Question about moving vent stack

kudzu9March 27, 2014

I am putting in a small, new bathroom in an area that was previously rough-plumbed as far as the vents and drains are concerned. This area is slab on grade, and I have 2" ABS coming up through the slab for a sink drain/vent and another for a shower drain/vent, as well. There is also a 4" drain for the toilet, and there is a 4" vent stack. All of these are connected under the slab and all of these drains are where I would like them to be, except for the 4" vent stack, which is coming up exactly where a vanity has to go. I can conceal this 4" vent pipe inside the vanity base where it comes through the floor, but I need it to jog over about 18 inches so that it can come up inside a wall.

My question: Is there any reason I can't do a right angle at the floor, bring the pipe 18 inches horizontally over to the wall underneath the sink, do a second right angle and bring it up inside the wall to the roof? The other vents would, of course be connected to the vertical run of this vent pipe as it goes up through the wall. (I know about the limits for horizontal runs of drains, but I couldn't quite figure out whether there is any similar code requirement regarding a horizontal jog in the vent stack when the jog is below the flood level of the sink.)

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The vent stack must be vertical until it is several (6?) inches above the highest flood level of the served fixtures, in your case probably the sink. Vertical includes angles greater than 45 degrees from horizontal. You can angle it back towards the wall, but not go horizontal.

    Bookmark   March 28, 2014 at 10:28AM
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Thanks. I understand your answer. That will be complicated, but doable.

However, let me throw in one more wrinkle to see if it changes anything. This does not involve the main stack in the house. I am putting the bathroom in a standalone 20' X 20' building. The only plumbing going on in this structure will be the sink, shower and toilet for this small bathroom. The stack coming through the slab in that building ties in to my sewer below ground, but it is independent of any other stacks in the main house because it is a separate building.

Is your answer still the same? If so, I am curious if you know the code rationale for this: it's hard for me to envision how an 18" horizontal jog could compromise the venting of these few fixtures given the limited amount of water any of them could produce relative to the diameter of the vent.

Thanks. I'm not trying to argue. Just wanting to understand.

    Bookmark   March 28, 2014 at 2:56PM
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Maybe because methane is lighter than air, and would follow an upward slanted or vertical path but not a horizontal one?

    Bookmark   March 28, 2014 at 8:44PM
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A horizontal vent sounds more likely to become obstructed over time, even with some pitch. And if you ever have a clog downstream, it goes that way first.

Tangentially, this kind of reminds me of an old house in DC. That house had some sort of catch basin under the rear driveway for storm water, but it also drained the water off and into the main drain of the house. Driveway drain cover was 9", and the concrete basin underneath was roughly 4'x4'x3' deep. When they had a main drain clog, before it was discovered it had backed up into the trap to fill it. Among other things, I had to call a company to vacuum it out of there. Ok, so it's not a vent, but maybe the principle is the same. Tangent over. :)

    Bookmark   March 28, 2014 at 10:30PM
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