Combined waste/vent piping

[rglandon]

I'm trying to add a small bathroom to a cabin and have a limited crawl space on one side.
I have worked out having everything go to a 3 inch waste/vent vertical pipe. From top to bottom, the attachments would be a lav, a shower and a wc. By using sanitary tees, I think this will get the proper venting. (See the attached pdf).
I have 2 questions:
1. Can the lav use a p-trap and then run 12 inches horizontally (but pitched) and then drop vertically about 24 inches to a sweep el and then another foot over to the 3 inch vertical? This looks a little like an S-trap which isn't supposed to be used anymore (though the cabin has them elsewhere). Trying to conventionally vent this would be close to impossible.
2. Does this whole diagram work ok?

I'm new to this forum, but have reviewed all sorts of past postings so I think I'm not asking what has been covered before.

Any help would be greatly appreciated. Thanks!

[LazyPup]

First off, lt me welcome you to the homerepairforum. We certainly hope that you will return often.

From your illustration it appears that the two 3in 1/8 bends on the main drain are offsetting vertically. If so that is fine.

In your drawing the shower fixture arm is 7’. If you are under the International Residential Code (IRC) the 2” shower fixture arm is allowed to run up to 8’ so you would be okay, however if you are under the Uniform Plumbing Code (UPC) the maximum length of a 2” fixture arm is 5’, and opting for the code exception which allows increasing the line by one nominal trade size, which in this case would be 3” would allow you a maximum of 6’ from the trap weir to the vent stack opening.

When connecting a fixture to the vent stack in the manner illustrated the horizontal drain line and vertical riser should be one nominal trade size larger than the trap diameter. Your trap diameter is 1-1/2” so the horizontal drain line, vertical riser and 1/4bend at the top of the riser should be 2”. To prevent an S trap type siphon action we are required to have a horizontal section equal to 2x the trap diameter measured from the trap weir to the vent opening or in this case the vertical riser. (See attached illustration)

[rglandon]

Thanks so much for the reply .... I think I'm getting close to a solution.

The 2 offset (45 degree each) take me down thru a dugout section and give me a total 90 degree turn (vertical to horizontal) so I can run under the main cabin to the septic tank --- I think that is ok and I get a cleanout in after the first 45 degree so I could still snake it if I had to.

For the shower .... I could make my vertical stack closer to the the wc and that would get me only 4 feet from the shower drain -- but then I would have to have the vent portion of the stack offset about 3 feet horizontally to get to the wall to run up again vertically. Is that ok to do with a vent (I'm guessing it is since it still lets the air move as necessary) -- the vent would actually pitch slightly back down to the waste in that "near horizontal" section.

I understand your drawing on how to plumb so I don't have an S trap -- and the need to go up one size for the Lav waste pipe -- Thanks.

I really appreciate the help. I've been reading books about plumbing layout, but this is so convoluted I've not found any good sources for answers (until I found you!)

Thanks again.

[LazyPup]

Your first step should be to contact your Plumbing Inspectors office or the reference desk at your local library and find out which major national model code your local plumbing code is patterned after. If it is the IRC or IPC you are permitted up to 8' so your present configuration would be fine.

If your local code is patterned after the UPC a 2" is limited to 6' from trap to vent, but if you install an auxiliary vent the line downstream of the auxiliary vent would be classified as a vented branch and may run an indefinite length.

Under the UPC the auxiliary vent may be reduced to 1-1/2". You could then install a Wye & 1/8 bend on the horizontal drain line and run a 1-1/2" vertical up through the shower wall to the attic space, then back to the stack. The horizontal vent line in the attic space must be pitched 1/4" per foot downward away from the stack and towards the riser. The Tee where the horizontal vent ties into the stack must also be mounted upside down so the radius curve is turning upwards. (see attached illustration.)

[rglandon]

I've confirmed we are under the International Plumbing Code --- so that solves the shower drain.

I have another question ... since there are already 2 places where there are 3 inch vents through the roof, can I use only 2 inch venting off the top of my stack for this bathroom group and run that in the attic to connect to one of the existing 3" vents -- I can then avoid another roof cut -- and running the 2 inch would be better. I'm not clear if the WC requires a 3 inch vent all the way to the outside.

Thanks again for your expertise!

[LazyPup]

EVERYBODY STOP, and let us regroup a moment. In the original post you stated that you are adding a bathroom to a cabin, I must apologize to you because all of this time I have been envisioning a small hunting or fishing cabin, which previously had no bathroom. Now you say the cabin already has two 3” vents through the roof, so from that I conclude that this cabin is much larger than my mental image but, as a consequence this new revelation makes the project much simpler.

Under the IPC or IRC every structure is required to have one “Main Vent”, which must run undiminished in size from the building “Main Drain” through the roof. After the main vent is achieved auxiliary vents may be reduced to ½ the diameter of the line they serve, but in no case shall a vent be less than 1-1/4” and if the total developed length of the pipe exceeds 40’ you must increase one nominal trade size.. In addition, auxiliary vents may terminate through the roof, in the attic space, in some instances through the wall or you may use an unlimited number of AAV’s (Air Admittance Valves- commonly called “cheater vents” or “studor vents”). Even though we may reduce the diameter of an auxiliary vent, in regions subject to frost all vents terminating through the roof must be increased to 3” at least one foot inside the attic space to prevent frost closure.

All this time I have been thinking that the 3” vent in your diagram was the “Main Vent” for the structure, but now you confirm that you already have a main vent. The means that your 3” riser is an auxiliary vent rather than a main vent in which case once you install the 2” tee to tee off to the lavatory you could reduce the riser to 1-1/2” from there to the attic space.

We must have a cleanout on the upstream end of that riser so we now have four options.

1.Extend the 1-1/2” into the attic space and at least 1' under the roof increase the line size to 3" then penetrate through the roof in the conventional manner.

2. Extend the 1-1/2” into the attic space and terminate it 6” above the insulation. (in my opinion we should never make an open vent termination in the attic space because there is a high risk of water vapors from the vent collecting in the insulation or excessive sewer gasses building up in the attic space.

3. Extend the line into the attic as above then install a female thread adapter on the end of the pipe and screw an AAV into the adapter. The AAV would allow air into the pipe for venting but it prevents sewer gasses from escaping. The AAV can also be unscrewed to provide a cleanout access.

4.Extend the riser into the attic and install a Tee on the top of the pipe. On the end of the Tee you install a female thread adapter and a cleanout plug, then from the side opening you can run a horizontal line and tie into an existing vent through the roof. (The only consideration here is that if the total developed length of the vent from the tee at the lavatory junction to the existing vent where you are tying in exceeds 40’ you would need to run 2” instead of 1-1/2”.




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