Plumbing Vent Sizing
Vent stack size from your fixture units and vent length, trap arm length and pitch, the venting configuration rules, and the roof termination, all from the IPC tables.
A 12 DFU vent run 40 ft long is a 1-1/4 in vent stack, because IPC Table 906.1.1 vents 24 DFU at 1-1/4 in under 120 ft. A 1-1/4 in trap arm is limited to 8 ft of developed length, pitched at 1/4 in per ft, and a 2 in vent carries 42 DFU at 120 ft, 12 DFU past 500 ft.
How the math works
- Vent size (IPC Table 906.1.1): the table gives the drainage fixture units vented by each vent pipe size at five vent lengths (120, 200, 300, 400, and 500 ft and longer). 1-1/4 in vents 24 DFU at 120 ft, 1-1/2 in vents 42 DFU at 120 ft, 2 in vents 42 DFU, 3 in vents 110 DFU, 4 in vents 160 DFU, 5 in 210, 6 in 260, 8 in 590, 10 in 900, and 12 in 1,400, and every length column past 120 ft costs capacity.
- Fixture units (IPC Table 709.1): a private fixture counts lower than the public version of the same fixture: water closet 3 DFU, lavatory 1 private and 2 public, kitchen sink 1 private and 2 public, laundry tub 2, dishwasher 2, clothes washer 2, bathtub 2, shower 2, shower stall 3, bathtub with a jet 3, sump pump 2, water heater 2, garbage grinding unit 2, urinal 2 private and 3 public, combination washer-dehydrator 3, floor drain 2 at 1-1/2 in, 3 at 2 in, 4 at 3 in, and a group of 3 fixtures with a water closet on a 2 in drain is 6 DFU. Count your own fixture list against the table, on the private column for a house.
- Trap arm (IPC 709.3): maximum developed length of a trap arm: 1-1/4 in is 8 ft, 1-1/2 in is 12 ft, 2 in is 12 ft, 2-1/2 in is 20 ft, 3 in is 30 ft, and 4 in is 45 ft, measured from the trap to the outlet of the trap arm, with the trap arm pitched to the drain and the trap arm outlet one pipe diameter from the trap arm.
- Slope (IPC 704.3): 1/4 in per ft (2 percent) for piping 2-1/2 in and smaller, 3/16 in per ft (1.5 percent) at 3 in, 1/8 in per ft (1 percent) for 4 in through 8 in, and 1/16 in per ft (0.5 percent) at 10 in and larger, with the building drain sized on its own table. A drain pitched over 1/2 in per ft self-scours and leaves the solids behind.
- Configuration (IPC 905, 906, 907, 908): individual vents, stack vents, vent stacks, circuit vents, loop vents, drum vents, and air admittance valves (905.5). The stack vent goes through the roof and is open, an AAV cannot replace it, and a vent connection sits above the flow in the drain, not below it.
What size is a plumbing vent pipe?
A residential vent stack is 2 in for most houses, 3 in for a two-bath house, and 4 in on a three-bath house with a 3 in drain. The size comes from the drainage fixture units on the vent and the length of the vent run in IPC Table 906.1.1, not from a rule of thumb, and the fixture units come from the fixture count in Table 709.1.
How long can a trap arm be?
By the trap arm pipe size: 8 ft at 1-1/4 in, 12 ft at 1-1/2 in, 12 ft at 2 in, 20 ft at 2-1/2 in, 30 ft at 3 in, and 45 ft at 4 in. The measurement is from the trap to the outlet of the arm, and the arm has to be the same size as the trap it serves.
Can I use an air admittance valve instead of a vent?
At the fixture, yes in most IPC jurisdictions: an AAV on an individual fixture vent, 6 in above the flood rim, accessible, not on the building drain, not on the stack vent. The main stack still goes through the roof, and an AAV is not a roof vent substitute.
How high does a vent pipe have to come through the roof?
6 in above the roof at the penetration (IPC 905.3.1), 12 in where the roof carries a snow load, and 10 ft from any opening that can take air into the building. A vent that stops 2 in above the deck gets buried in the drift and blocks the stack.