The cast iron portion of the vent pipe in my attic has rotted off. I need some guidance on how to go about replacing the cast iron portion with PVC. Particularly, I'm unsure of what diameter PVC I should get, and how to connect PVC to the metal vent stack.

Overview image: enter image description here

Side angle of vent stack connection where the cast iron pipe rotted off from: enter image description here

Close up of the vent stack hole: enter image description here

I called several plumbers around my area to get an estimate. Typically I like to get at least 3 estimates, but all the plumbers required a service fee of $40 just to come out. While I don't mind paying people a fair price for their services, being out $120 before any work is done is something I'd like to avoid. While I have zero expertise with this type of work, this looks like a relatively straight forward job. The one place that reluctantly gave me a ballpark figure without coming out said it would be around $599...

Any help/suggestions appreciated. Thanks!

1 Answer 1


You should use ABS (black plastic sewer pipe).

I would remove (unthread) the iron elbow and thread on an ABS female adapter (it looks like 2" to me, but there is nothing in your pictures for size reference). From there I would glue up a 45° ell, a short piece of ABS pipe, then another 45° ell to accomplish the 90° turn to vertical (this will eliminate the "dead leg" of pooling rain water that likely caused the corrosion and failure of the old pipe), arranged such that the top of the last 45° ell is directly centered below the roof flashing.

Then, because it looks like there is a size change (to 3", or maybe 4", hard to tell...) glue up a bell reducer to transition to the larger pipe size. Then glue in an appropriate length of the larger pipe size to go up through the flashing. Roof tar around the flashing and done.

Because of variability by region, pricing is not on-topic here, but $599 seems like too much to me. The parts would be in the range of $35 - $70 US dollars.

Also, support the pipe with some metal strapping.

  • Thanks for the quick reply Jimmy! Very concise and easy to follow instructions. Really appreciate it.
    – nbatalla
    Commented Aug 29, 2020 at 19:49
  • I got all the pipes and pieced them together. I was a little surprised with the combined weight. Although the original cast iron pipe had zero support and definitely weighed more (which I assume contributed to it falling off), do you recommend putting together some time of support?
    – nbatalla
    Commented Aug 31, 2020 at 13:32
  • Yes, at the bottom of my answer I recommended supporting the assembly. I think you could buy a role of cheap plumber's strapping tape (it's a roll of galvanized steel strapping perforated with holes) and just make a loop around the pipe and nail it to the rafters. Commented Sep 1, 2020 at 2:16
  • Thanks again! Somehow missed your support recommendations on your first response.
    – nbatalla
    Commented Sep 7, 2020 at 1:12

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