Sewer Line Under Slab - New Construction

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Steve20A, May 23, 2007.

  1. Steve20A

    Steve20A New Member

    Messages:
    6
    Location:
    MA
    Hello. Steve20A here with a general question for the experts.

    I am building a new house. The houses previously built by this contractor have waste lines running around the perimeter of the unfinished basement, collecting waste from the kitchen and baths and eventually going to the sewer exit pipe. They use an ejector pump for moving waste from the basement level bathroom.

    My house will be at a high enough elevation so that the sewer exit line will be below the slab. I will not need an ejector pump. The current plan for the basement bathroom is to have it drain into a pipe under (in) the slab. There is an I-beam in the basement that bisects the house (more or less). It prevents any waste pipe from being run near the ceiling, where it would be out of the way - it forces the pipe to be much lower (to clear the beam) and on the wall (because you can't have a pipe suspended in the middle of the room) and it therefore intrudes into the wall space where I would put cabinets.

    My suggestion was to have a larger sewer line run under (in) the slab and have the left side of the house drain into it on the left side of the I-beam and the right side of the house drain into it on the right side of the I-beam, eliminating the need for a perimeter pipe. The basement bathroom could use that one pipe, as well.

    The contractor says his plumber doesn't like the idea. A larger pipe under the slab might break more easily; two entrances into the sewer pipe are "not standard" for him; some other, lesser objections. He suggests I just build a false wall to cover the pipes.

    My questions:

    1. Any reason why a larger pipe under (in) the slab would not be a good idea?

    2. Any reason why more than one entry point into this pipe is a bad idea?

    3. Would a pipe generally go under the slab or in the slab? If under, would it be protected in some way (concrete channel of some sort?)

    Many thanks in advance. I would just like to be more educated before I speak with the contractor again.

    Steve20A
    Last edited: Jun 1, 2007
  2. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    1. No. A 4" pipe should handle everything your house has to throw at it.
    2. No reason with a competent plumber.
    3. Under the slab, not in the concrete. Your sanitary line should run at least 8-12" under the slab.

    I'd be concerned about the plumbing contractor if he really has issues with your request.
  3. post a line drawing

    if i understand you correctly, where the big beam is will have no bearing on where you want to embed 3" drain pipes in the ground. Just to be sure, please draw this, so we can figure out what you really want to do. BTW, 3" is big enough pipe too, unless your house has many toilets. 3" carries water and solids longer than 4".

    david
  4. Steve20A

    Steve20A New Member

    Messages:
    6
    Location:
    MA
    Correct. The beam has nothing to do with anything under the slab. Only why the pipe cannot be at basement ceiling level (for the upper floor waste sources).

    I don't know if the contractor is planning on 3" or 4" pipe. It never came up for discussion. Thanks for this info, tho.

    My illustration skills are not good, but here is a representative drawing of how the pipes are proposed to fit together. My idea is just to dump all left side waste into the under-slab pipe on the left side instead of running it all around the basement and entering on the right.

    Thanks, again.

    Attached Files:

  5. Gary Swart

    Gary Swart In the Trades

    Messages:
    7,308
    Location:
    Yakima WA
    A waste line is less likely to develop a leak than water lines, but nevertheless, I would avoid plumbing under a slab if at all reasonably possible. That's my unprofessional opinion based on the numerous cries on this forum for advise on fixing leaks under a slab.
  6. good drawing

    the dotted blue line in the side view shows the upstairs stack turning and going down the wall on an angle. (is that right?). Not good; angle is too steep for toilet waste which could get left behind, stick to the pipe and even dry there and glue itself on, if you are away for any length of time; then you'd have the beginnings of a partial block.

    You don't need to occupy that wall with that pipe at all; you can send the pipe straight down to the underslab drain, or to the drain that you build just above the slab if you choose to. (The stack can be built with a slight offset in it at each floor, so it is not just one long straight upright pipe, but that is another topic. There are other ways to build it too).

    Right now the deep red lines on the upper floors are confusing. I'd send the waste from the second floor down to where the first floor bath is, then from there send it over towards that wall using the space the I beam provides to you ("wasted space"), and then turn down that wall there where the I beam and the wall meet. The kitchen drain can come in using any one of many possible routes, and the master bath too. Under the basement, the proposed underslab drain doesn't need to take that route you drew going out in a straight line; it can go over to the wall side and stay there hugging that wall as it goes out the building. That way both drains get connected at the wall floor junction, and you only have one drain from there on.

    david
    p.s. I may be missing lots of things here; I have not done this myself.
    p.p.s. can you show venting too?
    Last edited: May 23, 2007
  7. Steve20A

    Steve20A New Member

    Messages:
    6
    Location:
    MA
    <<the dotted blue line in the side view shows the upstairs stack turning and going down the wall on an angle. (is that right?). Not good; angle is too steep... >>
    Correct, but the angles and slope are not accurate. The idea was to show that the pipes could not stay near the ceiling, because they had to pass below the I-beam. The main point is that the pipes would wind up in the usable part of the wall where I want to put cabinets.

    <<You don't need to occupy that wall with that pipe at all; you can send the pipe straight down to the underslab drain...>>
    Yes. That's what I want to do. We basically agree here.

    <<Right now the deep red lines on the upper floors are confusing.>>
    Just meant to show where the waste is coming from.

    Not sure what routing you are suggesting, but I just realized there is a small labeling error in the diagram. In the basement, the left side arrow should say "from the 1st floor" not the "2nd" floor. And the waste from the 1st and 2nd floors - left side - would have been combined at that point.

    <<...the underslab drain doesn't need to take that route you drew going out in a straight line; it can go over to the wall side...>
    I thought a straight line was the best -- fewer turns = less likelihood of clogging.

    Thanks very much for your comments.
  8. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    Genie brings up a good point, venting. A 3" pipe will be big enough for most 2 bathroom houses, a 4" (with 2 3" stacks) would ensure you wont have limitations. For the venting I would wye off of the plumbing stack above grade and run the vent up parralel with the sanitary stack. Turn the vent up at a convenient location and use it as your vent stack.

    Having said this and my previous post, I live in Florida and stuff I've described is very common. Gary doesn't seem to care for piping under slab....maybe it's an up north thing. I would at least get a second (or third) opinion from other contractors.
    Last edited: May 23, 2007
  9. Steve20A

    Steve20A New Member

    Messages:
    6
    Location:
    MA
    Final Questions

    Thanks to everyone for responding. I have created a new thread to ask the final questions that resulted from this discussion. This thread can be considered closed.

    Steve
    Last edited: Jun 1, 2007
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