If roots find their way to drain lines, what could occur?
Hairy-type roots can grow into a leaking pipe. Time will pass and eventually it will block completely with toilet paper, grease, or debris.
Roots that are in the pipe will continue to grow until the pipe is forced to crack, collapse, or break. Your only option at this point is is to go ahead and replace the pipe.
The pipe types which tend to have a problem with roots are the clay, concrete, and PVC. Although PVC is less likely to experience a problem since it has fewer joints and is less likely to have leaks.