Several months ago the mainline backed up (flooded the basement laundry room and bathroom.) Called roto rooter to clear out roots and that fixed the problem. After a few months with no problems, while taking a shower in the main floor bathroom, both the laundry room and basement bathroom flooded again. I called to have the mainline scoped again. The plumber said there was nothing blocking it and the water was draining fine. He found nothing with his snake.
So the question is... what could cause 2 different basement drains to flood while taking a shower upstairs at 7am, but at 10am there is no blockage in the sewer mianline and water is draining fine?
Any thoughts?