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cast iron drain in slab

amo675 Member Posts: 7
Hi, we are pouring a 5" floating slab on grade with several cast iron drains penetrating through the slab.

the drain lines run in the compacted fill below the 4-6" compacted gravel, 6mil vapor barrier, 2" tongue and groove xps and the 5" 4000psi pour with 1/2 radiant pex throughout.

the assembly we are planning on was to cut a hole in the vapor barrier and attatch to the cast iron pipe with tape, then cut a hole in the insulation around the pipe and fill with spray foam and finally to wrap the segment of the pipe that would be in contact with the concrete with thin foam insulation..

does this sound appropriate?

thanks in advance


  • Zman
    Zman Member Posts: 7,572
    I guess it come down to how much the slab is expected to "float".
    If it significant, you might consider these https://www.fernco.com/plumbing/flexible-couplings/expansion-joints
    They can be used on the vents and vertical pipes in the house to prevent the slab from cracking them. I suppose they could be used between the drain and the trap although I have not tried it.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • GW
    GW Member Posts: 4,699
    I'm not sure what your exact question is. The piping will not be affected by concrete contact, you're getting a bit more detailed than most. Any minor movement won't be an issue either, that pipe will simply move at the same time.
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
  • amo675
    amo675 Member Posts: 7
    Hi, thanks for the responses, i think the verdict is we should be ok wrapping the vertical pipes that will be cast into the slab in some sill seal insulation before pouring the concrete, i will include our diagram:

    GW, i was mainly just asking for opinions on our assembly, didn't really have a direct question :)

  • SWEI
    SWEI Member Posts: 7,356
    edited May 2016
    What is the intended use of this slab? 60 PSI foam would be considered somewhat unusual for application under a slab.