We are on a septic... had it pumped two years ago.
Ever since we have been in this house, 2001,
Our toilets will bubble when we have a significant rainfall,
1" of rain or more.
They don't like to flush well during this period of wet
Weather.
Otherwise... everything else seems fine.
What would be the best course of action to solve this problem?

Thank you for any insight.

jedijohnjohn

The toilet is bubbling because it wants to vent
Two basic possible things here If the main vent line was not plumed so the rain going down the vent stack above the roof line gets filled up with enough water and it doest drain to tank it has a slight "water trap" in it and is slowing down the venting for the toilet and the toilet bubbles and or flushes slowly.
If it was the drain field backing up then that would show up in the sinks at a lower level
Where are these toilets located? In elevation to other fixtures and stories?
I'm betting on the vent out of roof.
To test that theory in time go up on roof and take a 90 or a U bend and this will stop the heavy rain from entering the vent. Give the vent stack time to dry out. If this solves the toilet problem then someone plumed the vent for that bathroom not to drain rain water down through the vent to septic.
If the drain field backed up past the tank and int the house to that bathroom the all drainage would be stopped so its back to venting
Get back on toilet elevations

Signed 21 Boat

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We have one toilet near ground level, it is affected the least. Two toilets upstairs. The one closest to the vent pipe (Master Bath) is less affected than the other Bathroom. The other bathroom is affected the most. The water level in that bowl is usually low also. Is that another indicator of a vent problem?

The one toilet at ground level is probably venting a bit better from the septic tank or a clean out between the tank and house.
The one at master bath is best and that closer to stack am be effected the least because it his more suction to help over come a water trap in the vent line from there to roof.
The worst bathroom is not able to create that pressure difference to overcompensate the diff
Now the last bath does it have its own vent through roof which for that would be a 2" size vent pipe through roof?
If not that is struggling for air the most
Everything here points to vent problems and the big rain is the catalyst here for vent draining
There is really know way to unclog a stack vent other than capping is an blowing air down from the top of the roof and water can't be snaked out. And it isn't a snake out the drains because it fine in dry season.

Check for another stack in that last bathroom ans while your at it see how big the main stack is for the house out of roof

Signed 21 Boat

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To me it sounds like you have two problems. One problem being that thee is something wrong with the commodeupstairs that does not work right, or the line going from it to the main line is partially clogged.

The second thing I think of is that on the days that you have the rian the field gets saturated with water and therefore as you flush the commode downstairs the water has no where to go so it act as though it is a glogged main line. If this is the problem the only fix for it is not good and that would be to add more field tile to your septic system.

Thanks 21boat. I covered the vent with a large cashew jar with a wire cross 1" from bottom of jar so that the bottom doesn't rest flush with the vent pipe. 1 year and many rains later... not a single bubble... my wife kind of misses them... she thought it was funny. My dog misses the bubbles also. Thanks for a great answer.