I participate in a real estate investor discussion group and the topic came
up about old cast iron sewer pipes tending to crack in properties that have
been vacant for a long time.

One theory offered was that the "acid" from the waste lays in the bottom of
the cast iron pipe and erodes the pipe and causes it to crack. Maybe that's
true, but my hunch is that with the house vacant, the liquid in the pipe
will dry up and no longer be acidic rather than just lay there and eat
through the bottom of the pipe. I think that maybe if the bottom is
cracked, it is due to having been weakened over the years by the prior
constant flow of acidic waste running along the bottom of the pipe.

Another theory is that without liquid waste flowing in the pipes while the
property is vacant, the already-corroded pipes tend to dry out, and then
they crack at weak points due to the drying process and changes in
temperature in the vacant property. That seems maybe a little more feasible
to me, but I have no way of knowing if that theory is correct.

I have noticed that in two out of two properties that I bought that were
either vacant before I bought them, or vacant afterward, the vertical cast
iron pipes in both had longitudinal cracks in them and had to be replaced.
One horizontal cast iron sewer pipe also had longitudinal cracks, but those
cracks were on the top of the pipe, not along the bottom where acid
allegedly would have eaten through.

So, I am leaning toward the theory of old, corroded, and now dried out, cast
iron pipes in a vacant property tend to crack due to being dried out and the
temperature changes while vacant.

Does anyone know if any of these theories are correct, or what the true
cause of cracked cast iron sewer pipes is in vacant properties?