Let me ask this -- do you have a clue where the roots might be coming from? The reason I ask is that I have a few hundred feet of 4" black plastic corrugated pipe in my yard to drain from the gutter downspouts and several french drains. Everything has been working well for 12+ years, however, roots will enter the exits of the outlet pipe and gradually grow upstream into the pipe. I have to remember to yank the roots out every 1-2 years (and if I forget and we get a big storm, the newly forming lake in the front yard reminds me). It's a simple matter of grabbing onto the root bundle with a pair of vice-grips and pulling backwards. The longest root bundle I pulled out was about 25 feet long, perfectly formed to the inside of the pipe, so it looked like a pre-historic serpent. That took some serious yanking, and now I know to check and clean it out more frequently.
So, bottom line, check the exits and see if that is the entry point. If the pipes were installed carefully and have not been damaged or pierced, that could be the source of the problem. From experience, I can tell you that the roots don't look like much as they enter the pipe (you'd never spot them unless you knew to look) but they sure as heck balloon out once they get into the pipe.