View Full Version : Distance between trap and shower drain

04-21-2007, 08:40 AM
Installing a center drain shower pan where tub was. Old tub was elevated so trap and drain were above the floor (mobile home). I would rather not cut the floor under the shower to install the trap. Can I run about 25-30" of horizontal drain line (with 1/4"/foot angle) from shower drain and then put the trap in before before connecting to the vertical drain/vent where tub was connected? Also, this will be 2", not the tubs' 1 1/2" line. So, does the trap have to be right under the drain?


04-21-2007, 08:49 AM
and the consequences are that the piece of horizontal pipe before the P trap will eventually let smells float back up out the drain, as a certain amount of hair, skin cells, soap scum and other organic matter deposited on the bottom of the pipe will biodegrade and smell. I think it's not as serious as sewer gases, which are dangerous.


04-24-2007, 10:08 AM
This is a similar question to the issues I have on my 2" drain post on this form.
Without giving a DIY (me) a bad name, I pose the question..... Could you have a redundant trap? Meaning, put a 2" trap in the shower drain (correct location) & plumb down to the existing 1-1/2" trap, obviously converting from 2" to 1-1/2" along the straight section. The reason I'm asking in my case, getting to the original trap is a pain, due to its restricted location & short stub. Is there a downside to 2 traps in series?

Thanks, FJK

04-24-2007, 12:21 PM
no can do. illegal and dangerous. not code.

it's fine to ask. Sometimes the responses are clear; in a few cases they are a bit dodgy because it is one of the few things that could be debatable, like "best practices" put into code, and not straight out safety issues.

You also cannot reduce a drain pipe size part way down the line. That will cause blockages.


05-12-2007, 08:26 PM
I have a derivative question. We are rerouting the drain from a bathtub in a basement ceiling in my brother-in-law's house. Originally the trap was directly under the tub, which then put the drain about 8 inches or so beneath the joists making a very unsightly intrusion from the ceiling where it then went diagonally about 5 feet to the stack on the other side of the wall. We are thinking of shifting the trap to the end close to the stack, allowing the drain to proceed close to the joists rather than a foot beneath them, perpendicular to the wall and then making a right angle to the stack behind the wall. Would it be okay to have a second, shallower trap directly under the tub to block any smells you mentioned in the answer above? Or alternatively, could one use an "s" trap under the tub, instead of the original "p" one, that would allow the drain to leave then take a right angle directly against the joist?

05-12-2007, 08:53 PM
If I understand it correctly, an S trap has the inlet and outlet parallel with each other. The arm coming out of the p-trap should be basically horizontal - a bend is okay to get it where you want it. I'll defer to a pro on this, but I think what you want is okay. Just where is the vent, though?

05-13-2007, 06:28 AM
no you cannot have two traps in one drain pipe; they fight against each other; they do not coordinate themselves to agree on how much water to leave in the trap to keep the trap seal. The description above is confusing and even mentions an S trap. If the person posting wants to try again, he can use words like horizontal and vertical to explain the shapes geometry and image.


05-13-2007, 07:44 AM
S traps are not allowed by any code now, as far as I am aware. Double traps are also not permitted. Also probiteted are inline or running traps.

All these things have been used before, sometimes allowed by code, but have been found to be indadequate, dangerous, etc. and so are not used today.

05-13-2007, 07:56 AM
Thanks for cutting down the options. So I guess I'll just go the way of the first posting, which is to put one trap downstream of the tub and bear what may. My brother-in-law wanted the drainpipe to go flat against the joists from the tub to the wall (to make as little intrusion into the ceiling as possible), then slope downward from there. I thought that it must have downward slope the whole way. Is this right?

05-13-2007, 08:32 AM
ok, now a dumb question from me: why does the tub's existing P trap have to be so low as to end up 8" below the ceiling of the basement? I thought the P trap could be higher. The higher it is, the higher the tailpiece pipe coming out and going to the stack.

By the way, that pipe is and remains horizontal (with 1/4" /ft slope) whether it is straight or with an elbow. Not slanted up, not sloped upwards. Slightly sloped downhill, not less than 1/6" per foot, ideally 1/4" per foot. ((If slanted too steeply, it requires a vent near the P trap; depending on the distance to the stack. This is hypothetical information only, as no-one ever installs it this way)).

So far you have not mentioned venting. This is serious.

If you place the P trap so high up in the space between two joists that the tailpiece is now too high to pass under the joists, you have to drill holes in joists to keep that pipe horizontal (with 1/4" /ft slope). Can be done, has been done, not a new thing, not against code. If you really want to, you can do it. Is something preventing you, which I am not aware of? It is best to keep the same trajectory, same path, instead of lengthening it, in my opinion so far. If you go on the path you mentioned in your first post, with a 1/4 turn elbow, you will have increased the total length of pipe from about 5' to 7' or more. Then you may have another problem because this increased distance to the stack may then cause a requirement for another vent, regardless of slope. Is the tub pipe 2" or 1.5" diameter inside?


05-13-2007, 10:04 AM
The old arrangement ran about 4' diagonally beneath the joists from the P trap to the stack, using 2" pipe several inches below the joists requiring the P trap to come even further below that, probably because it was constructed in an unfinished basement and they didn't care where the pipe went. Because it was 2" pipe, I guess code did not require a secondary vent (5' max, right?). Somebody subsequently finished the basement and built a rectangular box around the plumbing that came down a good foot from the ceiling. We were hoping not to have to drill the joist but run 2" pipe perpendicularly to the wall across 3 joists, I think, then make a right turn inside the wall and put in a trap and run about 3-4 ft or so to the stack from there. Is there something wrong with that? We could drill through about 3 joists to run a diagonal as well, but the hole would have to take a 2" pipe to stay in code without a secondary vent, I think, which would be a larger hole than the 1-1/3" notch or 2" hole allowed for a joist, no?

05-13-2007, 10:24 AM
makes sense (what you just wrote). Clear too.

Taking the pipe over to the side wall and then turning it 90 degrees ("a 1/4 bend") makes the total distance longer, but it turns out that in this case the distance is small enough so you don't need more venting. It's still below the maximum -- assuming the amount of the slope is right and not too much, which is 99.9999% likely to be true now, but which may not be later if you were to raise the P trap and then connect the pipe to the stack at its old connection point.

Ah ha ! That is why I kept mentioning the slope. You have to redo some of the stack, too. Unless there is secondary venting on that line.

You can either drill through the joists diagonally to get a straight line of sight pipe, or drill straight through the joists to make a drain pipe shaped with a bend in the corner.

The joists are 2" by X" , but that information is not described in your post. Joist size determines whether or not you can drill a hole big enough for the pipe.

If you can't or don't want to rework the stack, another option is to raise the P trap as described and add an AAV, which gives the air needed when the pipe goes down too steeply to the stack. This is a second best solution, which has a few disadvantages, that may or may not be significant to you, your brother in law and the next owner of that building.

I am not a plumber, so I'll defer to any plumber who steps in here to correct anything.


05-13-2007, 10:39 AM
I recall that the joist is 2x8. Because the P Trap is inside the wall, I don't think I need to raise it. I should be able to adjust it to fit in.

05-13-2007, 01:52 PM
It is immaterial whether two consecutive traps are legal or not, because if you do it, the shower will not drain regardless of whether it was approved or not.

05-14-2007, 10:43 AM
I have received an answer to my main question from Howe Brothers, our local DIY plumbing shop. He said the trap can be no more than 15 inches from the fixture. Otherwise, the force of a surge of water coming down the pipe can push the water out of the trap. Though this can be dealt with using a deeper trap, he advised against it as a deeper one is more likely to clog up.

Regarding two traps, I had read that either in a book or on a website, which had said that if you had a shallow trap you'd need another one down the line someplace. It did seem a little screwball, and we did not do that. My brother-in-law went with one trap near the stack. He'll still have a lower section of ceiling to accommodate the pipe, but now only few inches. I'll advise that he put a removable panel on it until he finds out whether he's satisfied with the new arrangement.

08-24-2012, 11:53 AM
Due to limited floor framing depth, the homeowner would like me to locate the P trap away from the shower drain so as to not require a soffit in the room below. Due to limited headroom above in the shower, the shower pan can not be raised to effectively increase the floor framing depth. The shower drain would connect to a 2" sweep 90 and then run with the required slope to the P-trap, about 5 ish feet, as I understand would be allowed with a wet arm drain to a vented stack. Yes, I understand that even with a wet arm, the P-trap is conventionally below the fixture. Even if this does not meet code, what problems are likely to arise?

08-24-2012, 04:36 PM
A long arm to the p-trap means a likely smelly installation...festering hair, dead skin, soap scum, etc. sitting in the arm which would normally be blocked by the seal in the trap. Worse comes to worse, you can modify the joist structure to accommodate the trap where it should be.