I am going through a major renovation in my house which involves excavating 7'6" basement and pouring seismically strong 15" slab as a floor. The house is pretty high on the hill and something like 50' from the property line. I have a sewer line currently going under floor joists and in the middle of the space that will become a basement. So it will be above one's head after basement is done. In future, I want to consider getting a small powder room (toilet + sink) in that basement. So I decided that I want to reroute the sewer line under the slab using trenches excavated for drainage. Those trenches will be something like 6'-7' below the grade line.

Since we are on a small hill, per measurements, there should be enough slope for this sewer and lateral to be below the slab.

I will have plumber to do it but what are the things that I should look especially carefully considering that the entire house drain will go through this line and that fixing it may be ridiculously expensive?

Should I require specific access boxes installed at every join of the pipe? If so what is the recommended size and kind? How should I evaluate the quality of all connections? I think pressure test is required anyway.

  • A bit off topic, but I'm flabbergasted by the notion of a 15" thick slab. That's four times as thick as a typical slab around here. I'd like to hear more about the reasoning behind it.
    – isherwood
    Commented Oct 16, 2020 at 19:55
  • 1
    @isherwood We have deep landslide. That is we have soft and expanding soil going pretty deep. We are also 1/4 mile from major earthquake fault line. 25 years ago they poured 20' deep piers under the house but they never reached the rock. Newer geological science says that piers are not good as a foundation in this area, should be removed and instead they engineered mat/raft foundation with an idea that it moves with the soil like a raft so house is not torn apart. I didn't decide on it. Structural and geotech engineers did.
    – Uncle Meat
    Commented Oct 17, 2020 at 20:32

2 Answers 2


I'd suggest asking your plumber plenty of questions until you're comfortable with the way the installation is planned. If your plumber isn't a Q&A kind of plumber, feel free to hire a different one; some are personable and good with homeowners who like to be involved with their projects, and some aren't.

Including some clean-out fittings in your plan is a good idea. When I had a main sewer line replaced, I got one inside that was integrated with a back-flow preventer, one outside near the footing, and one installed where my service line met up with the city sewer connection. This cost maybe $100 more in materials & labor and allowed me to rule out any problems with the new line when troubleshooting a different problem just a few months later.

You don't need clean-outs at every turn or fitting in your under-slab sewer line, though you should consider having them installed everywhere a line comes up from the slab.

Consider getting a floor drain or two as well.

Also, while updating your drains, make sure whatever water heater tank(s) you have include catch-pans & drains.

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    Floor drains are a mixed blessing - if they are not used regularly they need a trap primer, and if backups are an issue (perhaps not on a hill?) a backflow preventer as well. But they are great if you mop the floor regularly and like to rinse it down the drain rather than having to mop it all up.
    – Ecnerwal
    Commented Oct 16, 2020 at 18:50

Use 4" - don't cheap out and use 3" because you can technically get away with it. That will instantly limit likely problems, and costs very little to do up front. Small lines (sink drain, vents, buried traps) are required to be at least 2" diameter under concrete IIRC.

Unless you have some absurdly torturous path in mind, a cleanout every 50 feet is generally adequate. If you are going to make absurd corners, more cleanouts may be advisable, but not making absurd corners is more advisable. You certainly don't need one every 10 feet, and providing the required access to them every 10 feet might screw up your basement plans anyway.

Remember to allow for future finish when roughing in your powder room access - we've had a few questions come by where the fixture rough-in was spaced from the unfinished wall, and there wasn't enough space when the wall was finished and a few inches were lost in the finishing. Also remember that vents are needed as well as drains. Much easier to put them in from the start than to try and retrofit after concrete is poured.

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