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Thread: Change tub drain pipe

  1. #1
    DIY Junior Member Collin's Avatar
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    Default Change tub drain pipe

    I am replacing a standard tub with a tiled shower. I understand the typical drain line width for a shower is 2". The current pipe is 1 1/2" and is in a concrete slab. The vent pipe is about 18" away, so I assume the stack is close, within about 2'.

    Do I need to bust up the concrete and replace the 1 1/2" pipe with 2" pipe all the way to the stack?

    If I do need to bust up the concrete, the copper supply pipes are also routed in the slab and come out close the drain. Do I need to take any special precautions? Are the supply pipes usually routed near the drain pipe? I'm worried about breaking one of the supply lines.

    I want to do this right, but not do unnecessary work.

    Thanks in advance. The guys at johnbridge.com recommended you.

    Collin

  2. #2
    Plumber jimbo's Avatar
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    Hopefully, your pipes are UNDER the concrete, not IN it. Of course, they come up THROUGH the cement.

    You will need to deal with the cement, because the tub drain is probably not located where you want the shower drain, and even if it was, it would be difficult to connect a shower drain to an existing tub shoe. You also need to get rid of the tub overflow. When you remove the tub, you may find that this is not as difficult as you anticipate. There is probably a large dugout opening in the slab where the tub waste and overflow piping are. A little slab busting will let you move it to where it needs to be.

  3. #3
    DIY Junior Member Collin's Avatar
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    Yes, I've got everything stripped down to the slab. I cut the tub overflow pipe and put a cap on it but am realizing this might not be the best solution.

    I see where the drain pipe runs down vertically into the slab. I am assuming my slab is very thick (single story home, no basement, slab foundation) and that the pipes are actually in the slab rather than underneath it. Does that sound right?

    I think I can get everthing connected and move the drain to the center of the shower without removing the existing 1 1/2" pipe all the way back to the stack. From what you are saying I should remove everything back to the stack and replace with 2" pipe, correct?

    Does the horizontal drain pipe typically connect to the stack just below the top level of the slab?

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    The drain needs to slope 1/4" per foot from the p-trap to the stack to allow proper drainage. The p-trap should be essentially directly below the drain assembly. For a shower, this should all be 2". If the new location for the shower is closer to the trap, you might be able to cap ythe old one rather than tear it out, but basically you'll need to bust the concrete up from the new drain location to the point where you can connect to the stack.

    For tiling info, suggest you go over to www.johnbridge.com . Depending on the actual size of the shower you are going to do, check out the Kerdi system on www.schluter.com . They have a kit for tiling a space where there was a tub. If it fits your application, it makes a great shower. The system can be used with a custom shower pan as well.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Junior Member Collin's Avatar
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    I only need to move the drain about 18", so I should have no problem meeting the 1/4" per foot guideline.

    My real question is if the existing 2' stretch of 1 1/2" pipe is a problem. If not, I could use 2" pipe for the new 18" extension and P trap, and connect that to the 2' of 1 1/2" pipe.

    I am assuming only 2' of 1 1/2" pipe because the vent is only about 18" away.

  6. #6
    Plumber Cass's Avatar
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    No, you can't connect 2" to the 1.5" without causing a problem.

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    As I understand it, code never allows you to have a restriction in the line - if the drain is 2" it must remain 2" until it flows into something BIGGER, never have a section that is smaller in the path. Otherwise, you end up with a place to let gunk accumulate and you can't ream it out.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  8. #8
    DIY Junior Member Collin's Avatar
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    Ok, I rented a chisel hammer and busted up the slab to the 2" pipe. Fortunately, the original 1 1/2" drain pipe was coupled to a 2" P trap directly below the drain, so I didn't have that much digging to do.

    Where should I cut to remove the 1 1/2" pipe? I have a wire saw so I should be able to cut anywhere. The current joints are all glued. Can I cut the bottom of the P trap and put in a compression fitting?

    The sewer side of the P trap is under a wall and don't reall have good access to it. I could bust up the slab in another room but would like to avoid that if possible.
    Last edited by Collin; 12-26-2005 at 10:56 AM. Reason: editing question

  9. #9
    DIY Junior Member Collin's Avatar
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    Ok, I cut the entire P-trap off using a wire saw. Wasn't very easy but I will now have a quality plumbing job with no half-a** fittings.

    My last question is about covering the excavation I made. The original drain pipe was routed through a small square cutout in the slab, and the the cutout was filled with tar (?). I imagine the purpose of this was to allow movement between the drain pipe and the slab.

    Should I do something similar to replace what I took out? I planned to fill the excavation with the dirt I took out, then fill with some of the busted up concrete, and then lay concrete to the same level as the original slab. Do I need to allow for some sort of non-rigid filling between the pipe and the slab? I am using PVC pipe.

    Collin

  10. #10
    Plumber jimbo's Avatar
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    They make foam collars to surround pipes penetrating concrete. They definitely need to be protected. The tar was probably for moisture and bugs. You could use caulk.

  11. #11
    DIY Junior Member Collin's Avatar
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    I looked at some caulks and most seemed to have a maximum joint width of about 1/4".

    I bought a 4" coupling that is about the same thickness as my slab. I want to put the 4" coupling around my 2" drain pipe, then pour some concrete. I bought some sprayable insulating foam that claims to be watertight. Would that be a sufficent seal between the 2" drain pipe and 4" coupling? I am using PVC pipes.
    Last edited by Collin; 12-26-2005 at 05:31 PM. Reason: adding info

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