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Thread: How to sequence closet flange install vs. CBU

  1. #1
    Geotechnical Engineer Fistor's Avatar
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    Default How to sequence closet flange install vs. CBU

    Hi all -

    I am replacing the closet flange as part of my bathroom reno, and thought I had everything figured out, but now I am scratching my head about sequencing of installation. Here is my dilemma:

    - I removed the old flange (was beyond reuse) and the uppermost portion of the old cast iron riser
    - I plan to add an ABS flange on an ABS pipe section that I join to the remaining portion of the cast iron riser. Have not done any of this yet.
    - I will be putting down cement board for flooring, and have cut a nice hole in the middle of it through which the new ABS pipe/flange will pass
    - I thought that I'd connect the new section of ABS pipe first, then place the cbu, then add the flange after the flooring is down. However...

    My concern: It seems like it will be hard to properly glue the flange on to the pipe, given that the joint will be slightly below the base of the cbu and I'll be working through a hole that does not have excess clearance around the edges.

    If the cbu is installed, there will not be a lot of clearance for screwing around with the cement, or making sure that the joint is good/proper. Once the cement begins to fuse, I'm done. With the cement board mortared and screwed down, there are now easy "re-do's", as I see it.

    Any thoughts? Should I instead install the pipe and flange, then add the flooring in sections around the sides of the riser/flange? Also, I guess that regardless, prior to gluing the flange on, I'll need to pass the closet bolts through, because once the flange is down, there will be no way to pass these bolts under and up...

    confused....

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    If the pipe is 4", then you can go right up to the pipe and use an inside mount flange. If it is 3", you need to leave enough gap around it to insert the flange on the pipe. If you leave that gap, you should be able to get enough glue on it through that gap.

    If you have access from below, you could attach the flange to a stub of the abs, then anchor it to the floor and attach to the existing cast iron after. A little careful measuring and it should be easy.

    When tiling, it's easier to notch the tile where the anchor bolts go than to try to drill through the tile afterwards - you'll probably need a diamond bit.

    When the flange is installed at the proper location, getting the closet bolts in or out is easy.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Removing the upper portion of the cast iron was probably not necessary and makes your task more difficult. The flange should be the last thing installed. You will have an annular ring around the pipe so applying cement to it should not be a problem, since the pipe should be even with, or slightly above the finished floor's surface.

  4. #4
    Geotechnical Engineer Fistor's Avatar
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    Quote Originally Posted by hj View Post
    Removing the upper portion of the cast iron was probably not necessary and makes your task more difficult. The flange should be the last thing installed. You will have an annular ring around the pipe so applying cement to it should not be a problem, since the pipe should be even with, or slightly above the finished floor's surface.
    Thanks for the suggestions, guys - sounds like making a slightly larger hole is the way to go - although I take it I should absolutely make sure to leave the hole small enough to provide full support to the underside of (the edge of) the flange.

    hj, I cut the old pipe because the old flange was so old, crusted, dented, and beat up that it needed to be replaced, and there really seemed no neat or clean way to remove it from the old riser. But you're right - it's this height thing that's causing me this problem.

    So, I'll connect the ABS pipe, install the floor, then put the flange on last, as suggested, through a hole large enough to allow glue application, etc.

    Thnx for the tips!

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