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What is the minimum trap arm length?
Tuesday, November 16, 2021
The minimum trap arm length is two pipe diameters, as measured from the trap weir to the vent (not between the hubs of the fittings). So a 1-1/2” pipe would require a 3” minimum trap arm, and a 2” pipe needs a 4” minimum trap arm. Here’s an example of one that just barely squeaks by with a 3-1/2” distance between air admittance valve (vent) and trap with 1-1/2” pipes.
It is not stated directly in the International Residential Code (IRC), but as a reference to a prohibited type of vent at P3105.3: “Crown vent prohibited. A vent shall not be installed within two pipe diameters of the trap weir.” Essentially, a trap arm that’s too short creates the equivalent of an obsolete crown vent. See our article What is a crown vent? for details. Also, our article What is the minimum and maximum slope of the trap arm of a plumbing drain? explains trap arm slope parameters and maximum trap arm lengths.
Here’s an example of an incorrect configuration, where the trap dives before the vent. It is essentially a prohibited S-trap because there is no trap arm (also called a waste arm). "The risk is that if the sink is filled up to wash dishes and then drained it will siphon the trap dry (much like a toilet flushing) and then it can allow sewer gasses into the room,” according James Freeman, of J.W. Freeman Plumbing
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Field Guide for Home Inspectors, a quick reference for finding the age of 154 brands of HVAC systems, water heaters, and electrical panels, plus 210 code standards for site-built and manufactured homes, and the life expectancy rating of 195 home components. Available at amazon.com for $19.95.
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