S-Trap or Poorly Done P-Trap?

Would this be considered an S-Trap or a poorly design P-Trap? This was under the sink of the master bathroom. I believe it to be a P-Trap, but it resembles an S-Trap in its form.

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Supposed to be a P trap but somebody moved the wash basin almost a foot to the left and couldn’t figure out the waste line.

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Wouldn’t call it an s-trap, but an improperly installed p-trap. Specifically, the weir should not be above the vent opening.


Welcome to our forum, Justin!..Enjoy. :smile:

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It is not correct and needs to be redone. Should have extended the tail pipe to make a correct P-trap work.

What is the measurement between the bottom of pipe and the cabinet?

Looks like they piped it this way because in their mind there wasn’t enough room to have a longer tail pipe extension.


The length of the extension needs to be 2-1/2 times the diameter of the pipe in order to eliminate the possibility of siphoning.


That’s kind of what I was thinking. Was probably a handyman based repair/replacement. The setup in the other sink(this is a double sink vanity) is even more twisted around than this one. Thank you all for your assistance.

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Judging from the photo, the short ><2" waste line extension off the trap, which should/could have been longer, I would consider this an S-Trap.
As Larry Cage correctly indicated per usual is, the distance of travel between the trap and any additional 90° elbow is a minimum distance of 2.5 times the diameter of the pipe, 2" pipe, which would be 5" inch minimum.

Observation: S-Trap.
Recommend: A licensed plumbing contractor rearrange the drain pipes under the sink to create a P-Trap to avoid any chances of back siphonage and sewer gas entering the residence.

How to Replace a Plumbing S-Trap With a P-Trap


I would too. I’m not sure what the 2.5 diameter thing is all about unless somebody is referencing a crown vent. Very different animal all together.


Two things are needed to create an s-trap out of a p-trap that connects to a vertical drain pipe:

a) short trap arm (less than 2x its pipe diameter)
b) lack of vent

If you only have [A], then it’s a crown vent, also not allowed in most areas.
If you only have [B], then it’s a p-trap without a vent, not an s-trap.

Anything that does not fit exactly A&B I just call improper p-trap because there are things like, for example, a vertical leg drain. And so, to keep things less technically confusing and or open to debate, I just simplify it. Let the plumber call it whatever the heck he wants :slight_smile:

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I didn’t pay attention to it before but clearly the drain is having some issues. Preloaded with hair. :face_vomiting:

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Tony you just cannot win! if you have hair, it gets into drains, if you lack hair (bald) it’s also bad (think ladies)! :laughing:

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Looks like this one connects to a horizontal drain.:slightly_smiling_face: One thing we were taught in Chicago union plumbing school and a P-trap like this was a very good example. Every fitting every pipe transition after the P-trap counts. It doesn’t matter the length of the pipe if it’s 3 inches or 3 feet. This P-trap appears to go to a vertical transition then to a horizontal transition before possibly interacting with the vent.

After the trap weir everything matters. Distance to the vent you could have a crown vent. Every fitting every transition counts.

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Everything does count, but an s-trap is a specific type of a trap. According to what you are saying, among others, a vertical leg drain would also be an s-trap, but it’s not. A union is a not a final authority :slight_smile:

This is why those of us that are plumbers are educated for five years, tested and licensed. You know what they say about opinions.

The gap between the top portion of the S, must be a minimum distance of 2.5 times the diameter of the pipe. The ABS pipe is 2". Multiply 2" times 2.5? 5" inch gap.

Robert I don’t quite understand what you mean. Can you reference the code? I believe that’s 1.5 inch pipe.

Do you mean that a vent shall not be installed within two pipe diameters of the trap weir? P3105.3.

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