Len56 wrote:
I am replacing some 4-inch cast iron vertical and horizontal sewer drain
lines. This is in a 3-story, 3-apartment building that I own. The
horizontal drain line at the bottom that runs above the basement floor
and out to the street will still be 4-inches; I'm just changing it to
PVC. But, I want to change the vertical stack and the horizontal toilet
lines to the new stack to 3-inch PVC if possible.

Is 3-inch PVC for the vertical stack and the toilet lines that run to
the stack okay to use?

Probably not. You'd have to do a study of fixture units
but fixtures for 3 units probably exceeds the capacity
of 3" stack. (The rated capacity of 4" pipe is roughly 10X
that of 3")

Further, the use of PVC may be restricted by local ordinance.

Your call whether you ask the local authorities, who can give
you the true answer...

Jim