Apprentice roughed in a bathroom lav: 1-1/2 inch P-trap, trap arm runs 9 feet horizontally at 1/4 per foot to a sanitary tee on the 2-inch vent/stack.

Two major code violations (IPC/UPC):

1. Trap Arm Length Exceeded (IPC 1002.4 / UPC 909.1 / 1014.1): Maximum developed length for a 1½" trap arm is 5 feet (60 inches) to the vent connection. At 9 feet (108 inches), you have no vent protection. The trap will siphon dry. This is the primary failure.

1. Trap Size Mismatch (IPC 701.2 / UPC 701.1): A 1½" trap arm cannot connect directly to a 2" sanitary tee on the stack. The trap arm diameter must equal the trap outlet (1½") or the drainage pipe size (2"), but you cannot reduce the trap arm downstream of the trap. You need a 1½" x 2" increaser at the tee, or a 2" trap arm with a 2"x1½" reducer at the trap outlet.

Fix:

• Option A (Preferred): Move the fixture ≤5 ft from the stack.
• Option B: Install a vent take-off within 5 ft of the trap (vertical vent up through roof or tie into stack above flood level). The 9-ft horizontal run then becomes a drain, not a trap arm.
• Option C: Use an AAV (Air Admittance Valve) on a vent take-off within 5 ft of trap if local amendment allows (IPC 917 / UPC 603.5.2).

Slope Note: ¼"/ft is correct for 1½"–2" pipe, but irrelevant if the vent distance is illegal.

Tell the apprentice: "Slope drains water; vents protect traps. You have a 9-ft trap arm with no vent. Rip it back."

The problem is the trap arm length exceeds the maximum allowed for a 1½-inch trap under the International Plumbing Code (IPC) and Uniform Plumbing Code (UPC).

• IPC Section 1002.2 and UPC Section 1002.2 state: > The developed length of the trap arm shall not exceed 24 inches for a 1½-inch trap. (Note: Some jurisdictions allow up to 36 inches if vented within 6 inches of the trap weir, but 9 feet is never permitted.)

• A 9-foot (108-inch) trap arm is far beyond the allowable limit — even with proper slope (¼” per foot) and venting at the end.

Why it matters: Excessive trap arm length causes self-siphonage or momentum loss, breaking the trap seal during fixture use. This allows sewer gases to enter the bathroom — a health and safety hazard.

Correction: Relocate the sanitary tee (or install a vent) within 24 inches of the trap weir (measured along the pipe centerline). If the fixture is far from the stack, consider:

• Reducing the trap arm to ≤24″ and adding a vent within 6″ of the trap,
• Or using a wet vent or circuit vent if permitted and designed per code (requires engineering review for 9-foot runs).

Bottom line: “Sloped right and vented” is insufficient — trap arm length is a separate, critical limit. This installation is non-compliant and unsafe. Shut it down and re-route. (Always verify local amendments — some AHJs may have stricter rules.)