In reply to message posted by fourth year:
Probably not, but check it to see. Have someone flush the toilet while you listen at the vent on the roof. If you hear the toilet flush you do not have a blocked vent. You more than likely have a situation where your septic system is flooding due to the rain satuating the ground and your pipe into the house is filling with water possibly right up to the point where the lowest toilet will not flush properly. Then when the ground water recedes, it takes the water in the lowest toilet with it.
Good point, BUT suppose these folks have a bathtub in the same bathroom wouldn’t water seek its own level and thus the tub fill before the toilet did? If this other fixture did get water there would be some debris left as the water did recede.
I honestly do not think it wise to tell a home owner to go up on a roof and put their ear to a vent pipe as someone flushes a fixture.
Of course everyone has a different view on how to do the same job.