Slope requirement on 3" horizontal drain - 1/8" or 1/4" per foot? Drain design?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by Kubota_Kid, Sep 6, 2012.

  1. Kubota_Kid

    Kubota_Kid New Member

    Messages:
    3
    Location:
    Shelton, WA
    One source specifically states that horizontal 3"+ drains require 1/8" slope (< 3" require 1/4" slope). Another source says 3" drains require 1/4" slope. I am installing a 3" drain under slab that will accomadate a 1/2 bath in a shop. The main 3" drain will run approximately 26' before it enters the septic tank. Do I go with 1/8" or 1/4" slope?

    My design is to start the drain with a 3" closet bend. Down stream 2.5' from the bend I will install a 2" to 3" waste T with the 2" entrance at the top. From the waiste T the main 3" drain continues straight for 23' to the septic tank. At the waste T I will use a 2" long sweep 90 degree elbow that I will run horizontally from the waste T for approximately 2.5' before using another 90 degree long sweep elbow which will send this 2" wet vent/drain pipe vertical in order to drain the sink and vent the system. Comments or suggestions would be appreciated.

    Thank you,

    Kubota_Kid - Shelton, WA
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,641
    Location:
    Cave Creek, Arizona
    IT depends on which code is used in your area. But, one error is that you CANNOT use a sanitary tee in a horizontal line.
  3. Kubota_Kid

    Kubota_Kid New Member

    Messages:
    3
    Location:
    Shelton, WA
    How would you connect the 2" wet vent/drain into the horizontal drain down stream of the closet elbow? Thanks. KK
  4. johnjh2o1

    johnjh2o1 Plumbing Contractor for 49 years

    Messages:
    1,143
    Location:
    South*East
    Use a Y and 45.

    John
  5. kreemoweet

    kreemoweet New Member

    Messages:
    371
    Location:
    Seattle. WA
    There are no "sources" that matter other than your local plumbing code. As near as I can tell, that would be the 2009 Uniform Plumbing Code, possibly
    with local amendment. I don't have that version handy, but a somewhat earlier one states that 1/4" per foot is the minimum slope required for ALL drain piping,
    provided that 4" or larger pipes, under some unusual circumstances, and with prior approval of the local authority, can have as little as 1/8" fall per foot.
    Nothing prevents, and in some cases it would be desirable, to have greater slope. I have found that it is always desirable to have greater than 1/4" slope
    when running pipe in a trench, and for any distance.
  6. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,175
    Location:
    Maine
    Does Washington allow wet venting? I believe its UPC which unless it's been amended, does not allow wet venting.
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,641
    Location:
    Cave Creek, Arizona
    UPC has ALWAYS allowed wet venting, if it is done properly. If he is using the IPC, they would probably allow 1/16" slope if someone asks for it.
  8. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,175
    Location:
    Maine
    UPC in Maine DOES NOT allow wet venting. IPC minimum pitch for pipes 3" and larger is 1/8 ipf.
  9. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    UPC Washington State allows wet venting for a bathroom group. ( No kitchen sink, no washer )
    1/4" per foot grade or better.
    Wyes on the horizontal.
  10. Kubota_Kid

    Kubota_Kid New Member

    Messages:
    3
    Location:
    Shelton, WA
    Just want to say thank you for all of the post. Very helpful. When I first browsed this site I was impressed by the professionalism of the questions and responses. A great group of folks. Thanks again. KK
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