[HayZee518]

if the "stack" has any blockage, a bird for instance, the commode(s) will back up. I had a bird in mine one day. the bird must've fallen in the 3 inch stack and the bird got lodged in the t-y fitting and somehow got trapped in the bottom of the toilet. my stack is basically a striaght vertical pipe with "things" teed off the vertical stack - toilet, 2nd floor drains, sink drain at the lowest point - before the drain takes a 90 degree left turn and onto the main line to the street sewer connection. in your case it goes to the septic tank. if you have clean outs along your line - remove the caps and take a look inside with a flash light.