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Thread: Replacing Bathroom Drain Pipe

  1. #1
    DIY Junior Member bet3zzz's Avatar
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    Default Replacing Bathroom Drain Pipe

    We've always had problems with our master bath drains. They're slow and clog easily. I always figured that it was the galvanized drain pipe inner diameter slowly shrinking from 60 years of use. Some repair work I'd done previously seemed to confirm this. Well, that may have been part of the issue, but not the whole issue.

    I'm remodeling the bathroom, and figured I might as well replace the old pipes with PVC while I had everything apart. I got the old pipes out, and got the PVC dry fitted. I'm trying to fine tune everything and get the pitch right, and I'm noticing that the holes cut in the floor joists do not permit proper pitch of the tub drain. For most of the run (about 5 out of the 7 feet or so), the best I can do with the current holes is dead even - no pitch at all. The problem is the joists. They have sagged, except for the doubled joist in the center of the bathroom.

    The way I see it, I have three real options:

    1. Turn a couple of the holes into notches or otherwise enlarge them. The (approx 2" diameter) holes are about 1/2" down from the top of the 2x8 joists anyway, so I don't know that there would be much harm in cutting that out. I'd rather not do it, because I'm not sure of that.

    2. Reduce the 1 1/2" tub drain pipe to 1 1/4" to give myself more room. It would go back to 1 1/2" after it turns the corner and starts toward the sink drain. I figure that with the buildup in the old pipes, I probably effectively had 1 1/4" pipes anyway.

    3. Glue it up and hope that the slicker PVC and lack of buildup on the new pipes makes at least some improvement.

    Any thoughts?

    Thanks.

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    For seven feet, you need 1-3/4" of drop.
    Quite a bit for a 2x8

    If you had even 1" of drop, it would help. I would stay with at least the 1.5"
    And with the tub and the lav, it should be 2"
    Is there room to drop below the joist an fir down the ceiling or box it below?
    Sometimes you can tuck some of the run next to a wall.

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    Moderator & Master Plumber hj's Avatar
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    DON'T EVEN THINK about using 1 1/4" pipe for your tub drain.

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    DIY Junior Member bet3zzz's Avatar
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    Quote Originally Posted by Terry View Post
    And with the tub and the lav, it should be 2"
    Is there room to drop below the joist an fir down the ceiling or box it below?
    Sometimes you can tuck some of the run next to a wall.
    I expected to see 2", but 1.5" is what was there.

    Unfortunately, there's no real opportunity to go below... If I boxed it in, I wouldn't be able to open my kitchen cabinets.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    quote; If I boxed it in, I wouldn't be able to open my kitchen cabinets.

    In that case you either have a very low ceiling or very tall cabinets, since the average wall cabinets have a one foot space above them.

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    DIY Junior Member bet3zzz's Avatar
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    It's the cabinets. On one side of the kitchen, they go all the way to the ceiling.

    I think I figured out another option. It looks like if I go 1/4" deeper on a couple of the joists, I'll get an acceptable pitch. I could scab 3/4" plywood extending a couple feet on either side of the cut to reinforce.

    I guess that's more of a framing question than a plumbing question... But with respect to my other options, as far as reducing the diameter of the pipe, I guess the question is what's worse, smaller diameter pipe, or no pitch? Any thoughts on this?

  7. #7
    Forum Admin, Expert Plumber Terry's Avatar
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    You could also box the top part of the cabinet if needed.
    But it's your house, if it were to be done now, it would be a 2" waste line.

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    Moderator & Master Plumber hj's Avatar
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    You can live with no pitch, smaller pipe will be a lifetime of problems.

  9. #9
    DIY Junior Member bet3zzz's Avatar
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    To help paint a mental picture, the drain goes from the tub (through the joists) along the wall where the door is, makes a right at the corner, picks up the sink drain after a foot or so, and continues along until it gets into the opposite wall, where it turns downward and goes out of sight.

    I would certainly like 2", but I think that would also involve ripping out the walls in the kitchen too if I wanted to replace all of the 1.5"; I think it connects to the main stack somewhere in the kitchen, because it doesn't connect in the bathroom above or the basement below. If I started tearing apart the kitchen too, I would definitely risk bodily harm at the hands of my wife.

    I think I will shoot for no pitch from the tub with 1.5" pipe; the tub always drained reasonably well. I guess maybe it's because of the larger volume of water that drains from it (?). The sink has been the biggest problem. Since the run that picks up the sink runs parallel to the joists, I can adjust its pitch (it's currently near level) without cutting any holes, and maybe that will improve things with the tub as well.

    Thanks Terry and hj for your advice.

    Any other things I should consider?

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