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Thread: Where to connect lav in basement bathroom?

  1. #1
    DIY Member MushCreek's Avatar
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    Default Where to connect lav in basement bathroom?

    In working out my DWV system, I need to add any fittings I will need for a future basement bathroom. I had a plumber do the under-slab DWV, and rough-out stubs for a full bath. What I don't see is how/where to connect the lav. In the picture, starting from the left is the tub (with the yellow bucket on it), the toilet, a 2" vent (for the entire bathroom, I assume), the main stack going up to the main floor, and then out through the roof for venting, and a radon mitigation vent (not part of the system, but in the picture). I don't think I can connect the lav drain directly to the main stack, can I? Can I connect to the 2" vent? Then it becomes a wet vent. Or should I come off the main stack with a combi, which would have a vertical vent which can then tie to the 2" vent @ 42" above the floor, and long sweep ell out to the lav drain? All of that would have to go to the left, since the radon tube is in the way on the right. I do have enough room (barely) if I go left to house the lav in a vanity and still have over 15" to the centerline of the toilet. Thanks!


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    DIY Member MushCreek's Avatar
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    I think I figured it out. (Amazing what you can do when it rains all day). I can drain the lav directly into the main stack, BUT- I will have to vent it separately, since there is a toilet above it. I really want to add a laundry tub, too, so I'm going to come out of the main stack on a 45 degree to the right to clear the radon vent, then another 45 into the future wall on the right. From there, I'll connect the lav (in the bathroom) to the laundry tub (outside the bathroom) with a back-to-back san T. Going up will be a vent, which will cut across the ceiling to the basement bathroom vent. Going down, I'll use a long sweep ell. Although the drain will be outside the wall where it cuts across the corner, it will be inside the vanity for the lav. Clear as mud? See any problems with that plan?

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    Moderator & Master Plumber hj's Avatar
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    Only that a "back to back san T", actually a sanitary cross, is not the approved fitting for that application.
    Licensed residential and commercial plumber

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    DIY Member MushCreek's Avatar
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    What is the approved fitting for back-to-back sinks? A combi 'cross'? Or is back-to-back not allowed in this application?

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    Forum Admin, Expert Plumber Terry's Avatar
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    I would have vented the tub "before" it entered the main waste line with the toilet.
    I would not have used a double wye flat for the vent and tub.

    A double lav uses a 2x1.5x1.5x1.5 double fixture fitting.
    You can add lav plumbing to the waste stack, if it's properly vented. Without venting, toilets flushing upstairs will pull the trap water out.

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    DIY Member MushCreek's Avatar
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    Figures- the underslab plumbing was done by a licensed plumber, and passed inspection. I doubt the tub will ever get used much, at least while we live here. I'm not cutting up the floor unless a problem arises.

    I looked at double fixture fittings today; they look like a double sanitary T to me. All of my plumbing books spec a double san T for that application. Does putting a laundry tub, with an 1-1/2" trap back-to-back with a lav, with a 1-1/4" trap pose a problem? The books do say that you have to have the same size pipe on each side, but don't say anything about two different sized traps. I could stack them, but the fixture for the laundry tub will be pretty low.

  7. #7
    Forum Admin, Expert Plumber Terry's Avatar
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    Just because something passed, doesn't mean it was a good idea.
    I don't try to do the least for my customers or myself.
    I personally would not put a washer on a fixture cross with a lav. Have you ever seen those things pump out? I'm guessing you will go the easy way though and save four dollars.

  8. #8
    DIY Member MushCreek's Avatar
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    It's not a washer; it's a laundry tub (utility sink). The washer is on the main floor, on it's own 2" branch. I'm not trying to save any money; I want to learn the best way to do it. That's why I'm here, asking questions. Plumbing is frustratingly vague. I have a number of books, and they routinely show things that people say are not allowed. I've tried to wade through the entire IPC 2006, which is our code here, but even that is short on specifics. I never thought that DWV would be the hardest part of designing and building my own house from scratch.

    As for the pipes now buried under concrete, as I said, we'll see how it works. My current home in FL has terrible drains, and it was the developer's own house! This is my own house, built by me and for me. I like everything to be better than code. I wish I knew more about plumbing before we poured the slab and buried those pipes- I would have changed it, or rather, made the plumber change it. Sadly, my experience here in SC with inspectors has been only cursory inspections. When they inspected the framing on my barn, he walked in about 15 feet, and I offered him a flashlight. He said, "No thanks; you're fine." He then signed off on the work.

  9. #9
    Forum Admin, Expert Plumber Terry's Avatar
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    I never thought that DWV would be the hardest part of designing and building my own house from scratch.
    I was a builder at one point in my career. Some of the homes were either copied by other builders, or bought and moved into by other builders.
    I don't live in one. I had a bad year with 14% mortgage interest the last year I was building.

    In plumbing they don't let you touch waste and vents for over a year after working daily with a Journeyman. They want you to be constantly around it before they ever waste time trying to teach it.
    Here on the West Coast, they want 6,000 hours working with someone before they let you get a license.

    It's kind of simple, but not. It's a bit like learning language. I failed Spanish 1 twice.
    I can walk through a set of blueprints in my head though. I see everything easily, but it's a good thing I can get by with native English or I would be sunk.

    A laundry tray would slow things down considerably. The double fixture cross would be fine for that.
    Last edited by Terry; 05-06-2013 at 06:38 AM.

  10. #10
    DIY Member MushCreek's Avatar
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    Thanks so much for your help!

    I'm building my own house for 3 main reasons:
    1) It's a life-long dream
    2) I can't afford to have someone else build even a stick-built code-minimum house, never mind the level of quality I want.
    3) After 6 years of research, plus a lifetime of fixing up houses, I'm shocked at the stuff so-called professionals get away with. I could write a book!

    As I said, the under slab plumbing was done by a licensed plumber that came highly recommended. Now that I've learned more about DWV, I would have done it differently myself. I only hope it doesn't cause trouble. It's now buried under vapor barrier, 2" of insulating foam, and concrete. To really make it right, I would have to tear out a wall and part of a stairwell, cut out a sizable chunk of concrete, fix it all, and then put it all back together. I'm sure the plumber wouldn't help, since 6 months have passed, and it did pass 'inspection'.

    Our house is very small, with only one bedroom on the main floor. It's a retirement place for us. The reason for the basement bathroom is that we are going to finish a room down there for a den, and for a guest bedroom. We've reached that age where it's nice to have a bathroom nearby when we sit in the den to watch TV. Depending upon what life throws at us, that basement bath may never get built, but I need to have the stub-outs in case we do. If we do finish it, the toilet and lav will get used daily, but the tub very rarely, possibly never. I'm not sure why, but my wife wants a laundry sink in the basement. I've learned not to question the boss!

  11. #11
    Moderator & Master Plumber hj's Avatar
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    quote; they look like a double sanitary T to me. All of my plumbing books spec a double san T for that application.

    it should look "like" a double combination y-1/8 bend, if you are looking at the correct fitting, but even though it "looks like one" it IS different. Books are often written by people who THINK they know the best way, or at least want to make money by selling a book. Have you ever seen the list of "errata" that many books contain?
    Licensed residential and commercial plumber

  12. #12
    DIY Member MushCreek's Avatar
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    The only back-to-back at Home Depot looked like, and was called, a double sanitary T. The shape was identical to a single; I even held one up to it to compare. I might have to go to a plumbing supply house then. I'm thinking of using two separate fittings anyhow because of the two different trap sizes. I could put the one for the lav about 17-18" off the floor, and the one for the laundry tray would be about 6" lower. My current house has a lot of issues with draining two fixtures at once, and gurgling sounds. I even get soap suds in the toilet from the washing machine. I would think that two separate fittings would cut down on this kind of thing.

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