Trees are one of my favorite things in nature. But there is no doubt that they can be terribly destructive when it comes to sewer lines and pipes in general.
Years ago, actually when I first started as an apprentice plumber, the line from your home to the sewer or septic system was probably made out of clay. This was in reality an excellent conveyance for waste because of its resistance to the elements. Unfortunately, it was also fragile (don't ask how many I've accidentally broken), the shifting of the soil and roots from trees were serious enemies. Cast Iron pipe with hubs was the other popular means of getting waste from your home to the sewer lines in the street. These cast iron pipes were put together with lead joints and the lead was poured in the ditch. This process was labor intensive so clay tile was still popular. In the 60's (in Ohio anyways) a form of cast iron became increasingly more widely used. This type of cast iron was known as no hub due to the straight ends on the pipe. It was put together with a rubber and stainless steel clamp that was resistant to most of the elements.
Then the heavens opened up and deemed that plumbers were working far too hard and plastic piping was invented. It caught on right away in areas that had loose soil, a little more slowly in areas that had clay and a large rock content. Many municipalities feared that it would be easily broken or ruptured. I can say from personal experience that it was much more difficult to break than clay tile. 
Enough history, and if I haven't lost everyone yet, I'll get back to those beautiful trees. If your home has clay tile or a cast iron product (or an asbestos pipe that I may get into in another blog) you can bet that it is at least cracked at this point in time. If there are trees close to the line the plumber took to the sewer or septic system then you can bet that there are roots threatening the pipe.
The photo on the left shows a typical clogged clay tile with the arrow showing the only path the waste had left to travel. You can imagine the difficulty. The cast iron pipe, no hub or hub, actually fares worse than clay.
Another small problem that both of these types of waste piping is the ease with which the tree root pushes it around. Roots are super-strong and determined to go whatever way they want (in search of nutrients). The roots will push and push against any surface (clay, iron, or even rock) until they move it out of the way or break it. For some reason (and I don't have the foggiest idea why), living things (including roots) don't like plastic. So at least for now, those of us with plastic waste lines are safer from the dreaded root.
Jack Gilleland
Home Inspection Services, Clayton
Serving Dayton, Ohio and the Surrounding Area
For some more info on this subject go to Russels blog.
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