planning layout forbath with toilet,sink and shower, connecting waste line to cast iron pipe that looks like 4 inchs, was going to use 4 in pipe to this but is that a overkill or would 3inch pipe be better
Since your in Rhode Island ( I was stationed in New Port in 1968) I know it gets really cold up there so I would go with a 4" vent through the roof (VTR) to prevent hoarfrost as I have seen to many 3" vent blocked with ice