Our house was built in 1925 -- cast iron waste pipe and stack going to roof. Last several years, we've had a problem with sewer smell. Removed sections of several walls, found almost the stack joints seem to be leaking -- smell, but no liquid, ever. The lead seal is pushed up and separated from the pipe itself. We've been caulking the joints with exterior latex caulk -- lots of it -- on top of the joints. Seems to work-- no more smells. BUT: will the stuff under the metal -- the oakum -- swell up and push the new caulk out, thus making it leak again? Or has the oakum swelled as much as it's going to, and so the slathering of caulk on top of the joints a fairly permanent solution? How does old oakum behave?

Oakum is prtty much like an oily rope. It is driven into the collar with special tools that look like a broad cuved screw driver. You will see some soon because I am not going to recommend you learn how to lead a pipe. Leave it for old gezzers like me and the other pros. Either get that piep professionally re-leaded or torn out and replaced with a PVC stack. DO NOT CUT IT DOWN YOURSELF, it can weight a ton and come crashing down on your head. Its lie removing a chimney, start at the top and take small pieces.