Reduce 4" cast iron to 3" pvc?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by WiWilliam, Apr 8, 2014.

  1. WiWilliam

    WiWilliam New Member

    Messages:
    7
    Location:
    Wisconsin
    1st post here, been searching but I can't find my answer.

    Our house is in WI, ranch, 1 bath, kitchen, and laundry.

    Currently has 4" cast iron into the basement, previous owner converted to 4" pvc right where the sewer line enters the house. The 4" is continuous up through the roof. This is the only roof penetration for venting

    We are moving the location of everything in the house so all the pvc will be replaced. I want to use 3" for this but the cross sectional area of the main drain requirement for venting is throwing me for a loop...

    Can I reduce to 3" where the previous owner converted over to pvc, including for the vent through the roof? I would keep a 4" cleanout at the floor level in the basement right above the conversion from cast iron to pvc. I would then convert to 3" for inside the house, including the vent through the roof.

    Or do I need to use 4" because that is what is going out to the street/sewer?

    Thanks!
    Last edited: Apr 8, 2014
  2. Reach4

    Reach4 Well-Known Member

    Messages:
    2,490
    Location:
    IL
    While you are doing sewer changes, maybe you would want to consider converting to overhead sewers. Has anybody in your area had basement sewer backups after a big rain? As the people downhill from you no longer accept water, the water finds a new place to go.

    I don't have what you are asking for. Do note that you can increase vent pipes as they head upward. So you can keep your 4 inch through the roof.
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,180
    Location:
    New England
    The size of the drain pipes is determined by the fixture units of the things it needs to carry...each thing has an assigned fixture unit. You add them up, and that will tell you the size pipe you need.

    If you happen to live in an area that regularly gets below freezing, they often increase the size of the vent from a bit below the roofline and on out so that the vent is less likely to close off with hoar frost. The air in the pipe is warm and damp...when it gets to the roof, the moisture can condense and then freeze. Having that opening larger means it is less likely to close off.
  4. Terry

    Terry Administrator Staff Member

    Messages:
    15,146
    Location:
    Bothell, Washington
    Most codes allow three toilets on 3"
    Venting would be either 3", or two 2" vents and one 1.5" vent, which also equals 3"

    4" is needed when a forth toilet is added. Having a 4" main sewer line prevents a limitation in the future.
  5. WiWilliam

    WiWilliam New Member

    Messages:
    7
    Location:
    Wisconsin
    Thank you for the replies everyone!

    We only have 1 toilet in the house.

    I just want to know if the old cast iron 4" coming in from the city sewer can be reduced to 3" once it enters the house above the basement floor?
  6. MACPLUMB 777

    MACPLUMB 777 TROJAN WORLDWIDE SALES RP

    Messages:
    679
    Location:
    Houston, Texas, United States

    YES you can go from 3" into 4' but not 4" into " always smaller into bigger going down stream !
  7. WiWilliam

    WiWilliam New Member

    Messages:
    7
    Location:
    Wisconsin
    Perfect! Thank you.
  8. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,873
    Location:
    Cave Creek, Arizona
    You have to know how your code is written. There are several different venting requirements. Some require the vent to be "full size" all the way through the roof, in which case you WOULD need a 4" line. Others allow the aggregate areas, NOT diameters, of the roof vents to equal the area of the incoming sewer, in which case you could use any of several combinations, i.e., four 2" vents, a 3", a 2" and two 1 1/2", or any of several other combinations. But, others would allow you to reduce to 3" and compute the vent sizes dependent on that pipe. Only your local building department can tell you what you can do with the venting.
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