Because the drain under the slab is original cast iron in much of the 1960s-70s stock, and cast iron corrodes from the inside over fifty-plus years: the pipe scales, narrows, and catches roots at its joints until the lowest fixture in the house becomes the pressure exit. A single backup can be a clog. A repeating backup is a line telling you its remaining diameter, and the camera inspection is the document that turns the argument into a decision.
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