How do tree roots damage pipes?
Roots anchor and support trees, but they also seek and supply moisture and nutrients for continued growth. Roots normally extend around 1.5 times the length of an adult plant's branches. During dry periods, in particular, tree roots will search for any trace of moisture and nutrient, both of which can be found in sewer pipes.
Tree roots are very persistent and surprisingly strong, they will take advantage of any and every opportunity to get inside sewer pipes. Often this is achieved through small cracks or fractures, or through ill-fitting pipe joints.
Few people realise how aggressive the roots of certain trees and shrubs can be in their search for moisture. Tree roots start as fine fibrous feeding masses, with their 'tails' spreading long distances through the pipe, resulting in reduced flow rates and eventually complete blockages.
The degree of risk and damage caused by tree roots depends on the species concerned, its proximity to sewer pipes, and the composition of your sewer pipes (clay, pvc, etc).