Kitchen island sink not vented!

Is this an illustration of a crown vent? I don’t see what that has to do with an S-trap discussion.

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What does that mean? If you read the article this formula has nothing to do with the difference between a P-trap and an S-trap.

If the distance between the trap weir and vent is less than 2 pipe diameters it’s called a crown vent.

I’m still waiting for someone to come up with a real formula for what is a S-trap. It must come from a code book.

I found this:

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Yes. The discussion is about an island vent. S vent was added in later. My example shows one way to vent it. You could add an AAV or a circuit vent.

“ASSE 1051 - Type A devices may only be used as individual vents for a single fixture drain. ASSE 1051 - Type B devices may serve as branch vents for one or more individual vents, including the dry vent connecting to a common vent, wet vent or circuit vent.”

There isn’t a single way to do this. That’s all I trying to point out.


What modern code book is this from?

You could add a AAV but can you describe to me how to add a circuit vent to the OP’s illustration?

I was attempting to find a definition and contribute. I’ll be sure to follow the rules moving forward. SMH

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I’m looking for a specific definition of a S-trap from a code book only. Anything else is an opinion or speculation.

Not without seeing an iso

Well, maybe they need to inform us all!
This is all I see in the IPC so far:

1002.3Prohibited traps.

The following types of traps are prohibited:

  1. 1.Traps that depend on moving parts to maintain the seal.
  2. 2.Bell traps.
  3. 3.Crown-vented traps.
  4. 4.Traps not integral with a fixture and that depend on interior partitions for the seal, except those traps constructed of an approved material that is resistant to corrosion and degradation.
  5. 5.“S” traps.
  6. 6.Drum traps.

Yup it’s prohibited but was is the definition of a S-trap. Does it have specifics like a crown vent? Looking for code book only definitions.

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You got it.

Here is a close one: (from link)

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

What is the minimum trap arm length? (

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Crown vent Daniel :slightly_smiling_face:

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But I guess thats as close as we get with code, since it talks about 2x diameter.

Interesting stuff, glad you brought that up!


Daniel it’s food for thought as the IPC and UPC have no definition for a S-trap but they will tell you it is illegal.

Just trying to get those that say it is or it isn’t an S-trap to think.

There’s a clear definition of a P-trap with dimensions in both code books.

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There is no definition in the plumbing code. IPC or UPC. S trap is just common lingo. It is basically a non U-shaped piping arrangement.

Beat me to it

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So in my opinion the OP picture does display an S-trap. Prove me wrong :rofl:

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So now you have me curious what you were taught

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Agreed. Was it an opinion? He was one of Illinois 5 state inspectors so I’m going to lean towards his teaching.