Sewer gas smell where cast iron mains exit the house

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sethermc

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This is a truly vexing issue for me. I recently finished the basement and, after closing off most of the air leakage, I noticed an unpleasant odor that would only be noticeable at first and then, after a few minutes, would seem to go away. It is also stronger near the floor and is different then the smell of natural gas.
As I was poking around and I heard many scuttling noises I looked behind the main 4" cast iron waste pipe, between the wall and the pipe, right where it exited the building and saw close to 10 cockroaches twitching away. After running to the hardware store and dousing them with some poison, close to a hundred roaches died that day.

There was a small hole around the penetration and I sealed it up with a pest resistant great stuff foam. I haven't seen any more roaches and it seemed to eliminate the smell, but the same smell has come back a few weeks later.

I have a vent just after the house trap and I tried to snake it out to see if there was some blockage, but I wasn't able to come to any definitive conclusion.
The whole system seems pretty sound and all the connections are leaded hubs. There are no other openings in to the waste system in the room.

Does it make sense to get someone to look at the drain system with a camera? Or smoke? I was also thinking of spray foaming the entire area where the waste and vent lines exit the house to seal up any penetrations, but if that doesn't work, then it would add to a mess to fix any of the plumbing.
I would greatly appreciate any ideas as to what I should do.

Thank you
 

Jim Mills

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If spray foam cures the smell, then you definitely have a more serious issue. Could be a broken buried sewer line with an air void around it from settling allowing sewer gas to follow it back to the pipe penetration into the house. Just a guess...
 
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