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  1. #1
    Join Date
    May 2009

    Question Drilling Holes in Floor Joists for plumbing

    I need to reroute a kitchen drain line which involves drilling a hole thru 3 floor joists, what is the rule on the diameter of the hole size in relation to the size of the joist and what is the minimum distance from the edges and the end of the joist where the hole can be so not to effect the structure?

  2. #2
    Join Date
    May 2009

    Default Re: Drilling Holes in Floor Joists for plumbing


    Don't remember the exact rules... Codes vary by area. A lot of electrical books have stuff about it - my electrical are currently at another locale..

    Anyway - you should try to avoid the end of each joists. Staying 1/3 distance away from each end. Holes preferably are put in the middle 1/3rd

    Joists can take a lot of 'swiss cheesing'... but the bottom of any hole should have a min of 2" from the bottom of the joist. This helps prevent puncturing of plumbing lines, and provides sufficient edge margin.

    A note that does not necessarily apply in a flooring book (which I do have in front of me) states (about notching joists): You may need to notch the bottom edge so it can fit over the foundation or beam. If that's the case with your joists, cut the notches in the ends no deeper than 1/8" of the actual depth of the joist. For example, a 2 x 12, which is actually 11.5" wide, can have a notch (1 7/8") deep."

    The above is almost two inches - and this is being placed at the end of the joists - where they don't want you messing around...

    Your plumbing line is not going to significantly hurt the strength of the joist. If you add more than one hole to any 1 joist - keep the holes at least 4" apart...

    More specifics - go get a couple of books... and read up. Readily available at hardware stores.

    Hope the above helps.

  3. #3
    Join Date
    May 2009

    Default Re: Drilling Holes in Floor Joists for plumbing

    I also just reread - and realized your putting in DRAIN lines.

    Significantly larger than supply, for either water or electrical.

    A 3" or 2" drain line is going to have a large hole...

    You can also substantially help - by sistering in a Doubler.

    Purchase a board the basic same dimensions as the joists your working on.

    Cut a length about 2' for a 3" drain line... and then glue and screw it to the joist in the location where the drain hole will be located. Then drill the drain line hole in the center.

    The doubler properly fastened to each of the affected joists - will significantly help load forces flow thru the wood.

    You might be able to get by without a doubler - if the joist is wide enough, and your drain hole is not extremely large... Joists can take a lot.


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