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Thread: Advice for venting new full bathroom

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  1. #1

    Default Advice for venting new full bathroom

    Hi there,

    I have a new bathroom studded out and can easily get below and above for plumbing lines. I'd like some advice on how to drain and vent the following:
    toilet next to shower across from pedestal lav.

    The main waste line for the house is located 4' down and to the right approx 15' (opposite the entrance to the bathroom). I am planning on running a new 2 1/2" PVC vent to the roof since I have a clear shot.

    I think all the fixture vents can be vertical once they're above the flood height of the lav, right? The lav is what puzzles me. Will I have a long run of vent running 45 degrees through the wall?

    See attached diagram.

    Name:  Plumbing Floor plan.jpg
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    Any help you can give me is great. Thanks!

  2. #2
    In the Trades Tom Sawyer's Avatar
    Join Date
    Nov 2010


    This is why homeowners shouldn't plumb. What in hell is 2 1/2" PVC ?
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  3. #3


    2 1/2" PVC pipe. You know, the white sched 40 stuff. Maybe you've never heard of it. I can't tell from your reply. I was under the impression that you needed something larger than 2" for a full bath vent with shower, toilet and lav. I'll run 2" if that's sufficient. The main stack for the house is 3 to the roof. It's cast iron.

    No need to be rude either. Thanks for your advice.

  4. #4
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    I won't say for sure that there is no such thing as 2 1/2" pipe...there probably is. But it would be a custom order, minimum order quantity probably one skid, or about 600 linear feet. If you have a separate 3" vent stack already, the 2" would be ok for that bathroom group.....unless you are in a climate area which demands 3" for all vents.

  5. #5
    Test, Don't Guess! cacher_chick's Avatar
    Join Date
    Jan 2008
    Land of Cheese


    If anyone makes 2-1/2" PVC, it would not be commonly found anywhere in the U.S.

    The plumbing code varies depending on where you live.

    In any case, your shower vent riser must come off the trap arm before the drain connects to the 3" line.

    The vents must be vertical from the trap arm until they are least 6" higher than the flood rim of the sink. The lavatory vent could go straight up into the attic before being turned to combine all of the fixture vents before going through the roof. Horizontal sections of vents must be pitched to drain.

  6. #6



    I don't have a straight shot to the attic from the lav - the only path that's available is over near the toilet, so I was thinking that I'd need to run the stack in that wall. Could I connect the lav to the 2" stack by running through the wall or up and across the ceiling as long as I maintained a pitch to drain or is there a lateral distance restriction?

    And for vents, is there a minimum fall / pitch to be considered "not horizontal" (ie 1 in 12)?


  7. #7
    DIY Member Gordan's Avatar
    Join Date
    Feb 2013
    SE PA


    Depending on the governing plumbing code, there are restrictions on vents regarding maximal developed length for a given combination of fixtures and the largest drain size served. Beyond that, no restrictions on horizontal offsets above the flood level of the highest served fixture. Also... there is no minimum pitch on vents. They don't carry waste. They're not allowed to create a trap, however. You can actually pitch a vent downward to a vent stack as long as that vent stack can eventually drain somewhere.

    You'll want all dry vents to come off the drain at 45 degrees or more.

    Lavs and showers require 1 1/4" or bigger vents, the water closet will require 1 1/2" or bigger given the distances here. Bigger is not necessarily better, especially if it causes you to have to drill larger holes through structural members.

    What are you doing about cleanouts? Depending on your code, every horizontal run may require them.

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