Back to back toilets

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Old 11-25-20, 06:08 PM
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Back to back toilets

Hello,

I am doing a complete reno of our master bathroom. This started because i noticed water damage on the wall directly below the master bath toilet. Once the toilet was removed it was clear that it had been leaking for some time. The subfloor and flange both need replacing and the wax seal was in terrible shape. I took this opportunity to redo the whole bathroom which was in desperate need of it.

The master bath toilet is back to back with the toilet in the main upstairs bath. Previously i noticed that when one toilet was flushed the water in the opposite bowl would move. Now that the master toilet has been removed I notice the rag that I stuck in the pipe look to be pushed out from the pressure when other toilet is flushed. The rag is also always damp and I have even seen a little water be "pushed" past the rag and onto the subfloor when the toilet is flushed. With a terrible flange and seal it appears I found the source of the water.

Now here is my question. After doing some research it appears that this may be due to the toilets utilizing a double sanitary tee (which i can just barely see in the wall/floor). The increased velocity of modern toilets forces the water into the other toilet's pipes. According to the "online gurus" the solution is to change it to a double combination wye. Now this would be pretty intense job due to its location and that it is in-between the first and second floor.

If I were to replace the subfloor, flange, and seal to ensure a good connection, would this be enough to prevent the water coming back out of the pipe when the opposite toilet is flushed? I don't mind the water moving in the bowl, and the water level remains the same when the other toilet is flush, so I am not concerned about that. I just don't want the water to leak again and ruin the floor.

Side note: Before the masterbath toilet was removed I have never noticed any issues of water leaking in the main bathroom when the master bath toilet was flushed.
 
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Old 11-26-20, 05:27 AM
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I agree, you don't want to try replacing the double sanitary tee. Even if it goes well it would be a big, annoying job.

I don't think the double sanitary tee caused your leak or water problem. I think the toilet has had a slow leak for a long time. The floor got soft allowing the toilet to wiggle a little bit each time it was used. The movement caused the wax ring to fail and leak even more. Wax rings can be pretty reliable if the floor is solid and the toilet firmly attached.
 
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Old 11-26-20, 08:21 AM
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I don't think the double sanitary tee caused your leak or water problem.
Possibly, but there had to be a reason why the sanitary cross is no longer allowed in a back-to-back installation. 40 years ago a 3" Sanitary Cross with two 2" Side Inlets was standard for all good plumbers in every subdivision home.
 
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Old 11-26-20, 10:03 AM
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Yea, If doing a double sanitary tee now I think you must have at least 18" developed horizontal distance between the toilets and the T. With a standard installation of back to back toilets there is nowhere near that much horizontal. I assume they changed the requirement because of issues like this.

I switched our bathrooms to Toto toilets a couple years ago and the flush is very quick and must send one big slug of water down the pipe, something our older toilets didn't do. They both consume the same amount of water per flush but the new toilets send that water out in about 1/4 the time.
 
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Old 11-26-20, 07:58 PM
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So what is my best option? Replace the subfloor, flange, and seal and hope for the best?
 
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Old 11-27-20, 06:28 AM
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Yes, unless you feel like replacing the double sanitary tee in the wall.
 
 

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