Mysterious backup in mainline


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?
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Yes; the problem is beyond the reach of the plumber's snake.
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We thought of that, he snaked in 90 feet. It is only 75 feet from the house to the sewer line. Not to mention the fact that he didnt hit anything in the sewer line but things were flowing fine. Thanks

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There are two possibilities: not enough fall from your home to the main line or the main line has a problem.
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Charles Schuler wrote:

My vote is for a restriction further down the main line. The problem occurs in the AM when everyone in the neighborhood is taking their shower and with the restriction the main line can't handle the flow rate. Later in the day there is less load on the main line so everything is fine.
Pete C.
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Did they in fact scope the line? A scope is a camera that is feed into the line so that they can see on a video monitor what is inside the line.
If he ran the line, How big a cutter did he use on the line. If he didn't use a large cutter, the snake may have just passed thru the opening created by the last drain cleaning. Tree roots can be a timely endeavor to clear from the main taking several passes with cutter of increasing size. There's a new main sometime in your future even if you find a company that is willing to spend the time to clear roots properly.
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the sewer company will check their end from the nearest manhole. also check neighbors on both sides. look into your vent[s] in the lawn and see if the traps have a tree stick or dirt dropped into them by kids. a garden hose with the squirting balloon end gizmo may be of some assistance for flushing this, but we have had good results also with a plumber-made lawn vent stack plunger and plenty of garden hose water plus a gallon of spray nine to get any grease or oil moving. fix the sewer but double-check check the family's and visitor's feminine "flushable" discard habits. paper towels and tampons have been main sewer blockers here.
snipped-for-privacy@excite.com wrote:

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Is your blockage somewhere between the drains that are backing up and the clean-out where the plumber ran his snake?
A crude test to see about how far out the blockage might be to let the water drain out for as long as possible. Then run the water until you it starts backing up. If it takes several minutes then the blockage is further down the line. If it is backing up quickly the blockage is closer.

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