Stack size & venting, stack air admittance

[Keithwins]

Hello,
I've got a 4" sewer out to the street. It is a single-family detached house in a jurisdiction adopting the 2015 IRC.
It is an older house, but all the plumbing has been removed, so it's all new. There is a requirement in the IRc that the stack can not diminish in size... does that mean a 4" stack vent through the roof?!
Also: I think I want to do a 3" stack-type air admittance valve in the attic, and a 2" side-wall vent. Does that sound right?
I realize that stack-type air admittance valves are not very common in the residential world. What do you think of them?

 

[Jeff H Young]

I've never put a vent out the side . nor terminated in an attic. Why would anyone do that?

 

[Sponsor]

 

[Reach4]

There is a requirement in the IRc that the stack can not diminish in size... does that mean a 4" stack vent through the roof?!

You will not use a "stack vent", but you will use a "vent stack".

As far as getting bigger as you go up, that size starts where you vent something, and does not diminish on the way up. So a sanitary tee might have a 1.5 pipe into the top as a vent. Then the path heading to the roof can increase in size, but not diminish.

https://wabo.memberclicks.net/assets/pdfs/Plumbing_Venting_Brochure_2018.pdf is for a newer version of IPC, but most things will not be different.

 

[Keithwins]

I woke up more clear minded. I believe I can drop from 4 to 3, perhaps because it's IRC and not IBC... I have to do so before the beginning of the stack, I believe. I think that stack may officially begin at the first vented fixture as mentioned, or perhaps when it turns vertical?
I do believe I meant a stack vent, terminated by a stack type air admittance valve, different than an air admittance valve for a branch.
The reason one would do a sidewall vent is because one can't or doesn't want to vent through the roof, in this case the vent would exit onto a roof deck, which is possible if you extend it up I think seven feet or something but I think it'll prove to be simpler to do it with the side wall vent. Furthermore, with a stack type air admittance valve, that side wall vent can be just 2 in, whereas the vent through the roof would need to be 3 in and only offset above the last branch, etc...
Vent stacks are entirely different, and I may use one too, to allow me to join the vents and get down to just the stack vent and the wall vent...
This is how I'm remembering things this morning, but I haven't cracked open the code yet.

 

[Jeff H Young]

If you have a 4 inch vent use it. follow code. venting out side wall has restrictions 10 foot off ground , distance from windows etc.

 

[Keithwins]

Hmm... yes, following code was the entire point of coming here.
"if you have a 4" use a 4": the underlying implication is "bigger is better". That may not be true. For a single-family house, without huge volumes of waste, there are trade-0ffs. It's the same with DWV pipe slope: 1/4"/ft is sort of thought of as optimal, b/c it "floats away" waste that otherwise settles and coagulates. Bigger is not better: a 10% slope allows the water to wash away quickly while solids settle.
So... I think stepping down to 3" is not only ok, it might be the right choice. I very rarely see 4" in SFD houses around here: do other people/areas (this is the kind of thing that can be locally variable)?
Most of this thread was ultimately about avoiding a hole through the roof, and now I'm thinking "gee, it sure would be simpler to just put a hole through the roof". So I'm probably going that way. I'm concerned that I'll get in to crazy tussles with the local code officials about code-compliant-but-unusual installs involving stack-type air admittance valves and sidewall venting. I haven't heard anyone here particularly familiar with that approach, and I suspect on average people here are fairly informed.
Thanks for your attention!

 

[Jeff H Young]

I agree 4 inch vent docent really improve much . more common on old homes where they would just run one vent out. Maybe you got a steep pitch I'm not that keen about going up on roofs anymore myself

 

[Keithwins]

It is in fact a very steep roof, around 10 in 12.

 

[Jeff H Young]

Tell you the truth , not going up there , I don't have the ladders and safety equipment soon to be 64 years old , unless there are roofers on the job or other trades up there . I personally would likely put an AAV in attic even if temporary and try to set up to work with roofer to flash the vent. That's just me . plus I'm not good with heights but believe me I've been up on some crazy situations try to avoid them