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Thread: Should I connect a 3" or 4" flange to 4-in drain pipe?

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    Geotechnical Engineer Fistor's Avatar
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    Mar 2008
    Vancouver, BC

    Default Should I connect a 3" or 4" flange to 4-in drain pipe?

    Hi -

    The title says it all. For my toilet installation, in which I am installing a new ABS closet flange, do you recommend that I use a 3-in or 4-in flange (that is the underside diameter of the flange) to connect to the 4-in ABS riser?

    As I see it, the advantage to using a 4-in ABS is that it would connect around the outside of the riser. Disadvantage is the decreased width around the understand of the flange rim for mounting to the floor (i.e. tighter hole clearance), and perhaps that the top rim of the riser, inside the flange now, might be prone to catching or snagging material...?

    The advantage for the 3-in is that it mounts inside the 4-in riser, so no snagging, and as well, there is more width beneath the flange for floor mounting. The disadvantage (maybe) is that this is a restriction, relatviely speaking, to the rest of the drain, which is 4-in. I realize that 3-in is probably just fine for the new, low-volume flush toilets, but is a larger flange (i.e. 4-in) better?

    Let me know what you guys think, thanks!

    (extra: I am splicing this ABS riser to the old cast iron riser (need about 2-ft length) - I actually could go with a 3-in pipe spliced to the old 4-in cast iron, but I figured that keeping everything 4-in would be better - thoughts on this??)
    Last edited by Fistor; 08-23-2008 at 02:27 PM.


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