Mediocre_Pool_Rocket t1_ixns29l wrote on November 24, 2022 at 9:34 PM

Don’t seal yet. I would do a smoke test; that is, if they are available in Spain. Essentially banana ( or other scented) harmless smoke dropped in your vent pipe. We call them stink pipes on roof. You’ll know immediately where the sewer gas leak is coming from. Just did this myself recently and immediately saw smoke coming out of the wall as well as from under toilet. Re-seating toilet was easy, but also had to tear up wall. Found that only one side of vent pipe was capped and the other was wide open. That t can sometimes pop off or get knocked off when snaking the line. I tried literally everything before this fix. 3 plumbers just scratched their heads. They were all sure it was a dry ptrap. Did the bleach, vegetable oil, calking etc. I would say that if the sewer gas smell is big, then your problem is most likely big as well. Good luck.

otr_otr_otr OP t1_ixq3w9b wrote on November 25, 2022 at 12:38 PM

Thanks, that sounds like a doable approach.

The plumber came yesterday night and sealed the rear of the toilet with bitumen/ asphalt - so far it seems to work, but it is also a little too early to tell since the sewer smell might be hidden behind the asphalt smell right now.

I asked about the vent pipe - he said that pipe is there to allow air to enter the pipes, not to leave the pipes. And since there are no issues with anything being sucked dry, it seems to work fine.

Will see if it the sealing already works, and if not, the smoke test sounds good.