reply to Raphion
Re: Best way to join clay and PVC sewer pipe? I really think that if those originally installed Fernco joints allowed roots to enter then they weren't installed correctly. If the pipes were undamaged and the clamps tightened properly there will be no water seepage and no place for roots to enter. I suspect that the clay pipe either had cracks at the joints, there was debris on the pipe keeping the clamp from sealing, or the clamps weren't really tightened down properly.
You're right, as I said in more recent posts, and showed in pics (blurrily anyways), the terracotta pipe is broken roughly one foot upstream of the first fernco, letting in that incredible stuffing of roots (which I got mostly ripped out of there by gloved hands, trowel, and some sharp forkshaped gardening thing I don't even know the name of, that was really gross haha)