Plumbing Roof Vent Mods

I have 60 year old home with only one roof vent for the entire house. All the waste lines collect into one 4" vertical iron pipe line, in the basement, run horizontal (with pitch) then exit thru the foundation to the septic tank. Some previous owner added PVC to the top of this line and vented it thru the roof rather than installing individual plumbing vents. This stack is in the way of some interior modifications I want to make to the house.

Question: If I want to redirect this vent around the mods I want to make (a new door in an existing wall), can I go horizontal with the vent up and back over at an angle in the attic to the roof penetration? If so how far can go? Also can I make the vent smaller since air travels thru a pipe differant than water? If I could reduce it to a 2" I would only have to change its path by a few inches...as it is now its too fat to pass by in the wall near by.

Dave

Anyone have thoughts on this?

Under most codes, the aggregate area of ALL vents through the roof has to equal the area of the incoming pipe, so you could NOT reduce a single pipe to a smaller one, unless you modified the layout of the venting. As far as offsetting, as long as you maintain grade, you can pretty much do whatever you have, or want, to.

The various plumbing codes each have specific requirements based on the number of fixture units and overall length for the size of the stacks, and additional requirements for drain and vent stack offsets.
Without seeing exactly what you have, speculating on how do do what you want to do would be a crapshoot.