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Pex holes in joists

Is it best to run only one 1/2" pex per joist hole, or is it OK to run 2, provided the hole is bored large enough & each tube is isolated to prevent expansion/chafing noise?



  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I am not a structural engineer...

    but I would worry about making holes too large in joists. I do not know what "too large" is, but I am confident running an air duct through one is too large. A hole for one BX cable may be OK, or a piece of 1/2 inch, or maybe 3/4 ich copper or PEX must be OK, but there must be some point at which making holes weakens it too much.
  • icesailor
    icesailor Member Posts: 7,265

    I'm not an engineer but I've always been cautious about weakening a structure.

    I've always liveb by "The Rules of Thirds". You can't drill a hole more than a third of the thickness of a joist in the middle third of the joist.

    If you have a 12' span, and the joist is a full 12", you can drill a full 4" hole in the first and last 4'.

    There is a side to this that I have discussed with inspectors. If the span requires say a 2X 10 joist spanning 12', the above rules apply. Or, if you are installing and you see a 12' span with 2X10's, and you see a 8' span, the 8' joist could theoretically be a 2x16 if you understand what I am saying. Therefore, you could drill holes in a larger portion of the joist following the "rules".

    Structural I-Joists are another issue. You can drill pretty much anywhere except in the beginning and end of the joist, some distance that I don't remember right now but it is something like the height of the span going on a triangle from the edge of the support outward to an apex. You can not drill in this space because the strength of the joist is in the bearing space and to weaken this area can allow the weight to crush the joist. Then, you need to check but you can't have the top or bottom of the hole within 1" to 1 1/2" of the board that makes the top or bottom.

    Therefore, you can pretty much drill 1 1/8" to 1 1/2" holes anywhere in a wood dimensional joist within the rules of thirds. And about anywhere in a I-joist. except along the top and bottom of the web.

    Hope that helps.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    edited December 2010
    DIfferent rules for different parts of the country...

    When I was making holes in lumber joists, as Chris said, not more than 1/3 of the depth of the framing element, must be in the center third and not more than 3 times the beams depth away from a support lintel/header.

    With TJI fabricated joists, you MUST follow the manufacturers guidelines, and basically you MUST be at LEAST 3 times the elements depth away from a support lintel before you are allowed to take more than 1-1/4" out of the middle of the element, but if you are more than 3 times away , you can take the whole ply wood webbing out to within 1/2" of the chord.

    If in doubt, check with the code authorities or a qualified structural engineer.


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  • Gordy
    Gordy Member Posts: 9,546
    edited December 2010
    Mark is right

    Those are the guide lines. I will add that with TJI joists you actually can make a bigger hole the more you are in the center of a span....According to followed manuf. specs. The opposite of dimmensional lumber.

    In my area they like to see a plywood gusset around the area of the hole 12" X lumber dimmension to prevent splitting.

This discussion has been closed.