Add a toilet to an existing drain trunk?

tjdabombJuly 22, 2014


Looking for info on adding a toilet on slab construction.

Commercial space, looking to a add a bathroom abreast to two existing bathrooms - left to right, far right being the new bathroom. Existing bathrooms have a water closet and a sink. Got a bid of $9k to add new bathroom because the plumber says we need a dedicated line from our proposed new bathroom to the main. I say BS but I do not know the code, and, I am not sure if the existing toilet trunks are 3" or 4". I have read that ya cannot add a toilet to such an arrangement unless the new toilet is 15' away from the next toilet.

So..... what's the real deal?? Can a toilet be added by just wye-ing into the last toilet?? Can I do that if it is a 4" drain? Can I do it if it is a 3" drain?

Thanks much experts!!!


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We would like to take a look at this, and give you another bid. Do you have plans available via email? Send email to

Here is a link that might be useful: Our website

    Bookmark   July 24, 2014 at 5:39AM
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Thanks for the reply!!!

I was more interested in the code that states the regulations for the work that I have highlighted and not a contractor to do the work.

Any thoughts?

    Bookmark   July 24, 2014 at 6:17PM
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    Bookmark   July 28, 2014 at 4:29PM
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Codes vary by State. Your plumber, and apparently you, need to analyze what is in the slab now, overlay the specific code in your area and arrive at the correct decision. Before the code question you have to know the lay-out and size of the existing drain & vent system. I think you call BS rather hastily!

    Bookmark   July 29, 2014 at 12:10PM
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Yes, I know codes vary by state, but, there is a general rule of thumb or predominant code specification that is used in multiple jurisdictions, that is what I want to know. And as originally asked, if it is a 3" drain, what is the code? If it is a 4" drain, what is the code?

it's like pulling teeth around here sometime.

    Bookmark   July 30, 2014 at 4:45PM
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