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

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Fistor, Aug 23, 2008.

  1. Fistor

    Fistor Geotechnical Engineer

    Mar 18, 2008
    Geotechnical Engineer
    Vancouver, BC
    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: Aug 23, 2008
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    Nov 23, 2006
    disabled-retired industrial fabricator
    200 miles south of Little Rock
    Yes, but the discharge hole in the bottom of the toilet is only around 2", thereby restricting the actual flush going into a pipe about twice its size while joined at a 3" opening. Personally, I use 3" flanges and 3" pipe inside my house since the opening at the bottom of the toilet is less than the pipe i.d. and nothing is very likely to catch on the top edge of the riser as long as it has been de-burred.
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  4. Terry

    Terry Administrator Staff Member

    Aug 17, 2004
    Bothell, Washington
    If you have a 4" coming up, it would seem almost easier to just couple on some more 4" and use the 4" hub flange.
    You do need to cut a round hole if you go that way.
  5. Herk

    Herk Plumber

    Oct 28, 2007
    S.E. Idaho
    I prefer the 3" flange because of it's larger surface, too. The problem, as I see it, is that you have a 4" pipe in the first place. I can think of no reason for a residential closet pipe to be more than 3".
  6. Basement_Lurker

    Basement_Lurker One who lurks

    Jun 21, 2007
    I smash things and demand money.
    Victoria, BC
    Having a 4" pipe there is a luxury...take the easy way out and use a 3" internal flange..using a 4" flange will make no difference in terms of functionality of the "shit-hole"!
  7. Redwood

    Redwood Master Plumber

    Dec 15, 2007
    Service Plumber
    I like using a 4" flange the first time. It leaves other options open for flange replacement in the future.
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