Lowlight, I used to have the same problem in a house I used to live in, except my sewer line was 124' long, and about 5' deep at the end. Mine was so bad, that I was having to root it out every 4-5 months. Then, I got tired of that shit. Whenever I'd go out of town for the weekend, I'd mix up about 4 gallons of really hot water, and a half a bag of ice cream salt, maybe more. Dissolve the salt in the water. The very last last thing I did before leaving the house was pour the salt water down the toilet nearest the drain line and leave. Don't run anymore water nothing, just leave, and let that salt water sit on those roots for the weekend. I went from having to root out my line every three or four months, to every 2-3 years. May be something worth trying.