Hello, what does it mean when two horizontal metal drain pipes on both sides of a 2" cast iron tee have no bottoms for over three feet starting at the tee? Corrosion ate out the bottom area of the two pipes. The third pipe connected to the tee is a vertical cast iron drain pipe for kitchen sink and is fine.

Is this some sort of electrochemical reaction? Or is it due to some bacteria feeding on whatever organic sediment getting through kitchen drain? Needless to say that together with this 6 ft long hole I also found a sizable pool of water underneath it.

The pipes are connected to the tee with shielded no-hubs. Are flexible fernco's better as a replacement connectors for sewer pipes or should I stay with shielded no-hubs.

All feedbacks will be greatly appreciated, as always. Best, h.

What kind of metal running horizontally?
Cast iron can rust right through. Sludge collects in the pipes and rusts away. I've seen vertical cast iron rust through too. Also the casting isn't always uniformly thick, so it can be thin all along one side.
Other metals can corrode for the same reasons.

I can't answer the connector question with certainty, but I believe that most code requires shielded no hubs where metal pipes are used. Real plumbers will be along.

Thank you very much. I am planning to cut out the rusted runs of pipes and replace them plastic, at least as a temporary solution. Will check the codes, thanks. The rusted pipes are either cast iron or galvanized, hard to tell.

Most likely galvanized as galvanized pipe has threads (cast iron doesn't have threads on pipe). Here, galvanized pipe reacts with cast iron pipes (dissimilar metals) in a manner that creates an electrolytic effect and causes minerals to deposit and can lead to corrosion of the pipes.

Install PVC or ABS plastic as you suggested and use the shielded clamps... ONLY way to go!

Mark

Have replaced rotten pipes with ABS. Here on the first photo is the crawl space -- only 12" distance between bottom of joists and ground to move around. Can't see at this point how joist sistering can be done in such a limited space. The other photo is the rotten bottom of one of the 2" pipes. I suspect that the bottom corroded at this particular spot because of slight sagging, with all the trapped waste standing there for decades.

Ugh. I'd just cut 12.5" and 13" pieces of 4 x 4 pressure treat and jam as many of them as you want under joists as little posts. The house doesn't even look very old.

Thanks. The house was built in mid-fifties, when 2x6s were common for floors. What kind of footing would these posts take? I thought of trying jacking a bit and then nailing steel strap f rom one side underneath to the other side.