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 building trap?  

• 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)?

• What is a P-trap?

• 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.