Moving, reducing and changing 4" cast iron vent pipe

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by jbclem, Dec 9, 2004.

  1. jbclem

    jbclem New Member

    Messages:
    5
    I have a 4" cast iron vent from my toilet (with one sink vent going into it) that goes straight up to the roof (1 story bldg). I would like to recapture some of the space it goes through so I'd like to cut it, convert it to ABS, run it horizontally about 24 inches, up about 48", then another right angle back to join the original cast iron just below where it goes through the roof.

    There are some questions I have about doing this...
    1. Ok to run the toilet vent horizontal for 2 feet?
    2. What's the best kind of adaptor (cast iron to ABS)?
    3. Can I use either black or white(thiner) plastic pipe?
    4. Can I reduce the plastic pipe down to 2"?
    5. Will the adaptor(s) become a weak spot over time, subject to leaking and smells?
    6. Should I continue with the plastic pipe up through the roof, instead of reconnecting to the cast iron pipe?
    7. Ok to cut and adapt the cast iron pipe around the level of top of the toilet tank?...it's a standard 2 piece toilet.

    That's all I can think of right now, but I'd appreciate any advice, especially if someone thinks this is not the correct way to do this.

    John
  2. LonnythePlumber

    LonnythePlumber Plumber, Contractor, Attorney

    Messages:
    319
    Location:
    Wichita, Kansas
    Elbow Above Lavatory

    Your first fitting should not be until 6" above the floor level rim of the highest fixture served. This would be your lavatory. A no-hub clamp connection would not be a week point. You can install bends in your vent but the piping should grade downward. Reconnection to the cast iron at the roof is okay. You may be able to reduce the size of your vent but it depends on the total vents serving the house. You have to have the same vent area as the house drain which is four inch. If you have two other two inch vents you may be able to reduce. Schedule 40 pipe is the same size outside diameter. You cannot use thinner wall pipe. PVC (white) and ABS (black) are the same size.
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,815
    Location:
    Cave Creek, Arizona
    vents

    Everything okay until you got to the reduced pipe size. He would need four 2" roof vents to equal a 4" main sewer pipe.
  4. LonnythePlumber

    LonnythePlumber Plumber, Contractor, Attorney

    Messages:
    319
    Location:
    Wichita, Kansas
    3+2+2=4

    I meant reducing the main stack to a 3" if he has two 2" vents now. Two 2" is 6.28" plus the 3" at 7.06 is 13.34" which satisfies our 4" 12.56 requirement for aggregrate cross-sectional area.
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