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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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Here’s links to a collection of our other blog posts about PLUMBING PIPES:
• What is the minimum and maximum distance a P-trap can be below a sink?
• How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?
• How can I tell if I have cast iron pipes in my house?
• Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection?
• What is the difference between green and white sewer drain pipes?
• Is a washing machine drain hose required to be secured at the standpipe?
• What are the abandoned pipes sticking out of the wall in my house?
• What are the code requirements for plumbing vent terminations?
• What are the code requirements for layout of drain piping under sinks?
• What causes a gurgling sound when a bathtub or sink drains?
• What is a "combination waste and vent" in a plumbing system?
• What is a galvanized nipple?
• What are the pipes sticking out near my water valves?
• How do you accurately find a broken water pipe leak under the floor slab?
• What is the difference between water pipe and sewage (waste) pipe?
• Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water?
• If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?
• How can I tell what type of plumbing pipe I have?
• Why is there a flexible accordion pipe under the sink?
• What is the difference between PVC and ABS plumbing pipe?
• What is the difference between water service pipe and water supply pipe?
• What are the pipes on my roof?
• How can I find out what type of water pipe runs underground from the water meter to the house (service pipe)?
• Why is old galvanized steel water pipe a problem for homebuyers?
• What does polybutylene pipe look like? Why is it a problem?
• Which water pipes are an insurance problem and possibly uninsurable?
• Can you connect CPVC pipe directly to a gas water heater?
Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.
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