Drain slope and depth questions

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by JimLS, Sep 21, 2019.

  1. JimLS

    JimLS Member

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    New construction in St. Louis county, Missouri.

    Looking to build a house with walkout basement and behind the house a detached garage with half bath (corner lot with house facing street with sewer main and garage access off other street). Sewer is only 8' deep so elevations are an issue. The other houses in the area stick the main floor out of the ground a bit and ours will be similar. I can get 2% slope from house to sewer so that is ok. Issue is half bath in garage. I can get 1% slope for this run if I raise the building floor 6" which would be easy to do. Run is about 100' from half bath to house drain connection. Due to the slope of the lot I will need to run this line to the corner of the house opposite the street corner to keep it buried and across under the basement floor. I have been told running under the slab is to be avoided if possible because future work that requires access to the pipe will be much easier if it is not under a building. That seems reasonable but it also seems like the pipe would be protected by the building and not likely to have issues with tree roots, etc, etc. How big of issue is it to run the drain pipe under the floor? Also, how close can drain and potable water lines be? I have seen requirements of 10' for water and building sewer but I think that was for the section that starts 5' from the building which for some reason is treated differently for some things.

    How deep does the drain line between buildings need to be? Footings here need to be at least 30" below ground but sewer will be about 24".

    Other choice is to put in a pump for the garage. Would like to avoid that if possible but if that is the best way I will do it. I could avoid running the line under the house and also get more depth. Garage just will have a half bath and another sink on the back side of the wall with the half bath plumbing so perhaps I could get a toilet/pump combo unit. A complication is I have a condensing furnace/AC and water heater so current plans show a floor drain. The HVAC is hung from the ceiling so could drain to a higher trap, not sure what to do about the water heater. And the condensate is corrosive - could put in a neutralizer.
     
  2. James Henry

    James Henry In the Trades

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    You can read the plumbing code book for where you live online.
     
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  4. JimLS

    JimLS Member

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    I did but didn't see these specific issues addressed. The local code is UPC2009 with local amendments so the local code just lists the points that are different. Did a bit more digging and found this on the slope question so 1/8" per foot might be ok. Still have the other questions. Some are more a matter of preference than code.

    708.0 Grade of Horizontal Drainage Piping.

    708.1 General. Horizontal drainage piping shall be run in practical alignment and a uniform slope of not less than 1/4 inch per foot (20.8 mm/m) or 2 percent toward the point of disposal provided that, where it is impractical due to the depth of the street sewer, to the structural features, or to the arrangement of a building or structure to obtain a slope of 1/4 inch per foot (20.8 mm/m) or 2 percent, such pipe or piping 4 inches (100 mm) or larger in diameter shall be permitted to have a slope of not less than 1/8 inch per foot (10.4 mm/m) or 1 percent, where first approved by the Authority Having Jurisdiction.
     
  5. James Henry

    James Henry In the Trades

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  6. James Henry

    James Henry In the Trades

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  7. hj

    hj Moderator & Master Plumber Staff Member

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    Two key points.
    1. It applies to 4" lines, and
    2. The AHJ must approve it.
     
  8. JimLS

    JimLS Member

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    Yes, I noticed the two key points. It could be a 4" line so that isn't an issue. Is going with the lower slope likely to cause problems for a half bath with 80' run? What about going under the house vs. around? These aren't code questions as they are allowed AFAIK but wondering how likely they are to be problematic down the road. Also the question of depth.
     
  9. James Henry

    James Henry In the Trades

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    1. Code requires 4" drain pipe and larger to have a grade of 1/8" per foot, it doesn't matter how far you run it.
    2. As far as pipe depth, your local building department should have all of that information for your location then all you have to do is the math.
    3. I just skimmed your local code books and found that the pipe needs to be 30" deep. The information's there you'll just have to weed it out.
     
  10. hj

    hj Moderator & Master Plumber Staff Member

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    I only put the lines 30" deep when the situation calls for it, otherwise any depth below the concrete is adequate
     
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    You also asked about water pipes...those must be below the frost line (which may not be very deep where you live) or, some minimum depth to help prevent them from easily being damaged.

    The hassle with trying to maintain an 1/8"/foot slope is that it's harder to ensure it is graded well with no flat spots or bellies, or reverse pitch. Stuff will flow slower, and anything that interrupts that like a flat spot or reverse pitch can create problems. Not that it's all that easy with 1/4"/foot, either. Takes some skill and craftsmanship.
     
  12. James Henry

    James Henry In the Trades

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    I appreciate that. it was referring to the building sewer.
     
  13. James Henry

    James Henry In the Trades

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    The frost line depth in St. Louis Missouri is 20". I'm starting to wonder why I bother. :)
     
    Last edited: Sep 22, 2019
  14. JimLS

    JimLS Member

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    It looks like it actually is 20" but local officials have deemed it to be 30". I am guessing that's why the 30" depth requirement. Building footings must be 30" minimum depth too.

    609.1 of the local code requires potable water to be minimum of 42" depth.

    https://www.stlouisco.com/Portals/8/docs/Document Library/Public Works/code enforcement/ordinances/09-upc-plumb-ord.pdf

    Still didn't find anything on depth requirement of drain/sewer.

    BTW, St. Louis city is not part of St. Louis county. Perhaps you were looking at city code?
     
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    One really doesn't want either of the pipes to freeze up, neither supply lines nor drainage ones. It can become a major inconvenience, and sometimes costly.

    If the lines don't leak, tree roots aren't as common - they'll be drawn to water if they leak. But, you still wouldn't want to put a tree over it. As the roots grow, it could push the pipe out of alignment, messing with the pitch.
     
  16. James Henry

    James Henry In the Trades

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    Look at section 718.3
     
  17. James Henry

    James Henry In the Trades

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  18. James Henry

    James Henry In the Trades

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    A building drain is inside a building. A building sewer is outside a building.
     
  19. JimLS

    JimLS Member

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    313.3 would seem to apply. Thanks!

    On building drain vs. sewer that may be correct but I am not sure it is that simple. Here a plumber can work on drains in buildings and drains between buildings but only to 5 feet past the building line. Drain layers can work on the sewer past that point. It seems the line between buildings may be considered a drain rather than a sewer. Not sure I have the terminology exactly right but that's what a licensed plumber told me recently. I am going to leave it all to professionals in that field...
     
    Last edited: Sep 22, 2019
  20. James Henry

    James Henry In the Trades

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    Usually the plumber installs the clean out outside the building which is approximately 5' outside the building then the someone else runs the sewer, but not always. hence, the 5' rule.
     
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