Question 1

What is the maximum vertical distance allowed between a trap weir and the fixture drain inlet (trap-to-vent distance is separate) — specifically, what is the maximum trap arm length for a 1-1/2" trap?

Accepted Answer

The maximum trap arm length for a 1-1/2" trap is 42 inches (3.5 feet) per the IPC. The trap arm is measured from the trap weir to the inner edge of the vent.

Question 2

What is the minimum water seal depth required for a fixture trap under the IPC?

Accepted Answer

The minimum water seal depth for a fixture trap is 2 inches. The maximum seal depth is 4 inches.

Question 3

Which type of trap is explicitly prohibited by the IPC in DWV systems?

Accepted Answer

The S-trap is prohibited because it is self-siphoning — the full flow of drainage can pull the water seal out of the trap, allowing sewer gases to enter.

Question 4

What is the maximum trap arm length for a 3" trap according to the IPC?

Accepted Answer

The maximum trap arm length for a 3" trap is 72 inches (6 feet), measured from the trap weir to the inner edge of the vent.

Question 5

Under the IPC, what is the rule regarding double trapping a single fixture?

Accepted Answer

Double trapping (installing two traps in series on a single fixture) is prohibited because the air space between the two traps prevents proper drainage and can cause the seals to be lost.

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