Exceptions to horizontal venting under floor?

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Crashcup

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Hi, I've been reading a lot on here, and have seen it clearly stated that vents cannot run horizontally until 6" above the flood level rim of the highest fixture.

But I'm confused by what I'm seeing in some books.

For example, these two illustrations. One side saying "never use dry flat horizontal vents", while the other side seems to show horizontal vents for a short distance before turning up vertical.


This illustration appears to show a short horizontal section of vent used to vent a toilet.

I wanted to ask if someone could explain what I'm missing in this. I am assuming that a vent pipe at 1/4" slope is still "horizontal", and that the vent isn't considered "vertical" unless it's 45 deg or less from actual vertical.

These aren't adding up in my head, and I was intending to vent a toilet drain in a new bathroom in a very similar way to the 2nd illustration above.

I am in MN, btw, and the local AHJ uses the MN plumbing code, which from what I can tell is similar to UPC. Although I have noticed differences, such as water closets must be vented 4' or less from trap (vs 6' in UPC).

Thanks in advance!
Keith
 
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hj

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MANY people who write "do it yourself books" do not know what they are talking about, but it looks good. For example, if he used a 60 degree elbow instead of the 45, he would NOT need the 22 1/2 bend. IF you are going to use a horizontal "vent" you should connect something, such as a sink, to it to keep it flushed out. Just "rolling it up", does nothing to keep debris out of it if the line plugs up.
 

Crashcup

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MANY people who write "do it yourself books" do not know what they are talking about, but it looks good.

One of the above books is written by Howard C. Massey, a "recognized expert on the plumbing codes"... a journeyman, then master plumber, then owner of a plumbing contracting business for 15 years. 14 years as assistant plumbing chief and plans examiner in Dade County, Florida. (So described by the back cover).

The other is a Taunton publication - usually good - by a "residential plumber in Albany, California."

I guess you just never know.

I finally talked to one of the local building inspectors, who told me "no flat vents!" I try not to bug them with questions, but in this case it wasn't clear from the state code, and he gave me a very clear direction. The vents here must be vertical (no less than 45deg from vertical) from behind the trap all the way till it gets above the FLR.
 

Tom Sawyer

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There's a couple of states like Massachusetts that do allow flat venting but neither the UPC or the IPC do. As for the illustration, it wouldn't pass anywhere around here. As for having a fixture upstream to wash it, great idea, then it becomes a wet vent.
 

Jadnashua

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All DVW pipe should have at least a 1/4"/foot slope to it under all conditions. That is what allows it to drain. A vent could end up with condensation or rain water in it, and it still needs to drain. If the main line backed up, it could go into the vent, and again, it would need to drain when the main was cleared. On a drawing, it's hard to differentiate true horizontal from one with a minimum allowed slope. There is a limit on how far it can go 'horizontal' based on the pipe diameter. WIth the required slope, you need an air channel to exist under any conditions and with the slope, that is what creates that limitation.
 

Gunn1

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This is a great post and something I have struggled to understand too, while I prepare for my own DIY plumbing project by reading everything I can on this great forum...

So, absolutely no horizontal vents? So then how would one vent a corner 5'x5' Jacuzzi whirlpool tub, with the drain approx.. 4' from either wall? Wouldn't you have no choice (at all) but to run a vent for some distance at 1/4"/foot slope under the floor before you get to a wall?

Similarly, what about a large shower (say 4' wide) against a wall, but with the drain in the center?
 

Jadnashua

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Code requires the vent to be within a certain distance of the p-trap. ON a 2" pipe, that maximum distance is 5' in most code cycles, gets longer with larger pipe, and shorter with smaller diameter ones. The key is that the vent MUST occur and service each trap individually (with a few very specific exceptions). IOW, for the most part, one trap, one vent. Vents CAN be combined as long as you follow the rules, but each trap still needs one. Some places do allow what is called wet venting, but that can only be done within a single bathroom group on the same floor and must follow some very specific guidelines.
 

LesP

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I know this is an old thread, but I continue to be baffled by how to vent fixtures that have a horizontal trap entering a horizontal drain. For example a shower or tub on the main floor draining into a horizontal run in the basement. Do you just put a vertical combo on the drain line underneath the vent wall close to the flat wye or combo that joins the fixture trap to the drain? If so, does it matter whether the vent combo is upstream or downstream of the fixture connection?
 

hj

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If I understand what you mean by "downstream of the fixture connection", then you might as well just put a ribbon and bow on it, because if will just be decorative.
 

Jadnashua

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The vent must come off of the trap arm prior to the trap arm turning down. There is a maximum length of the trap arm depending on the diameter of the pipe. Anything 45-degrees or higher is considered vertical on a vent, but any fitting on a trap arm would be problematic if it turned down. The length of the trap arm must be such that both waste AND air can be exchanged while waste is flowing, the trap arm (and all other waste pipes) must have a minimum of 1/4"/foot slope, and with slope required, that's where the varying distance comes into play. YOu must not be able to close off the trap arm and thus the vent while draining, or you can create a vacuum, which can suck a trap dry. IN the example of a 2" pipe with the maximum trap arm length of 5', 1-1/4" of the pipe could be full and still allow air passage. You might think 8', but then there'd be no room for waste to ensure the pipe still had an air passageway.
 

LesP

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Your response gave me a laugh HJ — I can just imagine ribbons all over my plumbing now. So does that mean that the pattern #3 picture in the first message is incorrect? The plumbing vent is “downstream” of where the the waste enters the drain. Compare that to this picture from Taunton’s plumbing book that shows the vent “upstream” of the closet bend. Does it matter where the vent is positioned relative to the fixture drain as long as the distance rules that Jim described are met?

35ADB820-17DA-4E8E-A886-F62CA6D733C6.jpeg

Jim, thanks for the explanation of the allowable trap arm distances to the vent — that makes a lot of sense. I have always wondered why the IRC allows for much longer distances than UPC (e.g., 8 ft vs 5 ft for 2” pipe). Does IRC assume a different slope or are they just more optimistic?

My main question is how to configure the vent for fixtures like a tub or shower where the horizontal trap arm enters a horizontal drain. If you have plenty of altitude, I guess you could drain the trap into a vertical sanitary tee like you would do on a sink and then drop vertically into the horizontal drain. But what do you do if you are working within a 2x10 joist bay? Any pictures or drawings would be greatly appreciated...

Thanks for your help!
 

Jadnashua

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This may be obvious, but, as long as the drain line itself has the minimum required slope after the trap is vented, it doesn't matter what you dump into the drain as long as you don't have more fixture units trying to be handled. The key is your trap arm lengths and what is a proper vent. The trap arm cannot go up or down, except the down component still needs slope to clear itself. You must vent it within the required distance, then, the drain can go anywhere you need it to within reason and physics.

Within a single bathroom group, you can utilize wet venting that shares a vent, but there are some specific rules on how that can properly be implemented. Maybe a simpler way to think about that is that the drain line ends up being larger to ensure there's still an air path for venting - i.e., the combination of things draining cannot fill the drain, so there's still room for air movement and the traps won't siphon dry.
 

hj

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Regarding Taunton's Plumbing Book, would you take advice from someone who installs a sanitary tee on its back for toilet connection? In any case, toilets are special fixtures so even though vents are installed for them, they DO NOT need, or USE, vents so you can put them anywhere you want to, as long as one of them goes through the roof.
 

Brian80

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Bumping an old post because I have the same issue with horizontal venting under the slab. Just so I'm clear, in practical terms the fitting that connects a drain pipe to a vent pipe needs to be under a wall so that the vertical vent pipe is concealed within that wall. Correct?
 

wwhitney

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Just so I'm clear, in practical terms the fitting that connects a drain pipe to a vent pipe needs to be under a wall so that the vertical vent pipe is concealed within that wall. Correct?
Almost. The plumbing codes (in the US, and presumably in Canada) consider a pipe up to 45 degrees off plumb to be "vertical," and the requirement for a dry vent takeoff is that it is vertical and stay vertical until at least 6" above the flood rim of the fixture served.

So in practical terms the fitting for the dry vent takeoff needs to be sufficiently close to the wall so that while rising at a 45 the vent can make it under the wall before it emerges from the floor. Assuming you don't want your vent pipe exposed above the floor.

Cheers, Wayne
 

Brian80

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Thanks. I'll be renovating a basement bathroom where all surrounding rooms have finished flooring, so I only have access from one side of the wall. I'll be adding venting since there was none before. I would have liked to run my drain lines within the confines of the bathroom (maybe 6" from the wall) and run short horizontal vents to the wall, but I'm not going to go against code just to make my life easier.
 

wwhitney

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As I mentioned, you can avoid going directly under the wall by tilting your vent takeoff fitting (a combo, or possibly a san-tee on its back, depending on what your plumbing code) 45 degrees off plumb. The shallower your pipe, the closer you'll need to be.

If you want more specific advice, draw up a cross section, showing the depth of the pipe, diameter of drain and of vent, slab thickness, wall framing, etc.

Cheers, Wayne
 
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