Vent Sizes

There are 4 basic sizes of vent in the US. The most common are 11/2, 2, and 3 inch. Older homes may have a cast iron 4 inch vent.
So why is this you ask. Well, prior to about 1965 or so, PVC or other plastic piping had not been invented, so cast iron was the most common pipe material around, and the builders would use this for their vent pipe, 4 inch being the most common. Between 1965 and about 1985 or so, the design parameters, at least here in Florida, was to have a small 1 1/2 inch vent at the homes drainage system entrance, a bunch of 2 inch vents along the way, and a big 3 inch clean out vent at the exit. Around 1985 or so builders just started to pretty much go to the 2 inch as a standard, and thats pretty much the way it is now unless you have a condo or apt where they tied everything into one 3 inch vent. Now you are a vent expert.