I just bought an old early 1950's home for my daughter and grandson. We are in process of doing a remodel. Some of the cast iron drain pipe is leaking. Rather than just replace the section that has the leak, I would like to just replace all of the pipe to PVC. It's a very small home and would be pretty inexpensive to replace it. That way I'm assured that we won't spring more leaks later. My problem is that the main vent is a 4" pipe going through a wall and then the roof. I would rather not replace this section. I can get a sawsall on this pipe to cut it free and just use a rubber boot coupler to tie to the new PVC but I'm afraid the pip might want to come down if I cut it loose. It's got to weigh a ton. Does anyone have any suggestions to keep it from coming down when I cut it loose? I was thinking of using a couple of pipe straps wrapped around the pipe and tied to the joist in the basement to keep it in place long enough to get the new fittings attached and a new strap or two to hold it there after. There is no water draining into this vent. It is the main vent only. Thanks!