Tree roots are frequently found in sewer lines that have collapsed or leak. Tree roots are not the cause of the problem. While seeking water, entry is gained through a previously cracked portion of the pipe. Cracks in a line may occur due to a deterioration of the pipe because of age, shifting or complication of the pipe bed, or because of the type of piping material used in the construction of the line. In older systems, the line was usually constructed with clay tiles. Modern sewer construction relies on PVC plastic pipe. For this reason, the City will not remove publicly maintained trees because of sewer problems.