The cast iron waste pipe in our basement developed a four foot vertical crack. We are working on replacing it with a length of PVC. Sounds like the shielded clamp is the way to go on the top, but on the bottom the crack extends to the cement floor. How do we connect at the bottom? Husband wants to use PVC couplers between PVC and cast iron and seal with silicone. I'm not in favor of that plan. He doesn't think using the clamps will support the weight of the stack. Help! He's about to start cutting!

Not to worry about the Shielded Clamp. I've put multiple storied condo using No-Hub cast iron and clamps.
Before hubby makes his cuts he MUST support the stack with a couple of 2X4s and a raiser clamp,( see image) If all that cast iron should come crashing down it could spoil your entire day.

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on the bottom the crack extends to the cement floor

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This bothers me. Does the crack extend below the floor so sewage could leak out?

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Husband wants to use PVC couplers between PVC and cast iron and seal with silicone.

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What are "PVC couplers"( do you mean couplings?) and how would he use them? Also if I were going to seal something I'd use Epoxy that hardens up. Let me know how you make out. Good luck, Tom

But you would use the shielded clamps, not the rubber ones with the two clamps...

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But you would use the shielded clamps, not the rubber ones with the two clamps...

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Think about it! Would you want all your cast iron supported by metal or rubber? Use Shielded Metal Clamps. Have you given any thought as to how far under the floor the crack goes? Regards, Tom

Guaranteed here that the only way a plumber would do this job is to use SHIELDED CLAMPS as Tom mentioned (see image). Those all neoprene rubber and SS band clamps were made for underground connections... not even supposed to be used for repair work, but that's where you see them most often!

Finally, in my area at least, you can't use PVC pipe here... need to install cast iron pipe into the vertical stack if cast iron is above the repair. You can buy single hub cast iron pipe in 5 foot lengths, or if you are going into the ground here (sounds like you should) purchase a 10 footer of no hub cast iron pipe (and other needed fittings/clamps) and cut the pipe using a metal cut off wheel attached to an angle grinder (or similar tool). Wear goggles for safety sake, OK?

Back to you...

Mark

Check with your local Building Department to see if PVC is acceptable. In my area it is and PVC will be a lot easier to work with then cast iron. Good luck, Tom

In your area buildings are only one story tall, Tom... ;)

If Nichivu's home is over 1 story, cast iron is best job, although as you said, it is always best to check with local codes!