Our house is 50 years old. Starting one year ago, when the toilet on the main floor is flushed, the bathtub trap, 4.5 ft. from the main stack, gurgles. Just prior to the start of this event, the asphalt shingle roof was replaced so I thought some debris was blocking the vent. A plumb bob goes down without problem and that made me think it was clear. What do you suppose could be causing the "new" gurgle and how could it be stopped? The gurgle does not happen with every flush and it does not happen when the upstairs toilet on the same stack is flushed.
Answer:
There's every possibility that the tub trap is starting to block. 4.5 ft. from the main stack is already starting to have an effect so perhaps the toilet is looking for more air during flushes and the easiest place it can get it is from a restricted trap nearby. The other thing is that a plumb bob is nowhere near the diameter of the vent stack so that too might be causing air starvation.
