No. I am not saying the plumber is a lying. In fact, I now agree that you are getting a good service for $80. I guess my creative writing went over your head. I should have been more careful. Make no mistake: I do not and never would recommend duct tape to repair this problem. It was a playful reference to the Âall purpose repair. But I said it, because that would actually get rid of the odors. The Fernco fitting is a rubber sleeve with a high-strength band that will be a truly adequate remedy and to-code. Since this is a not a professional site, and appeals to DIYers, I was sort of suggesting that you are already doing the most of the work by exposing this repair. It is one small step to repair the pipe. Then again, for 8 dimes, you can let him work in the cramped attic, cut cast iron, if need and make the repair. Probably he will guarantee it. Unfortunately my mind wandered far astray, in pursuit of the truth about corrosion. That I will stand behind. Cast iron pipe and the other black kind (ABS, which you don't have, I suppose) and other pipe rated DVW are especial suitable for their resistance to corrosion. In any case, it has to be an extreme rarity for this pipe to fail in the way you described. I have seen CI pipe corrode and leak where raw sewage has stood for a long time. This occurs when there is a chronic problem, such as reverse slope, or long-term clogging, where the effluent stands in the pipe  always a horizontal one. (A vertical would have to be filled too, but fail later.) So, I am sticking with my statement that sewer gases are not the cause of this huge, gaping hole. It makes no sense that any corrosive agent would be concentrated at one point, and predictably, " at this spot (right above the toilet connection.)" Assuming that a conventional installation, using a closet bend, is how the toilet is connected, that fitting will flow into a sanitary Tee. And the vent pipe just above that is inserted into the hub of the Tee. That, in those years, would have been a lead-and-oakum caulk joint. The pipe would have been cut with a stress-inducing chain cutter, typically. IÂm guessing, that the pipe was partially broken during the installation, years ago. And that the subsequent joint making, which packs lead tightly against the OD of the pipe, introduced a heavy permanent breaking forced. Finally, with aging, vibration and thermal changes, the old damage revealed itself as a hole. If the break-out is small, it may not matter a lot. If one large piece fell to the bottom of the stack, it could create a clog there. Keep your eyes open for that. Sorry to be so hard on the guy. HeÂs probably a pretty good egg after all. But IÂd look out for the other fellow. $400 for a Âsmoke test means heÂs keeping the good stuff for himself. Fine Columbian, probably. Pinoke

Did anyone inspect the vent pipes to make sure they were installed correctly? Try also posting this on the Plumbing forum. I found this on another web discussion site: "It may be that you have a leak in the sewer stack, and that the expansion with the hot weather is whats causing it. Cold weather contracts various things and may seal it up again. I too am the type that I can smell things in parts per billion (reason why a lot of times, the Superintendent would ask me how the plant was doing, because I was the first to be able to detect things at the Waste Water Plant). Another possibility is that something is in your attic and its coming down through cracks in the ceiling or through the walls, in your bathroom, (dead squirrels or some critter). Now, make sure, to check your TRAPS in the shower or bath. its possible, that you have a plugged up soil stack, (though the plumber should have found this, if he checked the roof), and that when the temperature gets warm, or something happens outside, it sucks down the water from the traps in your bathrooms, (like when you use your sink in the kitchen and it creates a vacuum and draws down the water from the traps if the pipes aren't big enough...)"

Thank you David! When having this house renovated post purchase, much of the bathroom was gutted and the sink I had installed is one I bought new some years before and was waiting to use (American Standard Standard Collection Console sink with oval glass shelf.) Because the works underneath are totally exposed I bought the best chrome P-trap and fittings a local plumbing supply house had. In a month or so I'll have my plumber back to install a pump for my utility sink, and he will need to add another vent stack I believe, so I'll have him see if the sink is connected to the bath vent, and if the vent has any blockage. I'll ask him about the air admittance valve - thank you so much for that information!

It may be your vent, or could be your sink drain. I found out my problem/cure from googling problem. We had recently replaced the sink stopper and faucet in bathroom. Shortly afterwards noticed sewer smell occasionally, few times a day. Yes, sewer smell is a skunky smell most times. My googling efforts said to check the overflow of the bathroom sink to see if clogged. I checked by using a funnel and pouring water into it to see if water drains. Sure enough it was clogged. When installing drain stopper we got excess plumbers putty into the hole that drains into the pipe. We are amateurs at home improvement and something usually goes wrong/not smoothly. After some fiddling with something to clean out putty and water drained properly no longer had sewer gas. It was explained on the Google info the clogged overflow sometimes causes some weird suction when toilet is flushed causing sewer gas to come up... Good luck.