In this fantastic bit of MS Paint artistry, I've shown an S-trap and a P-trap. In my humble opinion, the only difference between them is the horizontal run between the top of the upflowing bit of pipe (defining the end of the trap itself) and the bend to where the flow goes downhill to the rest of the drain system.

hand drawn images of an S-trap and a P-trap
Image source: Me! Copyright: Are you kidding?
Also, please ignore the sharp corner in the transition between the black circle and the red line in the P-trap example and assume, instead, that it's a smooth curve, as a store-bought trap would be, work with me here...

How long must that red section of pipe be in order for what is, effectively an S-trap, to be considered a P-trap?

My reason for asking is that I'm getting ready to do the plumbing for a new bathroom addition, and I need to plumb in the sinks which will be against an exterior wall. I don't want to put the drain line into the exterior wall and reduce my insulation there, so I want the drain to go into the floor. A floor-based drain would usually be served by an S-trap, but it seems those are no longer code legal.

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    IANAP: (though I play one at home) I think the other difference is that the P-trap line joins a vertical stack where there is air present to allow the line to drain out. I believe the problem with S-traps is potential suction or siphon from the line pulling out the trap water. The air break prevents that.
    – DaveM
    Commented Oct 17, 2022 at 15:21
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    Is there an interior wall you can reach from the sink area? How do you feel about plumbing chases you can see?
    – Ecnerwal
    Commented Oct 17, 2022 at 15:33

2 Answers 2


Those are both S-traps, since you have not drawn a vent, which is what actually makes the difference. Most P traps in finished locations have vents in the wall you can't see. Without a vent, if enough water flows in to fill the pipe beyond the trap, it siphons the water out of the trap, leaving it dry, which allows sewer gas to flow back through the drain. With a vent, if the pipe beyond the drain fills it starts to pull air in through the vent, protecting the trap seal.

While it's struck from adopted code in my area, there is a sneaky S-looking trap setup where the "down-and-out-pipe" is made so much larger than the "down-and-in-pipe" that it can't siphon, and venting is maintained to the trap weir. See section 917 in the IPC. Particularly 917.5

Air Admittance Vales (AAVs), also known as mechanical vents or "Studors" (a brand name of one such) which I personally hate with a passion, are the other approach to difficult venting. They have a "one way valve" which is supposed to only allow air to flow into them, preventing suction on the trap seal. If the "one way valve" in the AAV fails, sewer gas flows out through that, rather than it only allowing air in. I hate them becasue they tend to fail, somewhat regularly, and each time they fail they allow sewer gas into the house until somone tracks down the source of the stink and replaces them. Pipes to the outside with no valve in them fail much less frequently, if ever. If you must use an AAV, locate it as high as physically possible to reduce the frequency of failure by preventing direct contact with drain water.

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    If one can pop up to above the fixture flood line, one could also use a Chicago island vent setup... Commented Oct 18, 2022 at 3:23
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    Interesting. I didn't realize that the direct attachment of the vent to the top of the "down and out" pipe was what differentiated the two different kind of traps. Appreciate the code link - I'll look through there for venting requirements and options.
    – FreeMan
    Commented Oct 18, 2022 at 11:42
  • After thinking about it some more, if I were to install as an S-trap with a sanitary Tee (with the flow going down) below the floor, then a (sloped) horizontal run across the floor and then turning the vent up the exterior wall, that wouldn't be any better than just going straight into the exterior wall, would it.
    – FreeMan
    Commented Oct 18, 2022 at 14:09

As already mentioned, the P-trap requires a vent on the horizontal section, or it's still just a long S-trap. That said, you can't just put a vent on the top bend of an S-trap, since that results in what's called a crown vent, which is illegal in many places. Code may require a minimum of two pipe diameters between a trap and the vent that services it (IPC 2021 909.3) - to meet code, the horizontal section in red must be at least two pipe diameters long before meeting the vent, after which the pipe may bend to vertical.

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