We have a two story home. When flushed, a downstairs toilet stopped and nearly overflowed, and water backed up into a tub (also downstairs). Plunging didn’t accomplish anything. Upstairs I flushed a toilet, just as a test. It flushed fine, and the toilet and tub downstairs drained at the same time. Everything appears to be working correctly now.

We think this also happened a week ago but it is not a regular occurrence. We’ve never noticed any bubbling downstairs when things are run or flushed upstairs.

We had our sewer line snaked and camera about a month ago. It is unlikely we have roots or other blockages in the sewer line but it’s possble, I suppose.

Any thoughts on why this could happen?

1 Answer 1


The hydrostatic head water pressure on the sewer plug from the downstairs toilet is roughly 1 psi. The hydrostatic head from the upstairs toilet/drains puts roughly 6 psi on the plug. That is about 7 pounds force at the plug from downstairs or 42 pounds of force from upstairs ( with a 3" I D sewer pipe).

  • So, blacksmith37, what I’m hearing you say is that maybe there was a physical clog between the downstairs toilet and the sewer, and that flushing the upstairs toilet just exerted enough force to push it on through? Is that right? That definitely sounds possible.
    – Greg
    Commented Jun 4, 2018 at 19:17
  • Those head figures only apply if the pipe is full of water. A pipe that's mostly air doesn't have near the head pressure Commented Jun 5, 2018 at 4:45
  • Greg, yes. For the purists . those are the maximum possible pressures / forces. Commented Jun 6, 2018 at 3:15

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