Original sewer piping in Rhode Island and Massachusetts is typically of cast iron inside the foundation. In an effort to spare raw materials that are expensive and, in the case of this particular era, the cast iron usually transitions to clay 3 - 5 feet outside the foundation. The cast iron is married to clay gasket pipe. It is at this transition where the probability of root intrusion begins.
Roots find their way into a sewer line through broken or unsealed joints, cracks in the pipe or transition points between two dissimilar materials, which are often difficult to create a perfect seal at the point of transition.
In the following You Tube video, you will see the vitrified clay pipe broken and off-set in many places. It appears the pipe has been cleaned via hydro-jetting. However, the viewer will still be able to see the roots as they have been cut, growing into the pipe. This video is a perfect illustration for what a broken pipe with off-sets and entry points for roots looks like.