4.2 Trap-to-Vent Distances (Table 382.31-1)

Why the distance is limited

A fixture drain flowing full can siphon its own trap if the vent is too far away. The drop between the trap weir and the vent opening must stay small enough that the seal is not pulled. Wisconsin controls this with Table 382.31-1, "Maximum Developed Length Between Vent and Trap." SPS 382.31 (9) (a) requires each fixture trap to be protected by a vent within those limits.

Where the measurement starts and ends

• Separate traps: from the trap weir to the vent connection (9) (a) 1.
• Integral traps (water closets, many urinals): from the fixture outlet to the vent connection (9) (a) 2.
• Floor-outlet water closet: the point where the fixture drain turns horizontal counts as the fixture outlet.
• Minimum distance: a vent may not connect within 2 pipe diameters of the trap weir (9) (b). On a 1-1/2 inch arm, the vent must be at least 3 inches downstream of the weir.

Diameters are chosen from the smallest drain pipe installed downstream of the trap serving the fixture (footnote a).

Table 382.31-1 (feet)

Pitch is in inches per foot. NP = not permitted. Lengths for fixture drains over 4 inches need department approval. The "wye pattern fitting" means a tee-wye, a combination wye and eighth bend, or a wye plus eighth bend with no more than 1 inch between them (footnote b).

Three patterns to notice

1. Steeper pitch means a shorter allowed length. Doubling the pitch from 1/4 to 1/2 inch per foot cuts every allowance in half or more, because the drop across the arm grows quickly and can put the trap weir above the vent opening.
2. 1/8 inch per foot is allowed only for 3 and 4-inch fixture drains. Smaller drains must be pitched at 1/4 or 1/2 inch per foot (and the branch pitch rule in SPS 382.30 (5) requires 1/4 inch per foot for drains 2 inches or less anyway).
3. Vertical connections are shorter than horizontal ones, and a wye-pattern connection to vertical piping is shortest of all.

Worked examples

Lavatory. A 1-1/4 inch lavatory drain is pitched 1/4 inch per foot to a vent on horizontal drain piping. The table allows 5.0 feet. If the arm instead drops into a vertical drain through a sanitary tee, the limit is 3.5 feet.

Shower. A 2-inch shower drain runs 7 feet at 1/4 inch per foot to a sanitary tee in a vertical wet-vented drain. The limit is 6.0 feet, so 7 feet fails. The fix is to add a vent closer to the trap, or reroute so the vent is on horizontal piping (8.0 feet allowed).

Water closet. A floor-outlet water closet on a 3-inch fixture drain is measured from where the drain turns horizontal under the floor. At 1/4 inch per foot to a vent on horizontal piping, the limit is 12 feet. At 1/8 inch per foot it is 24 feet, which is why long water closet runs are often pitched at the minimum.

Where else Table 382.31-1 applies

The same limits are written into other venting methods:

• Common vents on horizontal branches: the drain from the common vent to the farthest trap must meet (9).
• Wet vents: both fixture drains in a vertical wet vent must meet (9). The lower drain's connection is treated as the vent connection.
• Circuit vents: each fixture drain's connection to the circuit-vented branch is treated as its vent connection.
• Combination drain and vent systems: fixture drains to the stack, and building drain branches, may not exceed the table.
• Garage catch basins: trapped drains discharging to a basin must be within the table or be vented.
• Chemical waste basins: the drain between a submerged basin inlet or outlet and the vent must meet the table.

Exam strategy

Check three things in order before you read the table: the diameter (smallest pipe downstream of the trap), the pitch, and the type of vent connection (horizontal, vertical sanitary tee or vertical wye). Most wrong answers come from reading the horizontal column for a vent that actually connects to a vertical pipe.