OK, we had a video scan done on our sewer connection. The house outlet is iron pipe. When connected to city sewers a decade or two ago, they evidently did ABS (recycled plastic resin) out to 75 feet from the house, and then switched to SDR35 (PVC) the rest of the way out to the street. It is written that SDR35 is often used for the final run to the city sewer. My question is WHY? Why don't they use the same kind of pipe the whole way? The pipes are in great condition, but there is root infiltration at the joint between SDR35 and ABS, which suggests that joinery isn't that tight between different kinds of pipe.