Back to Back Sink Plumbing/Venting Help

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10%Joe

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I recently bought a duplex. Much of the plumbing had been updated poorly or still had the existing cast iron/galvanized steel pipes. I had a plumber come and replace the old cast iron/steel drainage pipes with PVC.

The attached photo is a second floor back to back sink with the neighboring unit. Both units share the same venting and drain. The drain is used only for the two bathroom sinks and runs straight down into the basement where it connects to the horizontal waste.

The initial plumber finished his work but left our sink drain rough in un-capped. Once, while this drain was un-capped, the neighbor used their bathroom and water poured out from the open drain on our side. We asked the plumber about this and the solution was to cap the rough in. I'm concerned that something about this set up is incorrect. Since then I've had multiple plumbers come and look. I get a different explanation/solution every time. After looking through codes and your forum I still have questions about the proper way to plumb and vent this location. I would greatly appreciate some expert feedback about this area before spending more money on rework or closing this wall.

1. What would cause water to back up into a sink drain that is located above the drain being used?

2. Does the PVC to Galvanized vent location need to be updated to a shielded coupling?

3. Are the drain lines properly plumbed as shown?

4. Is the vent routing properly plumbed as shown?

Back-to-Back sinks.jpg
 

Terry

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The wet vent portion between the lavs should have been 2", they reduced down too soon.
The lower trap arm has a fitting that changes the 2% grade, that should have maintained a 1/4" per foot grade. The fitting they added changed that.
It does make you wonder if the pipe going down is clogged. Has anyone run a snake down that?

Back-to-Back-sinks-Joe.jpg
 

10%Joe

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Really appreciate the responses. The entire vertical waste had just been replaced when it overflowed. The only non-new pipe would have been the horizontal waste in the basement. If the clog was in the horizontal portion would other fixtures have also been affected? The waste has been updated since then, so everything down stream at this point is new.

Could I add a sanitary tee, plumbed to the horizontal venting, before the neighboring sink fitting to correct the wet vent issue?

If I'm understanding correctly It looks like the options are:

-Lower the neighbor P trap to remove the fittings. Size up the wet vent plumbing to 2"
-Update to double fixture fitting prior to the vertical venting?
-Re-route venting to create 2 separate dry vents (attached photo)
 

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10%Joe

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Edited for clarity:

I just finished digging through my partners phone and found this old photo from the day it overflowed. See the bathroom photo with bucket 'Before (Second Floor bathroom)'. Turns out I misremembered and only the supply had been replaced at the time. The waste was still the galvanized metal. It was in pretty bad condition, so I think you guys are probably correct with it being clogged.

Is using a T sweep fitting the preferred method in this case? Would keeping the waste lines separate have any benefits when it comes to maintenance (clogs, etc) in the future?
Before (Second Floor Bathroom).jpg
Before (First Floor).jpg
After (First Floor).jpg
 
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John Gayewski

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(Edit: The bold section is not correct as vertical wet vents need to be at least 2c in size per UPC not sure for ipc.)

As far as I can see the plumbing is correct. The wet vent is up sized to 1.5" as a lav only requires 1.25". Tees are commonly used as lav drains and they are placed vertically so there's no issue there. The trap arm's should be easily cleaned since they are short. The common drain they share could use an accessible cleanout but it'll likley never be used. Being as it's a rental cleanouts are helpful becuse tennants aren't the most mindful with your stuff.
The only real issue is, you do need a shielded coupling and probably a riser clamp on the floor as the pipe passes through it.
 
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Jeff H Young

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10%joe, you got a lot of input by others but your plumber did some goofy work I think he is a bit outa his area of expertise.
I'd recommend dealing with the blockage before repairing the faulty work of which isn't the cause of stoppage unless debris went down or dislodged and got stuck downstream. in any case it will drain even with the hokey work we se see. Probably a little blunt about the workmanship but its kinda dumb to a plumber but understandably to others might not know better
 

10%Joe

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Really appreciating all the feedback. We will definitely be adding the shielded coupler, riser clamp, and some general clean up on the blocking/pex mount clips.

As far as how to correct or improve on the current sink waste plumbing is there a preferred method? I'm trying to make this as bullet proof as possible. I've attached some options based on feedback and suggestions this far.
 

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John Gayewski

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Really appreciating all the feedback. We will definitely be adding the shielded coupler, riser clamp, and some general clean up on the blocking/pex mount clips.

As far as how to correct or improve on the current sink waste plumbing is there a preferred method? I'm trying to make this as bullet proof as possible. I've attached some options based on feedback and suggestions this far.
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