Back to back sink drains

[DanielD]

Hi - replacing some galvanized drains in a commercial building I own. There are back to back sinks in bathrooms with a shared wall.

The picture shows how I started but realized this is probably wrong. Please send me in the correct direction. The height of the horizontal drain is about 18" above the floor. Many thanks for any help, Daniel

 

[Tuttles Revenge]

Its totally wrong. What type of sinks are you installing? What height are they going to be? Can that horizontal drain be lowered?

IF the drain can't be lowered your best chance at getting a back to back sink installed correctly is to use a 2x1.5x1.5x1.5 fixture cross installed at a 45° angle using a Street45 bend from that horizontal 2" drain. then bring your vent up another 45° to plumb vertical. your stub outs will be around 21" above that floor in that configuration.

 

[Sponsor]

 

[wwhitney]

While I agree that it looks wrong, and wouldn't be allowed under the UPC, the IPC allows the basic idea and calls it "common venting".

https://up.codes/viewer/pennsylvania/ipc-2015/chapter/9/vents#911

Note 911.2 says ". . . vent connection shall be at the interconnection of the fixture drains or downstream of the interconnection."

If the fitting we can't see in the perpendicular wall is a san-tee, with a dry vent out the top, and each trap arm separately satisfies the length limit and the trap weir rule, then it would satisfy IPC section 911, to my understanding. Not sure if the increase in size of the trap arm before the vent is an issue or not.

Cheers, Wayne

 

[DanielD]

While I agree that it looks wrong, and wouldn't be allowed under the UPC, the IPC allows the basic idea and calls it "common venting".

https://up.codes/viewer/pennsylvania/ipc-2015/chapter/9/vents#911

Note 911.2 says ". . . vent connection shall be at the interconnection of the fixture drains or downstream of the interconnection."

If the fitting we can't see in the perpendicular wall is a san-tee, with a dry vent out the top, and each trap arm separately satisfies the length limit and the trap weir rule, then it would satisfy IPC section 911, to my understanding. Not sure if the increase in size of the trap arm before the vent is an issue or not.

Cheers, Wayne

The horizontal line ends in a vertical vent stack just out of view (about 30 inches away, horizontally) via a sanitary tee 2" opening. A urinal also drains into that vent. Originally said it was dry, but I realize it must be wet. It only serves this floor. Thanks, Daniel

 

[DanielD]

Its totally wrong. What type of sinks are you installing? What height are they going to be? Can that horizontal drain be lowered?

IF the drain can't be lowered your best chance at getting a back to back sink installed correctly is to use a os2x1.5x1.5x1.5 fixture crs installed at a 45° angle using a Street45 bend from that horizontal 2" drain. then bring your vent up another 45° to plumb vertical. your stub outs will be around 21" above that floor in that configuration.

The sinks will each be associated with a small vanity. You are correct - given the vent stack, I can't lower the drain any more. I think that is what started me down this path.

 

[wwhitney]

The horizontal line ends in a vertical vent stack just out of view (about 30 inches away, horizontally) via a sanitary tee 2" opening. A urinal also drains into that vent.

The urinal joins in above the sanitary tee for the sinks?

The urinal complicates things, and I'm not so familiar with the plumbing code on urinals. For example, urinals are not in the IPC definition of bathroom group, so I guess that means it can't be part of an IPC wet vent (IPC 912)?

If it weren't for the urinal, the lav common vent would be OK. Can you separate the urinal from the lav stack by using a short parallel vertical section for the urinal san-tee, with the urinal vent above rejoining the stack at least 6" above all the flood rims, and the urinal drain rejoining the stack below the lav san-tee?

Cheers, Wayne

 

[DanielD]

The urinal joins in above the sanitary tee for the sinks?

The urinal complicates things, and I'm not so familiar with the plumbing code on urinals. For example, urinals are not in the IPC definition of bathroom group, so I guess that means it can't be part of an IPC wet vent (IPC 912)?

If it weren't for the urinal, the lav common vent would be OK. Can you separate the urinal from the lav stack by using a short parallel vertical section for the urinal san-tee, with the urinal vent above rejoining the stack at least 6" above all the flood rims, and the urinal drain rejoining the stack below the lav san-tee?

Cheers, Wayne

I will take a picture when I stop by the building this afternoon. The urinal and the sinks enter the stack at the same height. I post a new pic ASAP. Thanks for you input.

 

[DanielD]

I will take a picture when I stop by the building this afternoon. The urinal and the sinks enter the stack at the same height. I post a new pic ASAP. Thanks for you input.

This shows the vent stack. This started when I bought the building a few months ago. The city mandates that sewer tie ins be updated. The plumbers pulled out the old bathroom fixtures when they replaced the old line with PVC. They connected the short PVC stub (leading under the concrete floor) with the cast iron vent serving the urinal on one side and the sinks from two bathrooms on the other. When I moved in, I started repairs and noticed these drains were galvanized. I decided to replace them, cut out another section of cast vent pipe and ended up where I am today.

 

[wwhitney]

The banded couplings in your photo are only for underground use. Above grade you need the ~2" long fully shielded kind.

With the double san-tee there, I'm not sure of the code status of your current arrangement. If the double san-tee counts as two separate individually vented connections, the current connectivity is fine, there's no wet venting, just the common venting of the lavs. [I think this is the right answer, but it's a long standing question of mine.]

If the double san-tee counts as a common vent, then you can't combine the two lav drains and common vent them, as common venting is limited to 2 fixtures, not 3. That could be resolved by adding a revent to the common lav drain. Which certainly wouldn't hurt but could be unnecessary work.

If your current connectivity matches what was there originally, then you certainly didn't make anything worse. I guess I would suggest seeing what your inspector has to say about all this. Or seeing what other people here have to say.

Cheers, Wayne

 

[DanielD]

The banded couplings in your photo are only for underground use. Above grade you need the ~2" long fully shielded kind.

With the double san-tee there, I'm not sure of the code status of your current arrangement. If the double san-tee counts as two separate individually vented connections, the current connectivity is fine, there's no wet venting, just the common venting of the lavs. [I think this is correct, but it's a long standing question of mine.]

If the double san-tee counts as a common vent, then you can't combine the two lav drains and common vent them, as common venting is limited to 2 fixtures, not 3. That could be resolved by adding a revent to the common lav drain. Which certainly wouldn't hurt but could be unnecessary work.

If your current connectivity matches what was there originally, then you certainly didn't make anything worse. I guess I would suggest seeing what your inspector has to say about all this. Or seeing what other people here have to say.

Cheers, Wayne

Thanks for all of your input Wayne! I wish I had discovered this forum earlier. What a great way to learn. I appreciate you taking time to reply. Best, Daniel

 

[Ridgerunner66]

I am also wanting to plumb in 2 drains for back to back sinks. 1 will be for a bathroom vanity and the 2nd is in case I would, in the future, decide to put in a studio apt. kitchen in the walk-in closet behind the vanity. I am showing 2 mick ups of what I think is possible and would like to know what others think is the best option or is it something else. I will adjust the final to be a better height from the floor and room vent up out of the top and connect it to the vent coming off the toilet.
Thanks in advance!

 

[Terry]

We normally would use a 2x1.5x1.5x1.5 double fixture fitting. You need a 2" cleanout for a kitchen sink.
With a disposer and a deep sink, the trap arm is at 16" from the floor.
The lav cabinet will determine what height you need for that.

 

[Ridgerunner66]

We normally would use a 2x1.5x1.5x1.5 double fixture fitting. You need a 2" cleanout for a kitchen sink.
With a disposer and a deep sink, the trap arm is at 16" from the floor.
The lav cabinet will determine what height you need for that.

I will be venting over to a 2 inch vent. Is keeping it all 2 inches acceptable?? So I take it that the cross is also acceptable, not having to worry about both sinks being run at the same time and causing an issue?

 

[Terry]

2" for venting is always fine.
We don't use a sancross for back to back.

 

[Ridgerunner66]

2" for venting is always fine.
We don't use a sancross for back to back.

I'm sorry, just realized that 2nd picture didn't load. Is this a better option, incorporating the clean out you suggested?

 

[wwhitney]

Seems like the second option may be better. That lets you put the lav san-tee higher than the kitchen sink san-tee, which is typical as kitchen sinks (particularly with disposers) are deeper than lavatories. And if you use a 2" pipe between the two san-tees, the UPC allows that wet venting with the lavatory on top.

Cheers, Wayne

 

[Ridgerunner66]

Seems like the second option may be better. That lets you put the lav san-tee higher than the kitchen sink san-tee, which is typical as kitchen sinks (particularly with disposers) are deeper than lavatories. And if you use a 2" pipe between the two san-tees, the UPC allows that wet venting with the lavatory on top.

Cheers, Wayne

So I should set the vanity at 18 or 19 inches from the floor and the kitchen at 15 or 16 with the clean out below?

 

[Ridgerunner66]

So I should set the vanity at 18 or 19 inches from the floor and the kitchen at 15 or 16 with the clean out below?

Also, the vent for the toilet comes up in this wall. I was planning to run my 2inch vent from top of San T over to the Toilet vent. I think that will keep it as a dry vent, correct?
Thanks!

 

[wwhitney]

I'm not up on the latest recommendations for sink stub out heights, I think it might be more like 18-20" for the vanity and 12"-16" for the kitchen sink. There are threads here on that.

On the dry vents, any horizontal segment needs to be 6" above the flood rim of the fixture(s), as should any joining of dry vents.

Cheers, Wayne

 

[Tuttles Revenge]

Determining the height of stub outs I start from my finished cabinet-undermount sink/counter top-top mount sink dimension.

Most manufacturers want the drain stub out 13ish" below the sink rim. You can vary this by determining whether you will use a lift rod stopper or a push pop up assembly. The lower the drain, the more options you have in a full sized vanity. We install a lot of sinks that are in super small shadow box style vanities where the drain has to be super dialed in really high off the floor like 22" which limits the drain options to only push pop ups and bottle traps.

For Kitchen sinks I determine the depth of the sink and whether it will have a disposer (and usually plumb for a disposer height regardless unless we're pressed for space). Most kitchen sinks we install are 10" deep. Add another 12 for the drain from that. Assuming a 34.5" cabinet and undermount sink that is 12" above the floor for the drain stub out. This would work for an undermount sink installation and top mount as well. I like to install my cleanouts above the sink drain so that it can be open with water running down the sink if cleaning is needed.