Kill tree roots in sewer lines using copper sulfate crystals or foaming root killer flushed down the toilet. Copper sulfate poisons roots on contact while foaming products coat pipe walls preventing regrowth. Apply twice yearly as a preventive measure. For severe root intrusion hire a plumber to mechanically cut roots with a rotating blade then apply root killer. Consider replacing damaged pipe sections with root-resistant PVC. Avoid planting trees within 10 feet of sewer lines.