I have just discovered a leak from my bathroom waste pipe. I can see the problem, which is it is connected to the much larger rainwater waste pipe. When I say connected, I mean it rests in the bigger pipe and the gap around the smaller pipe has been filled with what looks like flexible filler.

My question is, what is the best way to connect these two different size pipes together ( bearing in mind one is plastic and one is cast iron) so as to ensure a watertight fit?

:confused:

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I have just discovered a leak from my bathroom waste pipe. I can see the problem, which is it is connected to the much larger rainwater waste pipe.

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In my area, and many others, it's illegal to put anything into nbthe city sewer except sewage and gray water. You might want to check with your Building Department.

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My question is, what is the best way to connect these two different size pipes together ( bearing in mind one is plastic and one is cast iron) so as to ensure a watertight fit?

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That woulld be a Duel-Tite Gasket called a Neoprene Doughnut , (see image).
This gasket fits inside the bell of the cast iron and after greasing both the PVC and the doughnut is pushed in by levering it in with a Johnson Bar or shovel. If both the PVC and cast Iron's ends are bald you may use a No-Hub Band,(see image) to connect the two. Good luck, Tom

In my area it is illegal to connect rain water drain with sewer drain, too. Do you smell sewer around that area ?

Best way to connect those two is to cut the sewer line, insert "Y" fitting, connect this fitting with No-Hub couplings - and than connect your rain drain to it with Flex-Seal coupling.