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  1. #1
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    Default Maximum drain slope

    I've been told that once the slope on a drain pipe exceeds 1/2" per foot it should be sloped at 45 degrees or greater to ensure correct scouring of solids, however I can't find the code requirement mandating this. Is it a code requirement, or just good practice?

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    John Arnold's Avatar
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    Default Re: Maximum drain slope

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
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    Default Re: Maximum drain slope

    Wow... I've been around here long enough to have forgotten that I posted in that thread!

    Rereading it, however, I 'm not convinced the question really got answered...

    Michael Thomas
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    Default Re: Maximum drain slope

    Drainage fittings are required to be designed to maintain 1/4" per foot slope.

    Drainage piping is required to have a minimum slope of 1/4" per foot for sizes 2-1/2" and smaller, and, a minimum slope of 1/8" per foot for sizes 3" and larger.

    Table P3005.4.2 shows three slopes: 1/8" per foot, 1/4" per foot, and 1/2" per foot for calculating number of fixture units allowed.

    The maximum slope of horizontal branch piping is 1/2" per foot.
    - From the 2006 IRC.
    - - P3110.3 Slope and size of horizontal branch.The maximum slope of the vent section of the horizontal branch drain shall be one unit vertical in 12 units horizontal (8-percent slope). The entire length of the vent section of the horizontal branch drain shall be sized for the total drainage discharge to the branch in accordance with Table P3005.4.1.


    The maximum slope of a horizontal combination waster and vent pipe is 1/2" per foot.
    - P3111.2.1 Slope.The horizontal combination waste and vent pipe shall have a maximum slope of 1/2 unit vertical in 12 units horizontal (4-percent slope). The minimum slope shall be in accordance with Section P3005.3.


    Yes, it has to do with the water speed. Too much slope and the water speed will outrun the waste it is carrying with it, leaving the waste behind to clog the line up.

    If you need to exceed 1/2" per foot slope, then either use a 45 degree slope or a tie into a vertical drop.

    The above information is from what I have learned over the years and from very knowledgeable plumber (the ones who teach the courses for apprenticeship programs).

    Back up and supporting documents in writing? only what I've posted above.





    Jerry Peck, Construction / Litigation Consultant
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  5. #5
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    Default Re: Maximum drain slope

    I was talking about this with my plumber a few weeks ago when we were laying out my waste and vent lines. The fittings are designed to fit tight at 1/4" / foot, which is why if you start getting too crazy on your slopes, you have a much higher change of a leak at the fitting. I didn't ask him about the solids being left behind, but it makes sense at some level. He was quite picky about making sure things were either plumb or sloped properly at the 1/4" / foot designation.

    Jim Robinson
    New Mexico, USA

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    Jerry Peck's Avatar
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    Default Re: Maximum drain slope

    Quote Originally Posted by Jim Robinson View Post
    He was quite picky about making sure things were either plumb or sloped properly at the 1/4" / foot designation.

    Being "uniformly sloped" is quite important and is also a code requirement.

    You do not want a dip down, that's real bad (creates a bit of a trap), and you do not want a belly up, that's real bad too (creates a bit of a dam).

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Maximum drain slope

    Quote Originally Posted by fritzkelly View Post
    IMO: The fittings are designed to be installed with the pipe level with the fitting. Any slope should be done with sweeps, etc, not by bending the pipe down.
    Drain fittings are designed to start that 1/4" per foot slope, they are required by the code to be designed that way.

    Thus, yes, the 1/4" per foot slope fits straight into the fitting with no angle to it.

    Jerry Peck, Construction / Litigation Consultant
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  8. #8
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    Default Re: Maximum drain slope

    Jerry Peck has the answer "nailed". Great reply


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    Default Re: Maximum drain slope

    Quote Originally Posted by Jerry Peck View Post
    The maximum slope of horizontal branch piping is 1/2" per foot.
    - From the 2006 IRC.
    - - P3110.3 Slope and size of horizontal branch.The maximum slope of the vent section of the horizontal branch drain shall be one unit vertical in 12 units horizontal (8-percent slope). The entire length of the vent section of the horizontal branch drain shall be sized for the total drainage discharge to the branch in accordance with Table P3005.4.1.
    Uhhhhhhh, 1 in 12 is 1" per foot. No?


  10. #10
    Jerry Peck's Avatar
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    Default Re: Maximum drain slope

    Quote Originally Posted by Jerry Peck View Post
    The maximum slope of horizontal branch piping is 1/2" per foot.
    - From the 2006 IRC.
    - - P3110.3 Slope and size of horizontal branch.The maximum slope of the vent section of the horizontal branch drain shall be one unit vertical in 12 units horizontal (8-percent slope). The entire length of the vent section of the horizontal branch drain shall be sized for the total drainage discharge to the branch in accordance with Table P3005.4.1.

    The maximum slope of a horizontal combination waster and vent pipe is 1/2" per foot.
    - P3111.2.1 Slope.The horizontal combination waste and vent pipe shall have a maximum slope of 1/2 unit vertical in 12 units horizontal (4-percent slope). The minimum slope shall be in accordance with Section P3005.3.
    Quote Originally Posted by Dennis Webber View Post
    Uhhhhhhh, 1 in 12 is 1" per foot. No?
    I posted both sections, one is incorrect (as you pointed out) and one is correct (as you did not point out).

    Yes, "one unit vertical in 12 units horizontal" = 1" per foot.

    And, yes, "1/2 unit vertical in 12 units horizontal" = 1/2" per foot.

    I apparently got the fingers typing and made a typo - oops - thanks for catching it.

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    Default Re: Maximum drain slope

    Quote Originally Posted by Jerry Peck View Post
    I posted both sections, one is incorrect (as you pointed out) and one is correct (as you did not point out).

    Yes, "one unit vertical in 12 units horizontal" = 1" per foot.

    And, yes, "1/2 unit vertical in 12 units horizontal" = 1/2" per foot.

    I apparently got the fingers typing and made a typo - oops - thanks for catching it.

    I had assumed the vent pipe, which shouldn't be carrying any solids, could be steeper. But you say it was a mistype?

    Egbert Jager
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    Jerry Peck's Avatar
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    Default Re: Maximum drain slope

    Quote Originally Posted by Egbert Jager View Post
    I had assumed the vent pipe, which shouldn't be carrying any solids, could be steeper. But you say it was a mistype?
    The mis-type (typo) is where I said that 1" in 12" was 1/2" per foot, as brought up by Dennis in this post of his:
    Quote Originally Posted by Dennis Webber View Post
    Uhhhhhhh, 1 in 12 is 1" per foot. No?
    I included both sentences from my post to show the typo and where it was - that the typo was in the 1" in 12" was 1/2" per foot (1" in 12" is 1" per foot), not in the 1/2" in 12" was 1/2" per foot.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Maximum drain slope

    Quote Originally Posted by Jerry Peck View Post
    The maximum slope of horizontal branch piping is 1/2" per foot.
    - From the 2006 IRC.
    - - P3110.3 Slope and size of horizontal branch.The maximum slope of the vent section of the horizontal branch drain shall be one unit vertical in 12 units horizontal (8-percent slope). The entire length of the vent section of the horizontal branch drain shall be sized for the total drainage discharge to the branch in accordance with Table P3005.4.1.


    The maximum slope of a horizontal combination waster and vent pipe is 1/2" per foot.
    - P3111.2.1 Slope.The horizontal combination waste and vent pipe shall have a maximum slope of 1/2 unit vertical in 12 units horizontal (4-percent slope). The minimum slope shall be in accordance with Section P3005.3.


    I don't think you actually did make a typo...there are two allowable maximums depending on the purpose of the pipe. Horizontal branch piping can be up to 1:12 when it is a vent section, but cannot be more than 1/2:12 when its a combination waste and vent pipe.

    Not sure what one might gain by going that little bit steeper because, as you pointed out, the fittings aren't really designed for that.


    Egbert Jager
    Diamond Home Inspection
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  14. #14
    Jerry Peck's Avatar
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    Default Re: Maximum drain slope

    Quote Originally Posted by Egbert Jager View Post
    I don't think you actually did make a typo...
    I did make the typo, where I typed this:
    The maximum slope of horizontal branch piping is 1/2" per foot.
    - From the 2006 IRC.
    - - P3110.3 Slope and size of horizontal branch.The maximum slope of the vent section of the horizontal branch drain shall be one unit vertical in 12 units horizontal (8-percent slope). The entire length of the vent section of the horizontal branch drain shall be sized for the total drainage discharge to the branch in accordance with Table P3005.4.1.
    "one unit vertical in 12 units horizontal" is 1" per foot, not 1/2" per foot as I typed in the quote above.

    Jerry Peck, Construction / Litigation Consultant
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  15. #15
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    Default Re: Maximum drain slope

    I think the reasons for the max slope, stated in the 2005 thread (referred to by Michael) are valid but isn't it also because if the slope is too great, the water will flow so fast that it will drain any p-trap along it's path?

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  16. #16
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    Default Re: Maximum drain slope

    Quote Originally Posted by Doug Campone View Post
    I think the reasons for the max slope, stated in the 2005 thread (referred to by Michael) are valid but isn't it also because if the slope is too great, the water will flow so fast that it will drain any p-trap along it's path?
    Hopefully once the drain line has started on it's path to the final destination there are no more P-traps. But I suspect you are suggesting other traps that tie into the main along it's route. Sucking a trap dry is not rare but that's why there are venting systems.


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