Sanitary Drainage, Waste, and Venting
The Sanitary Drainage, Waste, and Venting domain is the heart of the master-level exam: slope, fixture units, trap protection, and vent geometry, often combined in one scenario.
Slope, sizing, and layout
- Minimum slope: 1/4 in/ft for 2-1/2 in and smaller, 1/8 in/ft for 3 in and larger, measured on the pipe, not the trench bottom.
- Size from DFU tables; the building drain may be larger than the stack it serves but never smaller downstream than upstream.
- Change direction on horizontal drains with long-radius fittings or two 45s; use sanitary tees for vertical-to-horizontal transitions.
Traps and trap arms
- Trap seal: minimum 2 in, maximum 4 in. One trap per fixture; no double trapping and no S-traps.
- The trap arm must be vented, and its fall may not exceed its own diameter. That is why a 2 in arm at 1/4 in/ft is effectively capped near 8 ft.
- Trap primers protect floor drains in boiler rooms, elevator pits, and rarely used fixtures.
Venting
- Every trap gets a vent sized per the vent tables; vents may reduce to no less than one-half the drain size where the code permits.
- Vent terminals extend above the roof at the height the code sets (6 in minimum generally, higher where the roof is a deck) and away from operable windows and air intakes.
- Wet venting is allowed only under the code's conditions: same bathroom group, correct sizing, correct developed length.
Worked example
A bathroom group with a 3 in water closet, 1-1/2 in lavatory, and 1-1/2 in tub. At 1/4 in/ft the lav trap arm falls its own diameter in 6 ft, but the table limit for 1-1/2 in is usually 5 ft, so the vent must tie in within 5 ft, at the wall behind the lav rather than mid-vanity. A 2 in lavatory drain may then wet-vent the water closet if the length and sizing limits are respected.
Traps
S-traps, over-long trap arms, an unvented tub, venting a floor drain from a stack below the flood level rim, and omitting cleanouts at the stack base.