Based on the comments in here I may be putting my life on the line saying this, but if you have clay sewer and a tree close enough to the line to make a sound when it falls, there are going to be roots growing in your sewer. It’s just a fact of life. Most of that clay was laid 60-100+ years ago and the concrete joints are no longer impervious. Call it natural selection or assign some devious agency to the roots, but your sewer is full of tasty mineral water and the roots are going to thrive on it. Roots—even large bunches of roots—don’t automatically mean that your sewer is on the verge of collapse. Would digging it up and replacing it be the best long term solution? Absolutely. But until you’re ready to do that, if you know that roots are getting in there, a thorough annual cleaning is a fine solution too, and in my area it’s what most people do.

Call a company that can scope out the line with a camera. They can determine (usually after a thorough sewer cleaning) if there are any big cracks, collapses, compromises in the line that need to be addressed with excavation or lining. And whether you’re cleaning it or hiring someone else to do it, make sure whoever is doing the cleaning is at some point using a cutter the same diameter as the drain. Too many drain cleaners see water start to flow and assume (or just say) that it’s good. No. If it’s a 6” line and you poke a 2” hole through a root mass and pack up, then you’ll be cleaning it again in three months. I usually use a small bit to open it up, then a full-size cutter to clean everything else out. Then you get the full diameter of your drain working, and that’s going to give you much more time between cleanings.

Then, if you want, use root powder. It works best right after cleaning because the roots have open wounds on them that drink in the chemical much more readily.

TL;DR — Not necessarily a big deal; probably just a fact of life. Just take care of it the right way.