Sanitary sewer systems are known for tree root problems.
Without costly replacement, the only aid that works for tree roots in sewer systems is copper sulfate. Copper sulfate is a chemical used for weed control in ponds, aquarium’s, and even used to treat burn victims. Flushing a cupful down your toilet, once a month, helps to retard the growth of tree roots. Because it is a crystal, it settles into the low spots in the sewer line, and activates when water flows through. This sours the water for the roots, so they don’t grow across the line.
Copper Sulfate is highly poisonous, though, so caution should be used when handling and during storage. It looks like blue rock candy, so it should be kept out of the reach of children and pets.