Shower drain rough in p-trap

[Xiaotian Guo]

Hi,
I am about to start my guest bathroom gut renovation, one thing still not very sure and hope I can get some advice on the shower drain p-trap installation -
I will replace the existing P-Trap since I need to move the center of the drain a couple of inches, also I am using Durock's shower kit (foam pan with drain kit and membrane). For p-trap I *think* have 2 options.

1. I can glue the U shaped pipe to the vertical straight pipe, then close the sub-floor. when the pan is laid, I can try to fit the drain kit piece and cut the vertical pipe back until my drain piece is flush with the pan. but since the vertical pipe is already glued to the U shaped pipe I will have to cut the pipe from inside using a drill with rotary blade - probably difficult to cut it very clean.

2. I can leave the vertical pipe section out, or at least not have it glued to the U shaped pipe, this way when I fit the shower drain piece, I can take the vertical pipe out, make a clean cut then put it back in, when everything fit perfectly, I can glue everything together.

Would #2 work? or I am missing something? I have never done this before and I'd really appreciate someone with experience can give some guidance. Thank you!

-X.T.

 

[Terry]

I'm having a hard time understanding what you have.
Do you have access from below? Normally the vertical from the p-trap goes to the shower drain.

 

[Sponsor]

 

[Xiaotian Guo]

Sorry I didn't explain it well.

I will have full access to the p-trap, I expect to replace the subfloor under the shower and toilet.

Here is the sequence of work I plan to do.
1. Demolition.
2. Circular saw plunge cut the shower area subfloor to open up the drain pipes including p-trap
3. Cut the horizontal drain pipe close the the p-trap. remove and throw away old p-trap.
4. Dry fit section of horizontal pipe with weir and U pipe, connect that to existing drain pipe using coupling. make sure the center of the U pipe is at desired position.
5. Glue the horizontal pipe, weir and U pipe, hold the horizontal pipe in place using a pipe strap. (note the vertical pipe is not connected to U pipe yet)
6. Put the new subfloor on.
7. Install cement boards as shower surround, rest on subfloor.
8. Put shower pan in place. verify the center is aligned with the U pipe.
9. Dry fit the vertical pipe, drain kit bottom piece and the U pipe, since the vertical pipe is not glued to U pipe yet, I can take it out and cut using pipe cutter or saw. should be easier/cleaner than cutting it from inside if the vertical pipe is already glue to the U pipe.
10 Glue the vertical pipe to U pipe, drain kit bottom piece to the vertical pipe.
11. Adding membrane....

Seems like all the video I found online so far had the vertical glued to the U-shape, then the contractor just cut it from inside using a rotary blade.

This might seems to be much fuss about a very simple task for an experienced plumber, but as a DIYer who has no prior experience I want to avoid situation like cutting vertical pipe that already glued to the U pipe - if I make a mistake I will have no access to replace the p-trap.

Thank you,

-X.T.