How much slope is too much?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by nosajha, Oct 31, 2007.

  1. nosajha

    nosajha New Member

    Messages:
    6
    I've received conflicting advice and thought I'd ask folks who have real experience with this. Has anyone ever seen a drain stopped up due to having too much slope? If so, how much slope is too much?

    If a 3" PVC toilet drain has a slope of 3/8" inch per foot, is that more likely to cause trouble or is it more likely that it will work ok in spite of being more than the standard 1/4?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    As long as you have the minimum slope or more, you are okay. There is an old wife's tale about liquids overrunning the solids. Gravity works on both the same...
  3. jimbo

    jimbo Plumber

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    San Diego
    If there were such a thing as too much slope, what do you do in a 10 story building? I suppose if you are talking about slopes in excess of 45º or so, then the same considerations as apply to high vertical stacks might come into play. What is your specific situation?
  4. frenchie

    frenchie Jack of all trades

    Over 45 is okay, gravity takes over like Jadnashua said.

    Below 1/4 per foot, things stay mixed & all move together.

    Between 1/4 per foot and 45 is where you can get into trouble, gravity's not enough of a factor to overcome friction (which acts diferently on solids & liquids).

    Related topic: there's such a thing as an oversized drain.
  5. nosajha

    nosajha New Member

    Messages:
    6
    I had heard what jadnashua says is a wive's tail about liquids outrunning solids if the slope is between 1/4" per foot and 45º.

    My specific situation is what I described with the 3/8 inch per foot slope. I'm installing a new pipe which has this slope unless I break out more of a nasty old wall made of concrete and large river rocks. But if it's just a solution looking for a problem, why bother.
  6. got_nailed

    got_nailed DIY Senior Member

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    Hmm there will be Teflon coated PVC in a year or to I bet.
  7. hj

    hj Moderator & Master Plumber Staff Member

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    slope

    as long as you meet or exceed the minimum 1/4" per foot slope you will not have any problems.
  8. nosajha

    nosajha New Member

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    6
    thanks

    Thanks for the info everybody. Much appreciated.
  9. Alectrician

    Alectrician DIY Senior Member

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    689
    Hmmmm...what DO they do?

    When you flush a toilet on the tenth (or 110th) floor does it just drop straight down? It must make one hell of a splash when it hits. Do they throw some kind of baffling system in place? Do they swoop it up like a roller coaster?

    Anyone???
  10. CHH

    CHH New Member

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    Location:
    Denver, CO
    Well now that's a pretty picture (not).

    The "liquid" flow will be on the pipe walls and maybe some solid (ish) slugs with air venting up the center. Call it churn flow and let it go at that...

    (There are people that develop numerical models in order to predict behavior of this type of flow. Ok, maybe the models don't include solids...)
  11. Winslow

    Winslow Plumber

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    Location:
    Hawaii
    When the waste water enters the stack from the drain it doesn't just freefall down the pipe. It hugs the wall of the pipe and drains in a circular motion. As the water flows down the stack it increases in speed until it reaches terminal velocity. It continues flowing down the pipe in a circular motion gradually closing up until the pipe is running "full". As the water flows down the pipe it is then pushing the air ahead of it. When the flowing water builds up enough pressure (from the air)it then "breaks up" and starts the proccess all over again. That is why tall buildings require yoke vents to prevent the system from building up enough pressure under the flowing water to blow the traps on the fixtures underneath it. The Sovent system is designed to prevent the water from reaching that point without the use of a yoke vent. It does however have a relief vent at the bottom of the stack. That is the general principle without getting into details. Doesn't matter if the water flows down 10 stories or 100 stories, once it reaches it's terminal velocity it will not fall any faster. Hope this helped your curiosity.
    Last edited: Nov 1, 2007
  12. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    velocity

    And some systems are designed with an offset in the riser every few floors to reduce the velocity even more.
  13. Alectrician

    Alectrician DIY Senior Member

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    689
    Thanks, it does help. Now I have the image of a 120MPH turd :)

    Can you describe a yoke vent?
  14. FloridaOrange

    FloridaOrange Plumbing Designer

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    SW Florida
    A yoke vent is just a vent that wye's off the main stack then turns up and goes through the roof.
  15. Alectrician

    Alectrician DIY Senior Member

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    689
    So if I were to cap off the vents on a hi rise, there would be some serious snit flying out of the ground level toilets?
  16. molo

    molo New Member

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    Location:
    cold new york
    1. Is there a problem with 1/4" foot - 45 degrees? Nobody has chimed in on this one, except HJ to say that anything over 1/4" is OK.

    2. As far as the oversized drain comment nobody has chimed in on that either. A plumber who did a carwash told me he plumbed the drains in 3" PVC. He said bigger drains would have allowed the sand/silt/etc to build up on the "flatter" portions of the bottom of the pipes.

    2 good points, what's the veridct?
  17. Winslow

    Winslow Plumber

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    Location:
    Hawaii
    A yoke vent takes off the waste stack below the floor rises above the same floor and ties into the vent stack. It's purpose is to prevent excessive fluctuations in atmospheric pressure in the waste system
  18. Winslow

    Winslow Plumber

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    450
    Location:
    Hawaii
    A yoke vent takes off the waste stack below the floor, rises above the same floor and ties into the vent stack above the flood rim of the floor its installed on. It's purpose is to prevent excessive fluctuations in atmospheric pressure in the waste system. It is installed every 10th floor (if my memory serves me correctly) in high rise buildings.
  19. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii
    Practically speaking having excessive slope will cause no problems unless there is an excessive amount of turns in the drain line. With minimal water the solids will be restricted more so than the liquid while negotiating the turns. Eventually the liquid will outrun the solids. On a fairly straight run of pipe the solids will remain suspended and travel along with the liquid.
    Last edited: Nov 2, 2007
  20. Alectrician

    Alectrician DIY Senior Member

    Messages:
    689
    ^^Did you mean "will NOT cause problems"^^
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