1. The system could not be a one piece unit but is assembled from several pieces. 2. Most of the weight is being supported by the various connections into in and the walls and floors that they are piped through. 3. The connection fittings into the stack should be cast-iron, but from there, they can be ABS, PVC, copper, cast-iron, etc.: We are having our bathroom remodeled, when they removed the walls and fixtures they said you have a problem. The old drain and vent system is a one piece cast iron unit. What I want to know is if it is possible to still use the old vent stack with new plumbing because the stack runs from the basement up through two floors and the attic before it hits the roof. We do have a union plumber working on this but he is not sure if he can keep the old stack. One because it is so heavy the weight might not be able to be held by any new fittings, and two are there fittings that will work. If any one has any ideas we would appriciate the info. Thanks for your time. Jennifer Kolowski