Shower Install -- How Downsize Existing Drain Hole Siize

Discussion in 'Shower & Bathtub Forum & Blog' started by unionmaid, Jan 19, 2010.

  1. unionmaid

    unionmaid New Member

    Jan 5, 2010
    Johnstown, OH
    We're installing a cement shower pan in our upstairs bathroom.

    The existing drain hole in the sub floor is too big -- 7.5" diameter.

    Our abs Oatey drain requires only a 4.5" hole.

    The sub floor is made of 3/4 plywood over joists.

    Any recommendations?

  2. VolsMan

    VolsMan New Member

    Jan 19, 2010
    West TN

    One way I would consider is to set the blade depth of circular saw to cut 3\4", remove the appropriate nails in the sub floor and cut a 16" square of the subfloor(center of one floor joist to center of other floor joist) that incorparates the 7.5" diameter existing hole. Properly replace with new piece of 3\4" and re-cut the hole the desired size.

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  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    That's a reasonable approach, but I'd also consider putting in some blocking on the cross joist ends of the patch otherwise, it will not be as sturdy, which is highly desireable under a shower pan. Check out for help on tiling. For maximum strength, a patch should bridge two joist bays and anchor to three joists. If you put blocking in, it's almost as strong.
  5. unionmaid

    unionmaid New Member

    Jan 5, 2010
    Johnstown, OH
    Thank you Jim and Bubba. We have been reading a lot on the johnbridge site and that has been helpful. We got the shower floor done by laying down another sheet of 3/4" plywood. Mortar shower pan is next.
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