Splice new vent into cast iron stack, or replace last few feet of stack with PVC through roof?

[Capt. Skinny]

I'm adding a branch vent that will run horizontally, in the attic, to the cast iron stack in the photo.

My initial plan was to make two cuts in the cast iron and splice in a PVC tee with Fernco couplings. But I'm so close to the roof that I wonder whether I would be better off just removing the last few feet of cast iron entirely, splicing into the cast iron once and running new PVC above it, through the roof, in its place.

What would you do? Any pros/cons to either approach?

I don't have snap cutters and it's not a safe area for sparks from an angle grinder, so I'd have to cut the cast iron with a Sawzall.

 

[Jeff H Young]

Id get rid of the cast and put a new flashing . thats a roof leak waiting to happen