Verify this tub vent is incorrect

[Red91sc]

I'm planning on adding a 2nd floor 1/2 bath in the future. Here is what I currently have in my 1940's bungalow. A full bath where I've replaced some of the old cast iron that became too small.

Before I get into other questions I wanted to verify that this galvanized vent location for the tub was incorrect. This should have to travel vertical until 42" or 6" above the sink before traveling horizontal, correct?

 

[Terry]

Yes.

 

[Sponsor]

 

[Red91sc]

The area directly above this line is bathroom floor. So the only way to run a correct vent would be to re-route the trap towards the closest wall, run the vent vertical, then have a few 90's to get back to the main location?

 

[hj]

HOW did the old cast iron "become too small"?

 

[Red91sc]

After 70+ years of soap scum, corrosion, etc it was about 3/4" diameter on the inside which resulted in slow draining.