I am constructing a porch with clawl space over the house to septic
cast iron drain waste pipe. The present pipe is 30 years old. I have
dug down to the pipe and it is in good shape. The pipe runs from the
septic to the house under a slab floor to the basement. A plumber said
that it would be a good idea to replace the cast iron from the tank to
the house slab with plastic. I am wondering if this would be a good
idea?

wrote:
I am constructing a porch with clawl space over the house to septic
cast iron drain waste pipe. The present pipe is 30 years old. I have
dug down to the pipe and it is in good shape. The pipe runs from the
septic to the house under a slab floor to the basement. A plumber said
that it would be a good idea to replace the cast iron from the tank to
the house slab with plastic. I am wondering if this would be a good
idea?

How did you determine how good a shape it was/is in? Did you run a
wall thickness test?

You're getting ready to cover this over to where it is going to be a
_REAL_ pita to get to if you ever need to in the future--or for whoever
is to come after.

Do it while you can. I'd even go so far as to suggest SCH 80 instead
of just 40. There is going to be some way to get to it for cleanout if
ever need to, I hope? You aren't going to affect access to the tank
for cleanout, etc., are you? (just checking...)

Have similar potential problem here w/ old tile waste from interior
cast...decided against covering it over and are rethinking the idea of
similar project with the thought of probably moving the tank/exit line
to avoid doing the same thing.