Property damage cause tree roots enter tru city sewer and cause line blockage flood house?

Question:had sewer line on property replaced two years ago . and had a sewer backup flood basment . walls and clothing damaged and cracking of foundation.edmonton risk management states no way to prove whos tree roots caused obstrution. the work crew stated it was city tree roots as some how their is about two feet of old cast iron pipeon the city side of water valve as mine replaced with plastic pipe to city valve. I need help with what if any recourse is available to me.thanks cliff.

Answers:
there are two ways to go about this first hire a sewer camera push it through the sewer line and document everything on a video tape or secondly hire a backhoe and dig up your sewer to prove to the sewer authority that the roots are in their pipe