Does a trap have to be directly below the fixture?

Hello:
I have a question for the plumbing pros here. I am working on a house my son recently purchased.
I roughed in a powder room sink with 1 1/2" schedule 40 pvc. I used 1 1/2" because the previous owner had a lead drain for about 10 feet, transitioned to 1 1/2" pvc with a mission coupling.

In any case, my daughter in law decided she wanted a pedestal sink in this powder room. It will be difficult, to say the least, to get a S-trap in the pedestal, and she would prefer the plumbing to not show behind the pedestal. Therefore, I am considering putting the trap for this sink in the basement, but, due to ducts that have been installed, the trap will have to be about 8 feet down the line, rather than directly below the sink. Rather than tubular, I would use schedule 40 pvc for this trap.
Is there any problem doing it this way?

https://codes.iccsafe.org/content/IPC2018/chapter-10-traps-interceptors-and-separators

I think most codes limit the vertical distance from a fixture to the trap to 24".
The IPC, as you can see, at that link, limits the horizontal distance to 30 ". I didn't see that horiz. limit in the UPC.
I know why the vertical distance is limited. It is felt that a slug of water greater than 2 feet could potentially have enough momentum to carry right though the trap and leave it dry. Not sure why the horizontal limit. Maybe something along the same lines.??????

You didn't mean to say S-trap, which of course is not legal.

There shouldn't be much of the drain piping showing. Just the arm going into the wall. How about the water service valves.
Use chrome plated waste line for a nicer look.

Thanks for your reply.

I did, in fact, mean S-trap. The wall of this bathroom is built directly on top of a floor joist. Therefore, there is no way to use a P-trap. The drain line MUST come up out of the floor.

I know that S-traps are less desirable than P-traps, but I don't believe they are illegal. If they are illegal, how can the orange big box sell them?

S Traps are prohibited by code. If you cannot achieve proper venting, use an air admittance valve under the fixture.

The reason box stores sell them is because while they are prohibited to install new, they are not prohibited to repair. Any newly installed S trap would fail an inspection 100%

Also under my code (Canada) the length of the fixture outlet pipe cannot exceed 1200mm or roughly 48”

Diehard said:

Sorry, I just assumed you were replacing an existing lav since you were talking about existing piping.

I don't have any ideas as to how to handle it.

What's on the other side of the wall?

Maybe one of the Professional plumbers can help.

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The other side of the wall is the kitchen, and we cannot "fatten" that wall to make room for plumbing.

I understand the vertical limit, but I don't understand why there would be a limit to the horizontal distance. Can anyone explain that limit to me?

Sounds like you’re talking about a running trap which isn’t up to code

Rickyman said:

Sounds like you’re talking about a running trap which isn’t up to code

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Not sure what it would be called, but I am talking about installing a trap in the horizontal drain run in the basement ceiling.

That is a running trap which as stated is not up to any code.

Can someone explain to me what the problem would be?

A running trap can have the trap “seal” sucked out and drained. This would nullify the reason for a trap. A trap protects the space from sewer gases . I believe the UPC allows a maximum 24 inch drop and 42 inch trap arm(1 1/2 pipe size). You can fight over “why”, we just state code.

RenewDave said:

A running trap can have the trap “seal” sucked out and drained. This would nullify the reason for a trap. A trap protects the space from sewer gases . I believe the UPC allows a maximum 24 inch drop and 42 inch trap arm(1 1/2 pipe size). You can fight over “why”, we just state code.

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Not fighting, just trying to come up with a solution.

Do you (or anyone else here) know which setup would be more likely to siphon the trap dry... an S -trap directly below the sink, or a "running trap" in the basement?

Well as I see it, unless the sink is filled with enough water to create a large enough slug of water when released, neither would lose it's seal.
The normal flow from a Lav through a 1-1/2" pipe should not exceed it's half full design limit.

Keep it at the sink and use an AAV to keep a "P" trap from becoming an "S" trap.

The Aav is your solution.
Diehard, in either of those situations, if you emptied a 5 gallon bucket of water into that sink you could easily suck the trap dry. I venture to say with a running trap, if you filled the sink completely and drained it at one time you’d suck that trap dry. You only have to
Lose a about half the amount of liquid from the trap to lose the seal. Not all of it.

Diehard said:

Well as I see it, unless the sink is filled with enough water to create a large enough slug of water when released, neither would loss it seal.
The normal flow from a Lav through a 1-1/2" pipe should not exceed it's half full design limit.

Keep it at the sink and use an AAV to keep a "P" trap from becoming an "S" trap.

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Diehard - thanks, but, as I stated before, this lav must have an S-trap, rather than a P-trap, due to the fact that the wall in the bathroom sits directly on top of a floor joist that runs parallel with the wall.

Are you a plumber???? Pick the answer tjat best serves you.

NO. then do what ever you want you have read the posts its your call.

YES. Then if you are you know that straps are illegal and the same with runnig traps

Since I am replacing what already existed, I believe I am allowed to use an S-trap. The problem is that the old vanity was not a pedestal style, so it had plenty of room for the trap. With this pedestal, that is not the case.

Since I have to choose one of these options am simply looking for advice on which would be the lesser of two evils.

I am not a plumber by trade, but I do plenty of plumbing on my rental properties..

But you’re not replacing what already is existing. You are turning a cabinet into a pedestal. Since you fancy yourself kind of a plumber, you should do it right. The problem with my trade is that people think they can do whatever they want and look to professionals to validate at best an unusual at worst a flat out unsafe solution. There’s ALWAYS solutions to correctly do a project.

jeffmattero76 said:

Diehard - thanks, but, as I stated before, this lav must have an S-trap, rather than a P-trap, due to the fact that the wall in the bathroom sits directly on top of a floor joist that runs parallel with the wall.

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Yes...I was thinking along the lines of dropping the legal minimum distance after a p-trap, and adding an AAV. That would be close to what you could get by with provided you can get your daughter in law to go along with it.