[LazyPup]

Kactuskid,
Although the differing codes take some radically differing views on venting there is one concept that they all seem to share. "The Main Vent shall run undiminished in size from the main drain through the roof." The only notable exception to that rule is that when the total length of the main vent is equal to or greater than 40' the main vent must be increased one nominal pipe size.

After the main vent has been established, additional auxillary vents may be downsized to 1/2 the diameter of the line they serve once they are 6" above the flood level rim of the highest fixture served, but in no case may an auxillary vent be less than 1 1/4".

In geographic areas subject to freezing temperatures (outside air 32deg or less) all vents must be increased to 3" at least 1' inside the attic space before passing through the roof. (Some local codes require 4")