Core TradesQuestion 93 of 700
CALCULATION: A vertical drainage stack must accommodate horizontal branch offsets. A trap arm for a 2-inch fixture drain is allowed a maximum developed length. If the code table permits 8 feet for a 2-inch trap arm, and the layout requires 11 feet to the vent, the installer should:
a.Ignore the limit; length does not matter
b.Relocate the vent or add a vent so the trap arm is within 8 feet
c.Increase the slope to 1 inch per foot
d.Use a smaller 1-1/2 inch trap arm
Explanation
If the required trap-arm length exceeds the code maximum for that size, the fix is to bring the vent closer (relocate or add a vent) so the arm is within the allowed developed length, preventing self-siphonage. Ignoring the limit risks trap-seal loss; over-steepening or downsizing does not resolve the length limit.
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