What is the minimum concrete thickness when you pour concrete over a newly installed drain pipe?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by Mini Me, Mar 26, 2021.

  1. Mini Me

    Mini Me Member

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    I am finishing the relocation of a drain after replacing the tub with a shower base and I will probably need to pour concrete back. The opening I created will be entirely under the shower base that I am installing so my question is this: How thick does the concrete have to be. The shower base is pretty sturdy (Kohler Ballast model 60x32) and heavy and it will be set in mortar as well.

    I would avoid pouring a very thick concrete layer if I don't have to as this is my first job of this sort and I would like to be able to easily break the concrete and fix whatever needs to be fixed if anything will ever need fixing. I guess ideally it would be nice to just fill the opening in the floor with sand. :)

    The shower base will be along the right side of the picture with the shower drain in the deeper area that see in the picture

    [​IMG]

    The plan is to set the shower base level and then add self leveling concrete around it so I can install tiles.
     
  2. James Henry

    James Henry In the Trades

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    How far down is the top of the pipe below the concrete floor?
     
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  4. Mini Me

    Mini Me Member

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    3" from the top face of the slab to the lowest point of the open segment of the pipe (that is to a point under the pipe, pipe is 1.5")
     
  5. James Henry

    James Henry In the Trades

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    If I'm understanding you correctly the top of the pipe is an 1-1/2" below the top of the slab.
     

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  6. James Henry

    James Henry In the Trades

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    If you pour a thin concrete cover over that hole it will eventually crack and settle from the weight of people taking a shower. A thin layer is only usually poured over the p-trap.
     
  7. pollymath

    pollymath New Member

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    3" is fine, especially for a pre-formed base.

    I had this exactly conversation with a concrete guy and my plumber, because I'm in a similar situation.

    In reality, as long as the rest of the slab right up to the pipe is 4" thick with good compaction under it, that little spot where the pipe is a making it a tad thinner isn't a big deal.

    My concrete guy explained it this way: "it'd have to be really shallow hand-formed pan, you'd have to be a big guy, know exactly where the pipe is, and break your heal trying to stomp on it in order to get it to crack." He thought 2" coverage was about the minimum coverage, and even then reinforcements or strategic doweling could help make it work - again, all with a hand-built pan in mind.

    Basically, any build-up required for a hand-built shower pan is going to make that area over the pipe thicker, and a many pre-formed bases will actually span plumbing rough ins that aren't even covered with relative easy. I was surprised to see my shower stall insert spanned a 1 foot square space without anything below it and I never noticed it being "soft".

    About the only situation where I'd get nervous is less than 2" coverage over a big/long piece of pipe with no-threshold style shower pan.
     
  8. pollymath

    pollymath New Member

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    Also, what's your impression of the Kohler Ballast? I'm eyeing that one in the 66x36 size.
     
  9. Mini Me

    Mini Me Member

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    Yeah that is a good point if the concrete is too think it could crack although this is a pretty good shower base and very sturdy. it currently lies on the floor in the basement and
    I am trying to find a solution for the situation when something goes wrong and I need to dig in again

    RE: Kohler Ballast -freaking awesome compared with what I have seen in the stores. I was looking for a low threshold 60x30 and I had to work with this 60x32 but it looks very good. Excellent quality but heavy like hell. Must have some metal inserted in
     
  10. pollymath

    pollymath New Member

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    Yea many of those preformed pans are designed to span soft areas, or open slab trenches. I didnt' realize how many freaking homes built in the last 20 years never had the plumbing rough-ins covered up with cement. Talk about a spider/rat den? *shudders*

    It's funny, my bringing up this topic with my concrete guy was for the same reason - I want to keep things relatively easy to access if I ever wanted to move things again or had problem.

    Talking with some other concrete pros, doweling is a good way to keep the new concrete from giving you problems, but will leave the patch easy to blast through later on. Again, pre-formed pans give your a lot of room for error, so I wouldn't worry about it in your situation. If years later someone wants to do a shallow hand-formed tiled base in your bathroom and the pull up the Kohler to find cracking and settling under, let them deal with it then!
     
  11. Mini Me

    Mini Me Member

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    Why is the layer thinner over the p-trap?
    I would say that the p-trap which connects directly to the shower drain has the highest chances to be dislocated when the user steps above the drain (his or her weight pushes the base downward and this in turn moved the shower drain and the p-trap)
    In my view that area should get the most support underneath and maybe on the sides
     
  12. James Henry

    James Henry In the Trades

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    A tub box is installed around the p-trap before the floor is poured so you can make any final adjustments to the p-trap if need be. I misspoke earlier, I was thinking of a bath tub p-trap, the boxed out p-trap is usually poured back with a couple inches of concrete just to keep the bugs out. On the shower p-trap depending on the shower drain design you might end up wrapping the pipe coming through before you pour the floor back so theirs a gap for the floor drain to go below the finish concrete floor. and yes you should pour it back around the drain at full depth.
     

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  13. Mini Me

    Mini Me Member

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  14. James Henry

    James Henry In the Trades

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    Yes, that's why you will wrap something around the pipe below the finish concrete to create a gap for the drain body to slide down on below the finish floor.
     
  15. Mini Me

    Mini Me Member

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    As you can see in the initial post I dug a hole there in order to be able to create room there for the p-trap
    As per the picture the p-trap is set in sand but in my case the p-trap will have its upper part within the three inches of concrete zone ...should I pour concrete there ? I guess I should and support with gravel and sand from beneath, otherwise there is no way the vertical piece of pipe that goes through the drain will keep its position if the user steps on the drain. It will probably move up and down and end up damaging the welding of the ABS that makes the P-trap and its connection to the main drain
     
  16. James Henry

    James Henry In the Trades

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    If you have a 2 lb. sledge hammer use that to compact the soil under the p-trap then wrap some plastic or cardboard around the pipe the thickness and depth you need to accommodate the drain body before you pour the concrete. If the soil is compacted good you shouldn't have any problems.
    DO NOT USE GRAVEL , USE FINE DEBRIS FREE DIRT. COMPACT GOOD. READ THE INSTRUCTIONS ON YOUR PARTICULAR / SPECIFIC SHOWER INSTALLATION BEFORE YOU DO ANYTHING.
     

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  17. Mini Me

    Mini Me Member

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    The Oatley brass shower drain came with no instructions
    The Kohler shower base has no instructions for what happens under the slab :-(
     
  18. Tughillrzr

    Tughillrzr In the Trades

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    Wrap it 4-5 times with sill seal, making sure to be above and below your pour level and tape it. You’ll be able to pull it out later after Crete is dry. Or as stated , You could box around trap and pour around it. This gives you a little adjustability later after pour. You just fill it in later with sand/stone flush with slab.

    if that’s correct model. Easy to search website. especially Koehler
    https://www.us.kohler.com/webassets/kpna/catalog/pdf/en/1185992_2.pdf
     
  19. Mini Me

    Mini Me Member

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    Yes that is the model and I also posted a link to that pdf around here but it is not clear, from that PDF, how you should (and if you should) cement the P trap or the 2" pipe that connects the shower drain to the p-trap. It just says connect the shower drain to the p-trap
     
  20. Mini Me

    Mini Me Member

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    For whoever drops by and needs an illustration of what was recommended here, here is a video


     
    Last edited: Mar 27, 2021
  21. Mini Me

    Mini Me Member

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    I am puzzled by the above advice, I am seeing gravel used here



    Ad I have also see ppl breaking the floor and excavating sand and gravel from under the floor. So really what is so bad with grave and sand under the slab especially if they are compacted and mixed ?
     
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