Master Plumber Practice Exam 2026 – Complete Prep Guide

The maximum horizontal distance of a trap arm is measured from the weir of the trap. The weir is the highest point of the trap where water sits before it enters the pipe leading away from the trap. This area is crucial because it establishes the water seal that prevents gases from entering the home.

Measuring from the weir ensures that the distance accounts for the gravitational flow of wastewater and maintains the trap's ability to function effectively. By focusing on this point, plumbing codes can set standards for the installation of plumbing systems that effectively prevent issues such as siphoning or backflow, which could occur if the trap arm is too long or improperly installed.