Most of the outside sewer lines are susceptible to growth of roots from trees and bushes. In the Chicago area the sewers are usually made of clay, and the pieces are only twenty four inches long, which gives roots a chance to enter into the pipe every two feet. Then, once the roots are inside the sewer the roots can grow further and expand to fill the entire 6" space.
Since the sewers carry fluids, they can be a source of water and nourishment for the vegetation that grows nearby. Roots can penetrate through the sewer line and grow into it, thus obstructing the flow of the waste water. When this obstruction clogs the entire sewer it then causes water to backup inside the house. To prevent the obstruction from happening, sewer rodding should be performed. Ideally, it should be done periodically as maintenance.
Rodding the sewer lines basically means cleaning the inside of the sewer lines with specially designed equipment. This equipment has blades that spin on a heavy-duty cable, used for cutting through roots from trees and bushes.
Rodding is done from an access point to the sewer line. The access can be inside the house (such as a clean-out) or outside the house (such as a flood control system, a clean-out, or a catch basin).
While rodding the sewer line from an inside clean-out may work, in many cases it may not be adequate to ensure the outside sewer line is in proper working order. Many inside clean-outs are far smaller than the diameter of the outside sewer line, and do not allow the plumber to perform the rodding with the blade of the same size as the outside sewer line. For this reason, an outside sewer clean-out of the same size as the sewer line can be installed.
An outside sewer clean-out, in addition to being the same size as the outside sewer line (usually 6"), allows easier, faster, cleaner, and less expensive rodding. This means the plumber can do a better and more effective job of cleaning the sewer line.