There should be a main sewer vent going out the roof??
I confess that I have not yet actually done the things recommended in this thread--that is, plug the opening and snake the pipe if that causes drainage issues.
What I DID do was call a plumber and ask for his opinion and estimate.
He said @Nealtw
is right, it's an old house trap like in the picture. He recommended replacing the house trap, though I'm not sure if that's because the house trap is actually causing a problem or if it's just because it's "obsolete technology", as he called it, which might as well be updated.
While he was there, I also showed him the vertical cast iron drainage pipe leading to this, which has a cracked fitting. I asked if it needed replaced, and he said yes, including some old lead cleanout above that and who knows what else.
The estimate he gave for replacing everything from the house trap to the roof vent is about $2000.00.
My dad, the true DIY-er (not me), hates to spend money on anything he could accomplish for less, no matter how much longer it would take. He says we could do all that ourselves.
But I am focused on overhauling my entire yard before summer is over, so I don't want to stop and spend a couple weeks on this project instead.
I would be willing to put off this project until next year after my yard is finished, if the project isn't critical. But something else to keep in mind is there seems to be a leak around the roof vent which drips into my bathroom cabinet when it rains.
I also want to turn my toilet 90 degrees, which I imagine will require moving some pipes. I don't care whether that happens now or a year from now, but I figure I'd better include it in this project rather than replace all this stuff and then have to come back and move it again later.
My last consideration is: What if I did pay the plumber the $2000.00 to replace the whole setup, but in doing so, something goes wrong with the sewer line under my yard, requiring even more work and excavation? On one hand, I don't want to spend the extra money. On the other, now's the perfect time to excavate my yard since I'm tearing it apart anyway.
Given this new information, do you have any advice? Should I pay the plumber or put if off to DIY another day?