The lower of the two toilets in our 2-story house occasionally overflows. It has happened only a few times, most recently (twice) when the upper one is flushed. The lower toilet feeds into a 3" ABS drain about 6' from the house, where the drain runs to the septic tank. The drain was installed with a 1/4" per inch slope; the two drains are connected with a standard "Y" fitting. After one of these episodes, the lower toilet flushes normally - no plunger needed. I dumped five gallons of water down, no problem.
Background: research tells me that each flush does not necessarily deliver all solids all the way to the sewer/septic tank; small, temporary "dams" occur, and the system depends not only on flushes, but also on waste-water from sinks, tub/shower, dishwasher, etc., to move solids and keep the drain clear.
IF this is correct, I believe the most recent incidents occurred because these small "dams" had a chance to dry a bit -only one person was in the house for a week or so each time, so less waste-water was used, maybe not enough to clear the line frequently.
The question is how best to deal with this. I'm considering moving the "y" so that the drain from the lower toilet joins the other 3" line as close as possible to the septic tank. Advice/questions/comments/suggestions welcome!