Sewer backup protection sounds great. We had major sewer problems last year and ended up paying over ten thousand dollars to have our yard and road dug up. Also we made the mistake of starting with a company that ripped us off... Otherwise we might have only paid half that. I would find a plumbing company in your area to diagnose the problem because sewer gases are dangerous. Check out the BBB website for a decent company that won't take advantage of your situation.

Sounds like you may not have a vent for your lines. The other thing you might check, is to dump bleach water down all the drains,(shower,tub ect.)
we had a problem when I worked with one room in the ICU unit that was not used often and would get the sewer smell, and this would solve the problem,as long as we put some bleach in the drain, and ran water down every week or two.

Never pour any strong chemical down a house drain.

Check the top of the vents on the roof to see if they are blocked by a nest.

Replace all of the toilet seals.

Check exposed vent and drain pipes in the attic, under sinks and in the basement for cracks or bad fittings.

Check the washing machine drain trap and any floor drains.

Might be a long shot on a house that old but a sewer gas riser pipe might be separated inside the wall. This happened to us. Figured it out because the gas smell came out the electrical plug receptical in the adjacent room.

The cause was a riser pipe joint that had not been glued. We tore the drywall off to get at it. I don't know if they glued pipes or used drywall in 1956 - somehow doubt it. Ours built 1981.

I don't think that's a long shot at all. I know a family who searched for months for the sudden source of a strong sewer smell in a relatively new house. Had all sorts of things checked, from septic to roof vents. Turned out that a former resident had improperly capped offf a sewer gas riser pipe in an intended bath that they never finished building. The smell came and went intermittently with the weather. When it was found, and addressed the problem was solved.

It can be tricky. All our riser pipes coalece into one pipe thru the roof. It was too close to the swamp cooler and that sucked in fumes. It was about 4 feet away and I think code was 10 feet. It was on the south side of the cooler and the prevailing winds are N and S. So it only sucked in fumes with the south wind.

I put an external extension on that pipe angling it up to the ridge so the opening was about 20+ feet away an several feet higher which solved it. It was on the back side plus I painted it same color as the shingles. Had that for 25 years till we re-roofed and re-routed the internal pipe.

All kinds of good info right here. Thanks so much.
We will def. get sewer protection although I don't think it's a factor at play here (we have no problem flushing i.e.).
We will probably look at replacing the gaskets on the toilet (specially the downstairs one where it smells more).
The vents in the attic/roof were checked by (a good) inspector - and they were fine. I'd be surprised if they suddenly got clogged (I'll check in the attic just to be sure, of course).
But the riser pipes disconnect behind the walls somewhere is intriguing and probably the best bet right now.
If I'm understanding this right - a bad sewer riser pipe would manifest itself as we observe it - a smell ONLY in the bathrooms (strongest in the downstairs' bathroom). Does that make sense?
Short of breaking the wall any other way to convince ourselves that that is a likely cause?
Thanks so much for this great advice!

It helps to know where the pipes are, of course. The piping drawing for the house would help if there was such a thing. Other than that just common sense. Probly #5 below.

1. Can you get into the attic and see where the bathroom pipe comes up?

2. Each fixture will have a riser probly centered on itself.

3. Those may be joined together with a horizontal pipe before going on up to the attic.

4. A professional pumbing forum might have some good ideas.

5. Google >how to find a sewer gas pipe leak inside walls sewer gas leak detection=================================
If the house was sold with the windows wide open the seller prolly knew about the leak. If it's an obviously old leak and you can prove that they must have known they might be liable for triple damages. 10-4?

This post was edited by mxyplx on Mon, Sep 16, 13 at 17:18