What size of ABS for toilet? 3" or 4"

Discussion in 'Toilet Forum discussions' started by hottubbrad, Jul 12, 2008.

  1. hottubbrad

    hottubbrad New Member

    Messages:
    9
    What size of ABS should I come up through the slab with for my basement toilet. I bought 4" for my horizontal waste drains under the slab. Is it OK to just 90˚ up with the same 4" for the toilet.

    What toilet flange should I use if I use the 4"?

    Thanks
  2. Gary Swart

    Gary Swart In the Trades

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    Location:
    Yakima WA
    4" will be fine. Your flange can be either the type the fits over the pipe or inside the pipe. You have the choice with 4", but with 3" you would not want to use the inside as it cuts the diameter of the pipe too much.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,139
    Location:
    New England
    As Gary indicated, with a 4" pipe, you can use either an internal or external flange. This means that you can leave the riser part long, put down your finished floor, then install the flange on top of the finished floor after cutting the pipe off if you use an internal flange.

    A 4" pipe would have a deeper (longer) elbow, meaning the pipe may need to be deeper than it would with 3".

    Using a 4" and mounting the flange on the outside would entail putting a sleeve around the pipe when you pour the concrete, but then would allow easier access to cleaning out the line, if that ever became necessary (otherwise, a 4" cutter head might have trouble getting through the 3" opening).
  4. Ravz

    Ravz New Member

    Messages:
    6
    I was planning on using a 3" inside flange, but also thought that it might cut the size of the pipe too much... then i realized that the opening at the bottom of the toilet is the same size as the opening size of the 3" inside fit toilet flange. whats the point of having a big opening at the flange if the limiting factor is the toilet?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    22,139
    Location:
    New England
    The nice thing about a 4" drain line is you can use the internal mount flange (although it wouldn't hurt to use a 4"). It is easier with the internal one since you don't have to be careful about maintaining clearance around the outside of the pipe for the flange fitting, since it fits inside. Leave the pipe long, then cut if off after the finished flooring is down.
  6. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,358
    Location:
    Yakima WA
    A 3" pipe is large enough, but a 4" will give you choices on the flange fitting inside or outside. The rule for sizing drain pipes is you never decrease the diameter going downstream so if you start with 4", you will have to stay 4" or larger. If you start with 3", you can stay with 3" or increase to 4" as desired. As far as one size being better than the other for draining purposes, there is no difference so it's your choice.
  7. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
    Cave Creek, Arizona
    toilet

    The toilet opening is the one place where you CAN decrease the size of the pipe, which is why I ALWAYS use a 4" riser by 3" outlet closet bend. If you are using 4" pipe, therefore, the logical thing would be to keep it 4" all the way. Use an outside the pipe flange with metal ring.
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