I have removed a tub to install a Tile-redi shower pan. The tub drain has a P-trap that goes into a wye. The other half of the wye goes to another P-trap for a tub in an adjoining bathroom. Both traps appear to be uniform in depth. Due to the location (under floor, floor joists, routing, etc.) and depth of Tile-redi shower pan's drain, I may have to lower the drain's P-trap. It is my understanding water in the P-trap must be 1-1/2" to 2" and no more than 4" max. Can I lower the P-trap safely within those dimensions without regard to the other wye's connected P-trap? What might be the issue(s) if any?

Hi A4d2n

What size drain pipes are we talking about here? Please be specific... what size to tub, to new shower and after the pipes combine, OK?

Back to you...

In my answer previously I stated:
1. House built in 1977
2. Drain is 1-1/2" PVC
You ask about the new shower drain and "after the pipes combine". Sorry I did not answer. Here is the scoop:
A. The shower drain accommodates a 2" drain pipe (I have a 2" to 1-1/2" reducer)
B. The wye drain is 1-1/2" input on each wye and 2" on the output.

Hi A4D2N...

Not to be specific on you, but you never mentioned that the house was from 1977 or that the drain is 1.5", OK? In your question you did say, " It is my understanding water in the P-trap must be 1-1/2" to 2" and no more than 4" max."... but that isn't a 1977 or an 1.5" PVC...:p

Now, in terms of the information you have now provided I would recommend that you cut out the 2"x1.5"x1.5" wye and install a wye fitting that will allow you to pick up the shower drain without having to use the reducer... should be a simple job as this is all PVC.

If you do that then I am thinking that a 2" PTRAP should lower the PTRAP inlet low enough that you won't have to worry about building your own PTRAP AND you won't have to worry about the drain backing up at the wye fitting where the fitting could be overwhelmed by a full tub draining at the same time as a shower which could cause issues, for sure!

This will also meet code requirements and that is a good thing.

With all that being said, I see no issues with lowering the PTRAP if you dare to stick with the 1.5" pipes, just as long as you do NOT exceed the 4" water seal at the PTRAP.

Back to you...

Mark

Thanks massplumber2008... I will see what is involved in removing the wye but it is located in a very restrictive spot. It probably will require removing kitchen cabinets and ceiling beneath the wye. The other (keeping 1-1/2 drain) might not be a big problem since my wife and I are the only ones living here now... kids are all on their own. Christmas holidays would only be a problem since all of them are home. Again thanks, I appreciate your expertise.