Stacked san tees and tub/shower vent tie-in

[WeRvenom]

Have 2 questions:
(1) Had a handyman modify the plumbing for a double sink from a single sink in our bathroom. He ended up stacking two san tees. All of the pipes are 1-1/2 inches. No fixtures on top of the vent. Would this set-up work?
I read from another post here where the person had stacked san tees with 2" pipes all around and was told would work as long as he used 1-1/2" drain traps. Do I need to change the vertical drain pipe under the san tees to 2" and leave the rest 1-1/2"? Or leave as is and just change the 1-1/2" p-traps to 1-1/4"?

(2) He also left out the p-trap for the tub/shower. I plan to put in a p-trap connected to the san tee as he had done but rather than capping the top I would drill a hole in the bottom and top plate to run a 1-1/2" vent pipe to connect to a 90 then to a tee into the vent pipe for the sink. Will this affect the flow of the stacked san tees for the double sink?

The "red line" is the proposed vent pipe that will connect in the attic. (Shows in the wall due to the picture being cut off)

 

[wwhitney]

(1) The UPC (California's plumbing code) allows a 1-1/4" drain for a single lav. So double lavs on stacked san-tees can use 1-1/2" pipe between the san-tees as a vertical wet vent, as it is one size larger than the minimum 1-1/4". Plus a vertical 1-1/2" pipe is good for 2 DFUs (2 lavs, say). But a horizontal 1-1/2" pipe is only good for 1 DFU, so the bottom of the lav stack would need to connect to a 2" LT90 and a 2" horizontal drain. I can't tell from your photo below the floor whether that is the case.

(2) The shower needs a vent on the trap-arm, which if it's 2" must be within 5' of the trap (or 42" if its a 1-1/2" tub trap). That vent can be a dry vent that is pulled off and connected like you drew the red line, as long as it comes off the shower fixture drain before it joins any other drain.

Or the vent could be a horizontal wet vent if the shower drain joins the lavs drain within the 5' or 42" limit. In which case the lav drain would need to be 2" starting at the upper lav san-tee, since it is now subject to the horizontal wet venting rules as well.

I can't tell from your underfloor pictures what the connectivity is. What is the drain on the right that is coming into the combo with the cleanout? And what is happening below the san-tee with the temporary rubber cap?

Cheers, Wayne

 

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[WeRvenom]

Thanks for the advice Wayne.
Will probably go with the 1-1/4" drain approach. The stacked san-tees connect to a 2" horizontal pipe under the floor.
The drain on the right of the "combo with the clean out" is the bottom of the stacked san-tees, the horizontal pipe on the left is another drain for a garage sink (not plumbed/used), and the san-tee with the rubber cap is the tub/shower drain that connects to the same 2" pipe downstream of the garage and sink connections. The 2" drain actually continues to the left but the view is blocked by the garage drain pipe. Attached is another picture showing the garage drain connected to the right of the san-tee with rubber cap.

"The UPC (California's plumbing code) allows a 1-1/4" drain for a single lav." >> How much of the sink pipe needs to be 1-1/4"? Can I use a 1-1/4" p-trap for each sink and keep the 1-1/2" horizontal to the san-tee?

Will use a 1-1/2" tub trap and connect the top of the san-tee to with 1-1/2" connected to the existing 1-1/2" vent.

 

[wwhitney]

I was not suggesting using any 1-1/4" pipe--you can do it all in 1-1/2". But the fact that the minimum size for a single lav is 1-1/4" (rather than 1-1/2") means that the minimum size for the pipe between the two san-tees is 1-1/2" (rather than 2"). Since it is 2" below the floor (and before the combined lav drain goes horizontal), looks like everything for the lavs is good as is.

Looking again the 2nd photo in the OP, I see now that the white piping is the tub waste and overflow. And it looks like you are missing a trap, that elbow just below the white piping should be a p-trap. Then the san-tee with the rubber cap above it would be the vent takeoff for the tub, and that pipe with the rubber cap would be the tub dry vent. The tub dry vent needs to be extended up and connected to another dry vent (such as the one for the lavs), at an elevation at least 6" above the flood rim of all the fixtures involved.

Cheers, Wayne

 

[Jeff H Young]

The Lavs should work fine , they won't meet code minimum vertical wet vent is 2 inch. To be legal you would need a 2x2 x1 1/2 bottom santee And 2x 1 1/2 x1 1/2 on top. (technically you might be able to run 1 1/4 trap arms and traps ) it would be pointless though .
weRvenom I think you said you'll be putting trap on tub and running vent See no issue with that good plan!

 

[wwhitney]

The Lavs should work fine , they won't meet code minimum vertical wet vent is 2 inch.

Thank you for the correction. I forgot the "but in no case less than 2 inches (50 mm) in diameter" clause at the end of UPC 908.1.1:

Chapter 9 Vents: Vents, California Plumbing Code 2019 | UpCodes

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So as Jeff mentioned, I was mistaken, and the 2" under the floor needs to be extended up to the upper san-tee. The trap arms and vents can be 1-1/2".

Cheers, Wayne

 

[WeRvenom]

Thanks Wayne and Jeff!