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Can I pass a sink drain pipe through a 12×12 wooden support beam in the basement?

longfieldl | Posted in General Questions on

I want to install a sink on one side of the support beam (on the floor above) and the main drain is on the other side of the support beam (in the basement). I can’t pass it above the beam because it is the middle of the hallway. If I pass it under, it will be so low that we will have to bend to pass under the drain pipe. Is it acceptable to drill a 1 1/2″ hole through a 12″ x 12″ support beam. I have seen that the diameter of a hole should be maximum 1/4 the height of a joist, but does that apply to a support beam?
Please advise.

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  1. Expert Member


    The IRC rules for drilling apply to beams, joists and blocking, so you are good to go with a few caveats.:

    - The hole must be in the centre of the beam (the neutral axis), not toward the top and bottom.
    - The hole should not be located close to the ends of the beam.
    - The hole shouldn't be directly under a point load.

    You probably need a bigger hole for a sink - 2" to 2 1/2" depending on if it serves a kitchen or bath - but that's still within the allowed limits.

  2. ar_t | | #2

    2015 IRC R502.8 Cutting, drilling , and notching. See also Figure R502.8

    "Holes" are limited to D/3 Max. where D= joist depth and minimum 2 in. from the top and bottom of the joist, or to any other hole located in the member.

    The reason for code minimums is occupant safety. I'm just a homeowner/builder/contractor. I see codes as a good thing.

  3. gusfhb | | #3

    I personally haven't seen a 12x12 in anything under 150 years old, unless a new post and beam.

    It just makes me think I would want to find a way around, personally.

    1. longfieldl | | #4

      Thank you very much for your answers. This is very helpful. Ketith's answer made me wonder about the dimensions of the beam. I had asked my wife to measure the dimensions of the beam but was not specific about what I needed so the 12" included studs nailed along the beam. I went and measured the actual dimensions of the beam and it is more 8" thick by 9 1/2" deep. According to the D/3 max rule, I would still be able to pass a 2" drain pipe (it is a bathroom sink) through.
      Thank you for your help.

  4. tommay | | #5

    Can I assume the support beam in the basement supports a floor above? Wouldn't that mean that there are floor joists resting on the beam. Is there a finished ceiling in the basement? If so, wouldn't there be a 10 -12 inch space between the beam and the floor, with possibly just a piece of blocking that you can either drill through or knock out, in which you can run your 1-1/2 pipe to be above the beam.

  5. gusfhb | | #6

    If it is actually 5 sistered 2x10's I have a lot less concern than a 200 year old old growth timber.

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Either way, the wood in the centre of the beam has virtually no forces acting on it unless there is rot.

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