My basement toilet erupts when the upstairs toilet is flushed. I am on a septic system, have it regularly maintained, have had the main line snaked twice... no relief from the eruption. How do I figure out which of the 3 vent pipes on my roof should be snaked? I don't want to get up on the roof more than one time as I will need to borrow a ladder.

You apparently have a blockage in the line between the basement toilet and the septic tank. Pull the basement toilet and snake from there.

You apparently have a blockage in the line between the basement toilet and the septic tank. Pull the basement toilet and snake from there.

Thanks, but that's not it. I had roto rooter service the main drain (twice) all was clear. They suggested a vent blockage. I need to know how to find the right vent pipe!

I assume that when you say the basement toilet erupts, you mean that it over flows. A blocked vent pipe will not cause that. The only thing that will cause the basement toilet to overflow when the toilet upstairs is flushed, is a blockage in the main line after the point where the basement toilet connects to the main line.

Sorry, I guess I haven't explained clearly. It does not overflow. It shoots water up to wet the closed lid. Nothing ever fills up or overflows. It shoots like a geyser.

Does basement toilet flush OK?