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Thread: Installing a flange over a concrete slab

  1. #1
    DIY Senior Member rsmith99's Avatar
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    Default Installing a flange over a concrete slab

    I pulled my old toilet today. The old toilet was set over a vinyl floor over a cement slab. The old flange was set about 1/4 inch above to slab. Too high for the vinyl and too low for the tile I am putting in.

    After reading posts on the forum it appears the correct way to install a flange on a concrete floor is to have it set on top of the tile and held down with screws. I have looked at a lot of new houses in the Oklahoma City and Edmond, OK area that are under construction. I don't recall ever seeing the flange screwed down to the slab. But I would like to do it the correct way so...

    My questions are;

    - How do I fill the area around the pipe with cement, but leave space for the new flange.

    - If the cement is filled up to the drain pipe, there is only about 1/2 inch of cement between the screws that will be put in to hold the flange and the drain pipe. Seems this would break the cement a soon as the screw is tightened.

  2. #2
    In the Trades Gary Swart's Avatar
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    Default

    You are correct about the flange being screwed to the slab. If it was over a wood sub floor, then it would screw to the sub floor. To fill in around the drain pipe, first use a removable wrap around the pipe. They actually make a foam wrap for this purpose, but you could improvise something else. Then use a quick set concrete to fill the void. This is really just a hard filler, it doesn't support anything. Smooth it level. Lay what ever flooring you are using then set the flange. There are a couple ways you could set anchor screws into the new concrete. My favorite way to anchor into any concrete is with lead ferrales. I drill a 5/16 hole for the ferrales then screw #12 stainless steel screws through the flance holes and into the lead. Others use Tapcon screws which do works as well. You would want to orient the flange, carefully mark the hole locations, then remove the flange while you drill the holes. I use a rotary hammer drill that must makes punching holes in concrete as easy as drilling wood. Some guy notch the tile when laying it to avoid having to drill though it.

  3. #3
    DIY Senior Member rsmith99's Avatar
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    Default Convert to 4 inch pipe

    Could I convert to a 4 inch pipe so I can fill all the way to the pipe and install an inside flange?

  4. #4
    In the Trades Gary Swart's Avatar
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    Not sure what your drain system is. You can not start with 4" then reduce to 3" of that's what you want to do. You can start small then increase in size, but not the other way. If you have a 3" drain, use a flange that fits on the outside of the pipe.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Default flange

    I ALWAYS transition from a 3" pipe to a 4" riser using a spigot closet bend. But I NEVER use an internal flange unless it is for a repair when the original plumber used an all plastic outside flange. Also, on concret slabs we never screw the flange down. The soil and ABC packed around the horizontal pipe prevents any movement.

  6. #6
    DIY Senior Member gardner's Avatar
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    Default confused

    Quote Originally Posted by hj View Post
    on concrete slabs we never screw the flange down
    Really? Why is it okay to not fasten down the flange on concrete? Packed or not, the pipe is just ABS or PVC. Tensioning the closet bolts could put a lot of force and break the pipe.

    I though it was required to be fastened to the subfloor -- whether that is concrete or whatever.

    Last edited by gardner; 12-10-2008 at 01:21 PM.

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