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Thread: Copper water supply under slab

  1. #1

    Default Copper water supply under slab

    I am removing a carport concrete slab and will pour a new slab for an attached garage. Currently my water well casing and the supply lines to the house come up through the existing slab. I need to move the suppy lines to a different location and thought I would replace the copper lines up to the house. My questions:

    1) What type of copper pipe should I use: Type M, L or K?
    2) Rigid or coil?
    3) Does code allow sweated fittings (couplings) under the slab?
    4) Should the copper be encased in something where it goes through the slab?
    5) How deep below the slab should the copper lines be placed?
    6) Should the well casing be isolated from the new slab? The current slab is cracked where the casing penetrates the slab.

    I will not know exactly what I have until the existing slab is removed so I may have more questions. There are 3 copper supply lines coming up through the slab. One is capped and the other two are connected to the well tank.

    Any comments will be appreciated.

  2. #2
    Engineer Furd's Avatar
    Join Date
    Jul 2007
    Wet side of Washington State


    1. Type K.

    2. Flexible.

    3. I don't know. Since all codes are made into law at the local level you would have to learn the local requirements.

    4. Absolutely do not allow the piping to be in contact with the concrete. I would encase the entire line under the slab in foam insulation and use the same as a sleeve when coming through the slab. Even better (if at all possible) is to not have the piping under sthe slab at all or at least not have to come through the slab.

    5. Below the frost line. I would have it no less that four inches below the slab.

    6. Again, you do not want any piping, including a well casing, in contact with the slab. Use some kind of spacing material around the well casing that will keep the concrete from touching the casing. Even wrapping several layers of corrugated cardboard around the casing (approximately 1/2 inch total thickness) will work and after the slab has cured you can dig out this cardboard and replace it with something water resistant.

    Often piping coming through concrete will have a wrapping of heavy thickness plastic tape to prevent contact with the concrete. While this is good (and I would also do it) the best installation will have some kind of sleeve or at least a space between the pipe and the concrete.

  3. #3


    Thanks Furd for the through reply. I really appreciate your time. I may wind up hiring a licensed plumber to do the job for code purposes. I had to get a building permit so I could get an electrical permit (electrical meter must be moved), so the city may require a plumbing permit when they inspect the foundation before the concrete is poured. I think I'm capable of doing the job but I must check with the city.

  4. #4


    I have received two bids for the plumbing job and I'm unhappy with both. Bidder #1 seemed more qualified and what he recommended is to code but the price was a little steep for 2 runs of copper approx. 30 ft. The bid was $2,200. Bidder #2 recommended using pex: sweating a barb on the exiisting copper pipe and running pex to the pressure tank. His bid was time ($78/hr) and materials plus permit fee, which is reasonable. However, code does not allow a sweat joint under a slab, it must be a brazed joint (I assume using a wrought copper fitting). I mentioned this to bidder #2 and he said brazing a joint makes the copper brittle. Is that true? A pex crimp under a slab seems like a poor idea in this layman's mind.

    Am I thinking straight about bidder #2? I plan on getting 2 more bids.

    I have no choice but to run the plumbing under the slab. I am a little concerned in that the existing plumbing has the copper pipe contacting the concrete at the kitchen sink and the washing machine. It's been that way for 42 years and the cost to correct that is prohibitive. Any ideas how long it takes for concrete to corrode copper?

  5. #5
    Forum Admin, Expert Plumber Terry's Avatar
    Join Date
    Aug 2004
    Bothell, Washington
    Blog Entries


    Brazing under slabs is what plumbers do. Above ground we use lead free solder.

    Any pipe under slab should be protected, I like the foam too. It allows for some movement.

  6. #6
    Licensed Grump GrumpyPlumber's Avatar
    Join Date
    Jun 2007
    Licensed Grump


    Brazing makes copper soft, not brittle...it anneals it.
    "The biggest regrets we have in life are the chances we never took."

  7. #7


    Thanks for the info. I really appreciate your forum, it helps a novice at plumbing like me a great deal.


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