Vacant property causes cracked sewer pipes?

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?
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RogerT wrote:

Theory:
Hairline cracks in the cast iron pipes are kept in a wet state when waste is flowing in the pipe, when the pipe is allowed to dry out, the moisture in the cracks dries up and in the process starts rust formation which continues and the expansion of this rust caused the hairline cracks to grow.
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That sounds very plausible to me. Also add in the original quality of the iron and the casting of the iron pipes may have been substandard.
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RogerT wrote:

The pipes you replaced: Did they offer a clue? Corroded? Rusted? Defective?
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The pipes that I replaced in both houses were old, corroded, and rusted. I just assumed that is why they were cracked and leaking. It never occurred to me that there may have been a connection between the fact that they were cracked and that they were in a vacant property. But then in the recent discussion group among real estate investors who buy and rehab properties, the idea came up that the cracking was caused by being in a vacant property.
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On 12/24/2010 2:03 PM, RogerT wrote:

I've seen and heard from plumbers that cast iron sewer pipes in vacant homes can be damaged by freezing but that doesn't appear to be what you're writing about. If the traps are full when they freeze, they can crack.
TDD
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I bought an old house (80-100 years) in the spring that of course had a cast iron sewer pipe running from the second floor bathroom, down to the kitchen, and into the basement. After finding water leaking from the basement ceiling, I followed the pipe up and discovered a 3 foot long crack in the pipe behind my kitchen cabinet. Interesting to note, the house had been sitting vacant for over a year through the up and down temperatures of the Northeast.
Plumbers putty is getting me through the holidays, but I'm thinking the whole pipe should just be replaced. Does anyone know how much it would cost to replace your entire CIP?
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AmandaL wrote:

You didn't say if the cast iron sewer pipe is buried in the wall or is in the room but not in the wall. Also, is it an exterior wall? If it is in an exterior wall, what type of siding is on the exterior side of the wall in that location? I just had to have a section of cast iron sewer pipe replaced in an exterior wall. Fortunately, it was a section below the toilet etc., so only the pipe from there down and into the basement had to be replaced and not all of the tie-ins etc. I had aluminum siding on the outside wall and the work was mostly done from the outside. They were able to remove and replace the same aluminim siding pieces. The whole job took two home improvement guys two days at $30/hour each plus materials -- so, $960 in labor plus materials. In my case, the sewer pipe was in the wall behind kitchen cabinets, and the cabinets are of a type that would have made removal and replacement of the cabinets to get access from the inside impractical (the cabinets are old built-in, custom made, formica covered cabinets).
My guess is that the pipe you are referring to is in the wall. That will cost a lot more to replace, and how it could be done will depend on what type of wall it is in, and what is on the other side of the wall. One of the problems is that the toilet etc. are probably tied into the cast iron sewer pipe with lead pipe. So, you may have to replace the whole cast iron pipe (probably with PVC) and run all new connections from the toilet flange, sink(s), tubs, etc. to the new sewer pipe.
Without more information or some pictures, there is no real way to even venture much of a guess as to how much it will cost to fix the problem.
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