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.