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Thread: Waste stack - 2 baths - upgrade to 4"?

  1. #1
    Design/Build fineline's Avatar
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    Default Waste stack - 2 baths - upgrade to 4"?

    Hey all,

    I am remodeling 1 full bathroom and setting up a tie-in for another future full bath. I removed most of the existing cast iron stack which was 3", and just as it enters the basement slab it transitions to 4". My plan was to reinstall the stack all as 4".

    If you look at the attached photo, the 4" hub you see right at the floor level is the top of a 90deg. fitting. I was thinking I good cut off the hub and use a rubber coupling to tie into it, this would still require me to open up the slab. My questions are:
    - do you see any issues with attaching a rubber coupling to the top of a cast iron 90deg sweep fitting (with the hub cut off)?
    - would I better off removing a larger section of the slab and removing that 90deg fitting completely - then tying into a straight length of cast iron?
    - am I making too much work for myself trying to upgrade the size of the waste line, and should just use 3" pipe and tie into what you see above the slab? (note I have already purchased all of the 4" pipe and fittings).
    - note that the 2" galvanized line you see coming in services 2 sinks and potentially a future washer machine
    - note the 90 fitting turns back into the house toward the direction the photo is taken from, so I don't have to be concerned with having to work in/underneath the foundation wall

    Thanks in advance for any advice,
    Sean

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    Moderator & Master Plumber hj's Avatar
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    There are times when bigger is better, but drains lines are not always one of them. The 3" line will have better "scouring" action than the 4" one.
    Licensed residential and commercial plumber

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If you keep it at 3", keep the hub and use a donut to make the connection. They probably make a donut that would work on 4" pipe as well. While the ID of the pipe is standard, the size of the hub is not, so you may need to order the properly sized donut.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Design/Build fineline's Avatar
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    Thanks for the responses. I realized after posting that if I really want the 4" stack I don't need to open up the floor - as long as I can remove those 3" fittings in the photo I can try to donut a new 4" line into the top of that 4" 90deg. fitting that enters the floor. I want to replace that 2" line and fitting above anyway.

    HJ -I'm assuming what you mean by "scouring" is the smaller diameter will allow the waste/water to "wash" all interior surfaces of the pipe as it passes through. Any reservations against going with 4" since I have all of the parts already?

    Thanks again,
    Sean

  5. #5
    Moderator & Master Plumber hj's Avatar
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    In the "old days" we did everything in 4", and did not even have any 3" pipe or fittings, so you should not have any problems.
    Licensed residential and commercial plumber

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    DIY Senior Member Reach4's Avatar
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    I was told that I should add a cleanout if I transition from 4 inch to 3 inch. A cleanout here might be a good idea, even if sticking with 3 inch.

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    Forum Admin, Expert Plumber Terry's Avatar
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    The base of the stack gets a cleanout.
    The cleanout can be at the floor, or above the fittings there.

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    Design/Build fineline's Avatar
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    Yes - a clean-out was definitely in the plan, was going to position it just over the tee where the 2" line comes into the stack. Thanks again for the advice.

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    hey all - re-built the stack and started the rough-in for the bathroom and was hoping to get comments or sign-off on the design. Some photos are below for your reference.

    The one question I do have is - can I wet vent the bathtub through the lavatory vent? I understand the concept of a wet vent, but not so fully that I am confident designing one. I hope the photos show enough for you to understand how that venting could work. If not, I could always run a separate vent for each fixture and tie them together in the attic and then into the vent exiting the roof.

    Any feedback or advice is appreciated, thanks!
    Sean

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  10. #10
    Forum Admin, Expert Plumber Terry's Avatar
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    Looks good.

    The lav can wet vent the tub if the wet part is 2" and they are on the same floor.
    The lav drain going down vertical would be the tub vent from below. That makes it about 23" in length.
    The trap arm can be 1.5"

    If you are coming up in two separate locations, the tie the vents together at 42" from the floor.
    Last edited by Terry; 10-17-2013 at 07:17 AM.

  11. #11
    Moderator & Master Plumber hj's Avatar
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    You show the bathtub drain pointing UP, (without a "P" trap),. If that is how it will remain, then you not only CANNOT wet vent it with the lavatory, but it will need its own vent.
    Licensed residential and commercial plumber

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    Design/Build fineline's Avatar
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    thanks for the confirmation Terry.

    HJ - I was planning on installing a p=trap, but now that you bring it up I realized it was going to end up more like an s-trap based on how close it is to the drain. I guess I could connect the p-trap to a tee and run another vent up - that is how the original installation was done. Just means some extra materials and time. Unless you have any suggestions...

    Thanks,
    Sean

  13. #13
    Moderator & Master Plumber hj's Avatar
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    quote; I realized it was going to end up more like an s-trap based on how close it is to the drain.

    It would not LOOK 'more like an S-trap", but it would BE an "S" trap. It has NOTHING to do with "closeness", it has everything to do with the pipe turning downward after the trap without an intervening vent.
    Licensed residential and commercial plumber

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