Having experienced several sewer back-ups because of roots from my neighbours tree, I can testify that replacing the offending line to the main works. The City took a little prodding to do it (the location was outside my property line) and were prepared to do chemical treatments and cutting but I was able to convince them that the replacement was in all of our best long term interests. They agreed that chemical treatments, and cutting, only provided temporary relief. My experience with some some private contractors, confirmed by comments from neighbours and City personnel, is that the contractor does a quick job, but sometime uses a cutter that is somewhat smaller than the i.d. of the pipe, leaving some of the roots inside, thereby reducing the time interval till the next (nasty) occurrence
Sewer back-ups are high on my list of least favourite events.
mac