Mg, my husband was a plumbing contractor. The problem we had was partially caused my the country drain at the road. It would fill with roots, etc. and they never cleaned it until in backed up into everyone's homes! We used to leave the cleanout (outside) cap sitting on top and not screwed down so it would overflow there first, but anytime we were not able to be home where we could watch it, we plugged the shower drain! We ended up moving because of it!
