[khamann]

In the lats two days we have a downstairs toilet which is bubbling during shower or sink (lower volume) use. Larger volumes of water (draining the tub, doing laundry, or running of the sump pump) causes back-up and/or overflow. Is this a blockage of the main drain or a vent problem or something else? THANKS MUCH in advance, KJ

[speedball1]

In the lats two days we have a downstairs toilet which is bubbling during shower or sink (lower volume) use. Larger volumes of water (draining the tub, doing laundry, or running of the sump pump) causes back-up and/or overflow. Is this a blockage of the main drain or a vent problem or something else? THANKS MUCH in advance, KJ

Rule of thumb; Bubbling= sewer blockage Gurgling= Vent blockage
You have a partial blockage in the main drain. What you need to do is localize the problem. Main house drain or the sewer line from the house to the street. To find this out, open up the clean out to the street and cycle the washer. If the water backs up out of the cleanout then the blockage is from the cleanout to the street and you will have to snake from the cleanout.
If the cleanout's clear and it backs up in the house then you must snake from the roof vent that vents the downstairs toilet. Good luck, Tom