Hello

I want to develop a basement bathroom that is diriectly under the main floor bathroom. The p trap from the main floor tub dips down about 4" below the bottom of my joists before curving back up to connect to a horizontal line that runs over to the main vented stack. I dont want to have to frame down around the trap or make the entire ceiling this level. can i move the trap 6" higher (to now be between the joists), add another 90 to turn down right after the trap, go down the 6" and then another 90 to connect to the horizontal line. i believe this becomes an "s" trap, no? will this create a siphon? by doing this only the horizonal line would be below the joist rather than the entire height of the p trap

PalmMP3

Yes it will. And that is exactly your problem.

In order to avoid making an S-trap, you will have to run the pipe horizontally for several inches after after the trap. After that, you can theoretically drop down a couple of inches. I say "theoretically" because you have just run into another problem: sharp 90 bends are a big no-no for drains (they're meant for vents), and should be avoided like the plague. Ideally, you're suppose to use a gradual sweep, but if you're only dropping down 6 inches, you won't have enough room for 2 sweeps...

cdnk5

thanks for the quick reply

is there a requirement for how long that horizontal piece has to be to be for it to be effective? would 12' work?

thanks again!!

PalmMP3

12" should definitely be enough. Our kitchen island sink's drain goes horizontal after the tee for about 12" or so before going vertical, so I believe it is okay.

What are you going to do about the new problem that I mentioned though? IMHO, two 45's might work better...

PalmMP3

To clarify what I just said: I don't mean two 45's forming a quarter-circle (which will result more or less in a homemade 90 sweep); rather, I mean like this:
Not only is it not as tall, but it has another benefit: the waste flows better. Instead of making a sudden shortstop, drop, and then picking up speed again, it flows smoothly all along, and the bend merely affects it like you would feel riding down a short hill on a bicycle.

cdnk5

I could actually go more than the 6", probable a foot.

PalmMP3

Wait a minute, I seem to be missing something here: why do you object to having the trap hang below the joists, but you have no problem with the horizontal pipe hanging below the joists?

cdnk5

the horizontal pipe is 1 1/2 and attached directly to the bottom of the joists. the trap extends another couple inches below that. with a low ceiling already i dont want to loose that extra couple inches. thanks

speedball1

"In order to avoid making an S-trap, you will have to run the pipe horizontally for several inches after after the trap"

Nice try but no cigar!! I don't care how long you make the horzizontal pipe the second you make a downward turn you have just built yourself a "S" trap, (lay it out on paper). The only way past that would be to put a vent between the trap and the first downward facing fitting. While Palm may have been a little hasty in advising you about the trap configuration he was right on target when he advised against using elbos and dog legs, such as the one you propose, in a drainage line that carries hair matted in with grease. This will just abouit guarantee clogs and blockages and using 1 1/2" instead of 2" just adds to it. Using the loop vent in his island kitchen as a model Palm attemts to justify building a "S" trap but that's comparing apples to oranges. Palm says," Our kitchen island sink's drain goes horizontal after the tee for about 12" or so before going vertical, so I believe it is okay."
What Palm failed to realize is while that his island vent goes vertical into the return vent, (forming a loop vent) yours goes down into a drain line (forming a "S" trap). ( Sorry Palm)

So to make a long story short, we address this problem by taking a hole saw and make holes or saw notches through the floor joists so we can connect the drain line without "S" traps or doglegs. I realize this may not be what you want to hear but to do it right and to stay within code you're gonna hafta do it one of two ways. Either drop the ceiling to accommodate the trap or do it my way through the floor joists. Good luck and I'm sorry if I rained on anyones parade., Tom

PalmMP3

Whoops! Sorry, forgot to mention the vent thingy - my island has an AAV, so I easily forgot to mention it, since a small device like that is easy to take for granted. Still, if AAV's are legal in your area, you might be able to make it work...

Either way, it wouldn't be a good idea in this case because of all the other problems Tom mentioned.

Anyway, I'm glad to know I was right about the whole elbo vs. sweep thing. Thanks, Tom.