7.2 Trap-to-Vent Distance (Table 909.1) & Vent Pipe Sizing (906)

Protecting the trap from self-siphonage (IPC 909)

Every fixture trap needs a protecting vent close enough that the fixture's own discharge cannot siphon the seal.

Table 909.1: maximum distance of fixture trap from vent

• 909.1: the slope and developed length of the fixture drain, from the trap weir to the vent fitting, must be within Table 909.1.
• Exception: the developed length is not limited for self-siphoning fixtures such as water closets.
• Developed length is measured along the centerline of the pipe and fittings, so bends count.

The one-diameter rule (IPC 909.2)

The total fall in a fixture drain due to pipe slope shall not exceed the diameter of the fixture drain, nor shall the vent connection to a fixture drain, except for water closets, be below the weir of the trap.

Table 909.1 follows this rule. For a 1-1/2-inch drain, 6 feet × 1/4 inch per foot = 1.5 inches of fall, exactly one diameter. For a 2-inch drain, 8 feet × 1/4 inch = 2 inches. If the fall exceeded the pipe's diameter, the vent opening would be below the trap weir and the drain could run full, so the seal would siphon out behind the discharge.

Example: a 1-1/2-inch lavatory drain run 6 feet at 1/2 inch per foot falls 3 inches. That is twice the pipe diameter, so it violates 909.2 even though the 6-foot length is within the table.

Crown vents (IPC 909.3 and 1002.3)

A vent must not be installed within two pipe diameters of the trap weir. Crown-vented traps are prohibited (1002.3), because the vent opening fouls with waste splashing over the weir. For a 1-1/2-inch trap arm, the vent fitting must be at least 3 inches from the weir.

Individual vents (IPC 910.1)

Each trap or trapped fixture may have an individual vent, which connects to that fixture's drain. An individual vent is "a pipe installed to vent a fixture trap and that connects with the vent system above the fixture served or terminates in the open air."

Sizing vents other than stack vents and vent stacks (IPC 906.2 to 906.4)

• 906.2: individual, branch, circuit and relief vents must be at least one-half the required diameter of the drain served. The required drain size comes from Table 710.1(2). No vent may be smaller than 1-1/4 inches.
• Vents longer than 40 feet in developed length are increased one nominal pipe size for their entire length.
• Relief vents for stacks with more than 10 branch intervals are sized under 908.2 (the size of the vent stack).
• 906.3: for these vents, developed length is measured from the farthest point of vent connection to the drainage system to the vent stack, stack vent or outdoor terminal.
• 906.4: a common branch vent serving several branch vents is sized from the horizontal drainage branch that would be required for the total dfu being vented.

Example: a 2-inch fixture branch vent with a developed length of 55 feet. Half of 2 inches is 1 inch, so the minimum is 1-1/4 inches. Because it is longer than 40 feet, increase it one size, to 1-1/2 inches.

Stack vents and vent stacks: Table 906.1

906.1: the diameter is found from the developed length and the total dfu connected, but is never less than one-half the diameter of the drain served or 1-1/4 inches. Developed length is measured from the vent connection to the open air.

Selected rows (maximum developed length of vent, in feet):

The 1-1/2-inch stack rows also allow a 1-1/4-inch vent: 50 feet at 8 dfu and 30 feet at 10 dfu.

How to read it: find the row for your stack size with a dfu value equal to or greater than your load. Move across to the first vent size whose length is equal to or greater than your developed length.

Worked examples

1. 4-inch stack, 120 dfu, vent stack developed length 180 feet. Use the 4-inch, 140-dfu row. A 2-inch vent allows 27 feet and a 2-1/2-inch vent 65 feet, both too short. A 3-inch vent allows 200 feet, which works. It also meets the one-half rule (at least 2 inches).
2. 3-inch stack, 18 dfu, stack vent 90 feet. Use the 3-inch, 21-dfu row. A 1-1/2-inch vent allows only 32 feet. A 2-inch vent allows 110 feet, which works.
3. Vent header (904.5). Two 4-inch stacks of 150 and 170 dfu join a common header. Size the header for 320 dfu, using the longest length measured from the base of the most distant stack.

Common exam traps

• Table 909.1 has one 3-inch row: 12 feet at 1/8 inch per foot.
• Trap arm distance is measured from the trap weir to the vent fitting.
• Water closets are not limited by Table 909.1.
• Individual vents are sized under 906.2 (one-half the drain diameter); vent stacks and stack vents use Table 906.1.