Sanitary Drainage, Vents, and Storm Drainage
Sanitary Drainage
Loads and sizing
Drainage sizing starts with drainage fixture units (DFU) from the fixture-unit table, summed on each branch, stack, and the building drain, then read against the sizing table for the chosen slope. Keep DFU and water-supply fixture units strictly separate - mixing them is the most common arithmetic trap.
Slope and fittings
Slopes are 1/4 in./ft for 2-1/2 in. and smaller, 1/8 in./ft for 3 to 6 in., and 1/16 in./ft for 8 in. and larger. Pipe may not reduce in size in the direction of flow. Changes in direction require long-turn or combination fittings sized to the pipe. Cleanouts are required at the base of each stack, at changes of direction greater than 45 degrees, at intervals not exceeding 100 ft, and with 18 in. of clearance for rodding.
Special drainage
Backwater valves protect fixtures whose drainage is below the crown of the public sewer or where the code official requires them. Grease interceptors serve food-service sinks; sand/oil interceptors serve garages. Sewage ejectors and sumps handle sub-sewer drainage and must be vented. Traps are protected against siphonage and back pressure.
Traps in questions
- Applying 1/4 in./ft to a 4 in. line rather than 1/8 in./ft.
- Assuming any stack can carry any load; the stack table caps DFU by size.
- Omitting a cleanout at the base of the stack where the drain turns.
- Forgetting the backwater valve on a below-grade fixture group.
Worked example
A branch serving two private water closets (3 DFU each), two lavatories (1 DFU each), and one floor drain (2 DFU) totals 10 DFU. The reviewer confirms the plan's branch size comes from the horizontal-branch table, not the building-drain table.
Vents
Purpose and types
Every trap must be vented. Know individual vents, common vents, wet vents, circuit vents, stack vents, and relief vents, plus air admittance valves. A stack vent is the extension of a soil stack above the highest fixture; a vent stack serves the drainage stack through a relief or yoke vent. Wet venting is limited by fixture units, pipe size, and which fixtures may drain past a wet-vented fixture.
Sizing and geometry
Vent size comes from the vent-sizing table based on the fixtures it serves, with a minimum of 1-1/4 in. A vent must rise vertically at least about 6 in. above the flood-level rim of the highest fixture served before it offsets. Trap arm length from the trap weir to the vent fitting is limited by pipe size and slope: at 1/4 in./ft, roughly 5 ft for 1-1/4 in., 6 ft for 1-1/2 in., 8 ft for 2 in., 12 ft for 3 in., and 16 ft for 4 in.
Terminals
Vents extend not less than 6 in. above the roof; where the roof is used for other than weather protection they must extend higher (about 7 ft). Keep terminals away from operable openings and lot lines as required.
Traps in questions
- A larger vent does not fix a trap arm that is too long - the arm itself must change.
- Air admittance valves must be listed, accessible, and cannot substitute for a required vent terminal.
- Offsetting a vent below the flood-level rim of the fixture it serves is a frequent failure.
- Wet vent loads are limited; two bathrooms back to back are not automatically allowed.
Worked example
A 2 in. trap arm runs 9 ft to its vent. The limit for 2 in. at 1/4 in./ft is 8 ft, so the design fails; move the vent 1 ft closer or upsize the arm to 3 in. (12 ft limit).