Toilet Gurgling in Shower Drain + marital debate

Chapter 1: We have two baths in second story of our 35 yo house. One has stall shower, other has bath tub. Plumbing is back to back with toilets on outside wall. There is a single vent pipe which appears to arise very close to the wall between the toilets. Each bath has two sinks. Everything flushes and drains well. It sounds like toilet is venting through shower. There is no gurgling when other toilet is flushed nor is there any sound coming from bath tub or when bath tub or sinks drain. Puzzle is why is there a gurgling sound coming from shower drain in bathroom 1 when toilet in same room is flushed?

Chapter 2: I treated the shower drain with drain cleaner and problem is now gone, but I am still curious about what could have happened. The wife is worried that the drain is clogged and some day something horrible will happen. I think not, but need some back-up. Help!

The problems your wife is concerned about seldom happen in a second floor bathroom. The sound you describe usually happens when the shower is not used for a period of time and the trap dries out. If that were the problem, then using drain cleaner restored the seal so the sound could not occur. Any other problem would not have been solved with a drain cleaner.

Pouring water in a p-trap will refill it. If the shower had been in use, the trap would have had water in the wier of the trap, that would have sealed it. A dry trap allows air to come into the room from the sewer system, and since it's an open pipe to your bathroom, it's like listening to everything that happens in the pipes on a speaker.

Either use the shower, or pour some water in every so often.

Thanks for replies, guys. Shower in question is in daily use, so dried out trap doesn't seem likely. Still puzzled by cause of sound and why treatment worked. Toilets aren't supposed to vent through showers.

Shower drain is in the middle of a 3x5' terrazo shower base so only access would be a big hole in the 1st floor ceiling. Looking around not an option. What do you thinn of a periodic treatment of shower drain with drain cleaner? Any risk?

Drain cleaner?
If the pipes are metallic, it could perforate the pipes. There are natural drain cleaners like Bio-Clean that is safe for pipes.

The shower is likely tied in past the toilet, so when the toilet is flushed, the water rushes by the wye or tee.