I'm planning on adding a utility sink and 1/2 bath in my metal shop building. Use would be minimal. Configuration would be all plumbing on outside wall, with utility sink on middle of wall, then about 5 feet to the right would be 5'x5' half bath with small vanity sink and then toilet. I want to avoid digging up too much of the concrete slab for DWV. My idea is to cut out about a 24"x6" hole where the toilet drain would be, install 3" toilet drain which would run below slab level towards outside wall , and then run a 2" vent stack from the toilet drain pipe up through a new framed 2x6 wall directly behind the toilet. Both the utility sink and the vanity sink would then drain into the 2" stack with all plumbing running above floor grade within the 2x6 wall. I would most likely use studor vents on the two sinks, and would like to use a 2" air admittance valve on the 2" vent stack also to avoid having to run it up through my metal roof. I know that residential code calls for at least one main vent stack piped to open air... but in my proposed shop application I think it would work fine. Thoughts... comments... advice? Keep in mind that I am not talking about permits and inspections for this project. Thanks.

Let me know a little bit more. Any chance of freezing? Where would the main off the toilet connect to? How about water? Where's that coming from?
As far as your venting goes, if you arm out on either side to pick up the lavatory and utility sink you won't need to install AAVs on them. The 2" toilet vent will take care of that nicely.
Although I would rather see the 2" vent exit the roof if you locate the AAV where it can be serviced and replaced if necessary it works for me. Good luck, Tom

Thanks for the quick reply. I have already run cold water undergound from my house to the shop, and already have a hose bib on the outside wall. Line is buried below frost line for this area, and I hope to have the shop heated above freezing temps for the few weeks we have when that could be an issue. The main drain will be going to a septic tank. The vanity sink will be about a 24" horizontal run to the 2" vent stack, and the utility sink will be about a 6'-7' horizontal run to the 2" vent stack (both drains will enter the stack on the same side... the longer run just below the shorter one)... do you feel neither would require an AAV?. that would be great! Thanks again for your help...

I think your sinks will be vented just fine without the AAVs. The utility sink's a few feet out of code but if you don't tell I won't. Good luck, Tom

Well, I finally got everything installed... saved the toilet until last... sinks drain fine, but toilet does not want to plush properly... water level backs up in bowl and you can hear bubbling in sink trap. I thought the studor vent might not be working, so I removed it, and the toilet then flushes, but not well. What I am finding is not a lack of vent air coming into the vent system, but in fact there is a positive pressure in the vent system when I try to flush... in other words air is being forced out of the 2" pipe where the aav is installed, not sucking in as would be expected.

The system is all 3" cellular core pvc from the toilet flange to the septic tank... the horizontal run from the building to the tank is 40', all at 1/4"/ft slope. I opened the tank... all is well there. The toilet is a jacuzzi "perfecta" model which has a 3" flush valve and 2.125" trapway... is it possible the 3" run to the tank is too small for this toilet, or just in general too small for this application? I am at a loss to understand what the problem is.

Are you sure you have no construction debris in the pipe ? Try to snake it - or at least put hose inside the toilet pipe and run it full blast for few minutes. See what happens...

I'll try the hose... I don't have a snake long enough