Flush 2 to 4 pounds of copper sulfate (soled in the store as "roebic root killer" I believe)And the do not run any water for 8 to 12 hours. Best done when you are all going to work or away for the weekend. It kills off roots. I had to have roots rotor rootered out a few years back and I use the copper sulfate every 6 months. No more trouble here.