I've got an older home with a cast iron vent stack which has some cracking near the base. If I'm going to replace it with ABS do I need to cut through the concrete or can I use a fernco above grade? Also, do I have to replace the entire cast stack to the roof, or can I have an ABS section in the middle?

It's not as easy as simply cutting a section out of the CI stack and inserting ABS. How many branches and fixtures come off that stack? Am I looking at a vertical stack in the pictures? Where will the cuts be made and how do you plan on supporting the upper part? If it has branches you can not let it sag even a little. I would also use Schedule 80 PVC if you have a lot of upper stack. More details please. Good luck, Tom

There are 2 toilets, 3 sinks, 2 bath tubs, and a washing machine that come off the stack. I intend to replace everything from the basement to the second story ceiling. There is bracing for the cast iron above the second story ceiling to support the section that goes to the roof. The stack comes out of the concrete at a 45 for about 4' the straight up through the rest of the house.

Thanks, B.

So far so good! I forget to answer you about converting to plastic above the slab. Yes you can and I would use a CI to PVC No-Hub Band to do it. Good luck, Tom