Why is my downstairs toilet overflowing when I drain the bathtub or washing machine upstairs?

Question

We've already snaked the drain, it doesn't seem to be clogged. There are two other toilets in the house (both upstairs), no problems with either of those.

Answer 1

You did not snake the drain far enough down the sewer line. You still have a restriction which means that water upstairs flowing down to the sewer pipe underground can not get out and backs up into the lowest point downstairs. If you had a bathtub or shower downstairs, it would preferentially come up in the lowest point first, the shower or tub drain, because water seeks its own level. However, since apparently you do not have a shower or bathtub, the lowest point becomes the toilet bowl. There are no check valves to prevent backflow, just loop seals to prevent gasses from backing up into the house. You need a plumber or maybe just a longer snake...

Answer 2

On your downstairs toilet you have a built in pee trap and it should connect to your drain line just inside the wall.  What is happening is that you are getting back pressure from the existing line and it is forcing the tub water to empty into your toilet instead of going out to your septic system.  You need to install a proper vent that connects to the existing vent on your plumbing system to allow the escape of the back pressure as your toilet is not properly set up to code.

Answer 3

sewage pipe blockage near the lower level of the house...you need a plummer...You could try to use draino all over the house so it gets in the sewage pipe when no other water does for about an hour...

Answer 4

Most likely there is a clog or partial clog somewhere else in the line, with all the water coming from upstairs its putting all the force of the draining water down to the first floor.  The water is just coming out at the point of least resistance thus the toilet.  You are probably going to have to get a plumber or rooter guy to come and do a more extensive cleanout at the other access points for the sewer line.