I am adding a basement bathroom in a 1926 house. The mainline is cast iron. I plan on cutting the ci below grade and adding a backwater valve. The house is two story with a basement. I currently have a 1.5" vent from the old laundry tray in the area. My question: Can I use the 1.5" vent to vent the toilet, washer stand pipe and a lav? What about an AAV? I am trying to avoid running a 2"vent up through two floors.

Have you checked to see if your city inspector will approve a AAV, many will not allow them. A vent stack works best if it reaches above the roof line but they can terminate outside along the exterior wall as long as they are a required distance from doors, window, vents, etc...

I am adding a basement bathroom in a 1926 house. The mainline is cast iron. I plan on cutting the ci below grade and adding a backwater valve. The house is two story with a basement. I currently have a 1.5" vent from the old laundry tray in the area. My question: Can I use the 1.5" vent to vent the toilet, washer stand pipe and a lav? What about an AAV? I am trying to avoid running a 2"vent up through two floors.
2" would be better but try going 1 1/2" with a AAV. If yousee the toilet bowl water bouncing or hear any gurgles when the washer discharges increase to 2". Good luck, tom

You will need at least one 2" vent. If you go with AAV, install at least 2 of them and split the load.

Milo, Where would you put vent #2 and why? Regards, Tom

Thanks for asking Tom...

I would run one 2" from toilet, second 2" from rest of the bathroom including washer. Since 2" vent caries 16 plumbing units, I would combine them in the ceiling and run single 2" to the roof.

But I am (already) aware of Code ( and general installation... ) differences between the Coasts. We want to believe that we do things "right" here... :D

We want to believe that we do things "right" here...
And the rest of us think that you "overdo" things in your area. No wet vents, no high loops,etc. Some of your code restrictions have a tendency to hinder the homeowner more then to help him. Just my opinion but I have lots of company with it. Regards, Tom