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Thread: Rough in toilet before pouring slab

  1. #1

    Default Rough in toilet before pouring slab

    I am trying to finalize my drawings which I will submit to the city for a permit.

    This is a new construction of a small office building in my backyard.

    The only 2 things requiring plumbing will be a sink and a toilet.

    Can you please describe to me what I will need to do to correctly install the drain, hold it in place, and make sure it is at the correct height so that when my concrete guy comes in and pours the foundation and slab it will all be correct.

    I know that I will have to vent the sink and the toilet drains through the roof.
    I also know that I will need to put a clean out somewhere in the line.

    What I don't know is exactly what needs to be installed and entombed in the concrete slab.

    Also please let me know what size pipe I need for the drain, is it 2-1/2" ABS?
    Thanks.

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The toilet needs at least a 3" and depending on the distance it has to travel to connect to the sewer, you might need a 4". If you use 4" for the toilet and the main drain line run, then all you have to do is make sure the pipe is in the right place and plumb. You can buy an internal mount flange and they can pour right around the pipe. There may be a reason to put a sleeve around it, and if so, then you could use an outside mount flange. Trying to install the flange now is not a good idea...it should be installed after the finished floor is in and sit on top of the finished floor. For the maximum flexibility, you want the pipe's center to be 12" from the FINISHED wall. Anything less and you may not be able to use a 'standard' toilet (although some will fit). Trying to find 10" rough toilets will restrict your choices and likely increase the costs.

    Not sure about the size of the vent line required for only those two fixtures...probably a 2", but wait for one of the pros.

    It is helpful to know where you are, codes can differ.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    Quote Originally Posted by jadnashua View Post
    The toilet needs at least a 3" and depending on the distance it has to travel to connect to the sewer, you might need a 4". If you use 4" for the toilet and the main drain line run, then all you have to do is make sure the pipe is in the right place and plumb. You can buy an internal mount flange and they can pour right around the pipe. There may be a reason to put a sleeve around it, and if so, then you could use an outside mount flange. Trying to install the flange now is not a good idea...it should be installed after the finished floor is in and sit on top of the finished floor. For the maximum flexibility, you want the pipe's center to be 12" from the FINISHED wall. Anything less and you may not be able to use a 'standard' toilet (although some will fit). Trying to find 10" rough toilets will restrict your choices and likely increase the costs.

    Not sure about the size of the vent line required for only those two fixtures...probably a 2", but wait for one of the pros.

    It is helpful to know where you are, codes can differ.
    Hey Jad,
    Thanks for the quick reply.
    I am in Southern California - Los Angeles Area.

    I can place the toilet 12" from the finished wall no problem.
    The finished floor will be the top of the slab (decorative concrete finish).

    So then the questions are:

    (1) 3" pipe stubbed up through the finished concrete with some foam wrap for "wiggle room" ? or attach that 3" pipe to some kind of flange and have the cement guy float up to that level?

    (2) What does this 3" pipe connect to before it heads towards the house (with the standard 1/4" per foot slope) and connects to the drain system of the master bathroom?

    (3) Since the toilet will be located against one of the exterior walls of this structure what type of fitting would I use which incorporates a clean out that can be poked out through the interior/exterior wall?

    (4) Is there an online resource which has illustrations that will help me understand the concept a little better?

    Thank you so much for your time and help!

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The specific details are best left to the pros. If you use 3", I'd still sleeve it and put the flange on after. Leave it high enough and with a cap on so it sticks out of the concrete and it is obvious if it gets knocked off plumb curing the pour. Then, cut it off when ready, drill some anchors for mounting the flange, then install it. I think, the size of the pipe will depend somewhat on how long the run is...you may have to upsize it to 4", and that means where it connects in the existing plumbing must also be 4" as well.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5

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    Quote Originally Posted by jadnashua View Post
    The specific details are best left to the pros. If you use 3", I'd still sleeve it and put the flange on after. Leave it high enough and with a cap on so it sticks out of the concrete and it is obvious if it gets knocked off plumb curing the pour. Then, cut it off when ready, drill some anchors for mounting the flange, then install it. I think, the size of the pipe will depend somewhat on how long the run is...you may have to upsize it to 4", and that means where it connects in the existing plumbing must also be 4" as well.
    Ok Great.
    The run is 30 feet to the existing master bathroom drain system.

    One other question I have:
    Can the water supply lines be copper? or since portions of it will penetrate the concrete will it have to be PVC or some other form of plastic?

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    SUpplies can be copper...all of the pipes should be insulated (sleeved) as they go through the slab as I understand it.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    Plumber jimbo's Avatar
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    The toilet flange need to be snug right on top of the FINISHED floor. So, you would just stub up the pipe and cut it down as necessary for final installation of the flange.

  8. #8
    DIY Senior Member MG's Avatar
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    If it were me I would spend the extra $$$ and go with 4" pipe.
    Note: I am a DIY'er and not a professional. My posts here are observations / opinions and may or may not be in accordance with your local ordinances.

  9. #9
    Commercial Plumber markts30's Avatar
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    If it were me, I would spend the extra and go with a plumber...
    Seems to me that you have no idea what is required for underground plumbing to work right and pass code...
    As well, the inspector might have issues with improperly installed plumbing...
    You are going to have permits and inspections, right?

  10. #10

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    Quote Originally Posted by markts30 View Post
    If it were me, I would spend the extra and go with a plumber...
    Seems to me that you have no idea what is required for underground plumbing to work right and pass code...
    As well, the inspector might have issues with improperly installed plumbing...
    You are going to have permits and inspections, right?
    Yes I am going to have permits and inspections.
    I have a pretty good idea of what is required for underground plumbing as I did a complete re pipe on the main house 2 years ago including all new ABS drains throughout as well as a complete copper re pipe.
    We used 4" ABS and had to connect to the original main which was also 4" under the front driveway.

    What I do not know about is what is required for covering it all up in concrete which I why I posted here, so if you have any good tips/tricks or methods, please do share, I would really appreciate it.

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