Tree roots typically invade older types of sewer mains, where cracks and loose fittings allow roots to enter. Old clay sewer mains are highly susceptible to root intrusion because the mortar that was used to seal the fittings has deteriorated with age and because old clay pipes often have cracks. As roots enter these openings, they grow thicker, causing increasing damage to the pipes and enabling further root invasion. Root growth into sewer pipes may increase during dry weather because seepage from the pipes may be the only available ground moisture. But regardless of wet or dry conditions, roots are attracted to sewer pipes because the effluent contains organic nutrients as well as water, and this provides an enticing meal for hungry trees and shrubs.

Clearing a clogged sewer line with a rooter machine provides a temporary respite at best because rooting merely prunes the root ends that have entered the pipe. Once the plumber’s machine is withdrawn, the roots begin to grow again, and six months later the plumber is back. Chemical products can be flushed down the drain to kill these roots, but such products are not nearly as effective as portrayed in advertisements.

The most common long-term solution is to replace the old sewer lines — obviously a very expensive answer to the problem. A less costly solution, offered by some plumbing companies, is the installation of a special synthetic lining in the old sewer main. With this method, the cost of excavation 
is eliminated and the seepage that attracts tree roots is terminated.