation and becomes the building sewer, which runs at code slope to a public sewer, septic system or treatment unit. Sewers need cleanouts at the building, at intervals, and at changes of direction. Fixtures below the elevation of the next upstream manhole are subject to backflow and need a backwater valve or a lift pump. Below-grade fixtures that cannot drain by gravity require a sewage ejector: a vented, sealed basin, duplex pumps where required, check valves and an audible alarm.
Storm system
Rainwater must be handled separately; connecting storm drainage to the sanitary sewer is prohibited. Size roof drains and leaders from roof area and design rainfall intensity, and provide secondary drainage such as scuppers or overflow drains. Area drains, downspouts and footing drains go to storm systems, drywells or grade. Sump pumps handling groundwater or clear water discharge to grade or storm - not to the sanitary sewer.
Worked example. A flat roof measures 60 ft × 40 ft = 2,400 sq ft. At a design intensity of 4 in. per hour, runoff ≈ 2,400 × 4 × 0.0104 = 100 gpm. Enter the storm drainage table at 100 gpm to select the roof drain and leader size, and add a secondary drain or scupper.
Traps. Tying downspouts into the sanitary sewer; dumping a sump pump into the house drain; sizing leaders by square footage alone without rainfall intensity; omitting a backwater valve on a below-street fixture.