Re: Sewer odor from shower drain
Sewer odors usually points towards a venting problem. Meaning that the drain line isn't vented correctly (not uncommon in slap-it-up new construction)... and therefore the vacuuum created by the water running down the drain causes the water to get sucked out of the trap.
Could also be that somehow the vent is plugged.
Had you done any work on the house or rennovations? Just trying to understand why it's suddenly become a problem now. Usually that means something has changed.
You could try pouring just a cup or so of water down the drain slowly after you take a shower. IF the trap is full, there should be no way sewer gases can come back inside.
Also possible that the trap is getting sucked clear by another drain on the same line. This is most often a toilet which has a large mass of water draining at once. Either way, that's still a venting problem.
Maybe someone else has another theory.
1925 Two-Story Stucco Beaux Arts Neoclassical