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I am in midst of finishing the roughed in full bath and want to put a bar sink on the wall behind the tub. I will add pictures later- once I am able to get some good shots....

The way it was roughed in- I have the lav sink on the left, and the toilet on the right. On the opposite wall is the tub drain. Here is a mock up: https://drive.google.com/open?id=0B3vdRz1ABOmaVmxCUWJ3dVZIYzA&authuser=0

I am assuming that the rough-in for the full bath is correct, and everything is vented properly. The solid red lines are what is there today as well as the vent stack- at least what I believe is there. I can only see the rough ins sticking out of the floor for the toilet, and the tub drain in the ground. The dashed lines are what I was planning to do. My plan is to tie the bar sink drain to the tub drain- since they share a wall. I was hoping that however the tub is vented- the bar sink would just use the same vent. But after reading many threads on here I am not so sure. If this is not a good idea, please help me to figure out what I can do.

Thanks!

MORE INFO: Thanks pdd. I am not sure my picture is as clear as it should be. The lines I had drawn were all flat- meaning they are under the slab so I am guessing where they go. I just assumed that the rough in was vented properly. So if I assume that the tub is upstream of the toilet and vented properly- can I tee off the pipe that is in the slab for the bathtub and connect the drain for the tub and the bar sink? And add a vent for the bar sink and run it over to the vent across the room that is for the bath lav?

    New vent                       |
 |---------------------------------|
 |                                 |    
 |                                 |  vent for
sink    tub                        | toilet and
   |     |                         | bath lav                                           |
   |     |                         |
   P     P- each has P-trap        |
   \     /                         |
    \   /                 toilet   | 
      |-  drain in slab     |      |   
      |                     |      |    
----------------------------|-----------------|
                                              |
                                              |
                       drain to septic system \ 
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Updated Answer

After watching your video, and looking at the photos, there seems to be quite a few assumptions being made, i.e. if the vent is actually a vent, what happens to the vent after it passes through the ceiling, how the toilet and bathtub are piped under the floor, and how they are actually vented.

In addition, there is also a few issues what would need to be addressed:

  • There is a semi-continuous fixture draining into the assumed vent (from the water softener). It’s load would need to be accounted for when sizing the wet vent. However the 2” wet vent would be maxed out with the bathroom fixtures and the bar sink alone.
  • The wye connection into the bathtub's fixture drain looks to be too close the the bathtub's trap (min should be 2x the pipe diameter, this is to protect the seal of the trap from being siphoned by the passing water when the sink drains).
  • The bathtub’s trap looks like an s-trap (against code where I’m from due to s-traps having the ability to self-siphone).

Codes vary from place to place, however due to these issues and the unknown assumptions, I don't think this installation would pass a plumbing inspection, defiantly not in my area.

If I was you and I wanted to meet code, I would further investigate the vent to confirm that there are no other fixtures draining into it. I would determine what the output flow the water softener is in order to determine what it's load to the drainage and venting system would be. I would investigate the arraignment of the below grade piping to confirm how they are piped and how they are vented (this can be done via video scoping the lines). Only after this was done could I determine if code could be met.

With that being said, you didn't ask if it met code, and I've seen far worse.


Original Answer

Every fixture requires a vent. Typically washrooms are installed so that the toilet, bathtub, and basin are all vented via the basin’s vent. This is called a wet vent as the basin’s drain line is sized to allow for both the drainage of its water as well as the passage of air required to vent both the toilet and the bathtub. In addition, the fixtures served by a wet vent need to be arranged so that the toilet is either the last fixture connected or is connected via a symmetrical double wye fitting:

       Bathtub
           \
            |
            |
           /
-----------\------\
            \      \
             |     |
          Toilet   |
                 Basin

In your sketch, the bathtub is connected downstream of the toilet, which would be incorrect if the bathtub is being vented via the basin’s vent as described above. However, if the bathtub has its own individual vent then it’s not a issue.

How you pipe the new sink will depend on how the bathtub is being vented. Without knowing the arraignments and pipe sizes, I cannot give a definitive answer, but this should get you started:

If the bathtub has its own vent, and it is of sufficient size, you can simply connect the sink’s fixture arm to the vertical vent in the wall. If the bathtub is wet vented, you’ll need to connect to the main sanitary line downstream of the last fixture in the wet vent, and add a new vent from the new sink back to the existent vent stack:

     /---—---—-- Sink (c/w vent piped back to vent stack, not shown)
     |
     | Bathtub
     |     \
     |      |
     |      |
     /     /
-----------\------\
            \      \
             |     |
          Toilet   |
                 Basin
  • Thanks pdd. I am not sure my picture is as clear as it should be. The lines I had drawn were all flat- meaning they are under the slab so I am guessing where they go. I just assumed that the rough in was vented properly. So if I assume that the tub is upstream of the toilet and vented properly- can I tee off the pipe that is in the slab for the bathtub and connect the drain for the tub and the bar sink? – Bob Mar 31 '15 at 16:59
  • And add a vent for the bar sink and run it over to the vent across the room that is for the bath lav? New vent | |------------------------------| | | | | vent for sink tub | toilet and | | | | | | P P- each with own P-trap | \ / | \ / | |- drain in slab toilet | | | ----------------------------|--------------| | drain to septic system\ – Bob Mar 31 '15 at 17:19
  • That would depend. Where I’m from, if the wet vent is 2”, it can handle a total of 3 fixture units of load in addition to the load of the toilet. A bathtub and a sink are 1-1/2 fixture units each, so in this case it would work (as long as the bathtub pipe is 1-1/2"). However, the wet vent will not act as the vent for the sink so you would still require a separate vent for the sink regardless on how the drain is connected. – pdd Mar 31 '15 at 17:20
  • Thanks pdd. How did you put those gray pictures in your response? Not sure if my "drawings" look okay- having a hard time putting them in here. The tub drain in the slab is 2". So I need to vent the bar sink up and over to the existing stack- is there a certain height I can tie it in to the stack? – Bob Mar 31 '15 at 17:29
  • The vent needs to stay vertical until it passes the heigh of the sink's flood level rim (the level of the sink's rim where water would overflow if plugged). After that height, it can be run horizontally to the vent stack. The connection to the vent stack should also be above any fixture's flood level rim. (The gray mono-spaced text is done by formatting the text as code, I've updated your post's formatting.) – pdd Mar 31 '15 at 20:51

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