Blockages can occur for two reasons. The first is the accumulation of material inside of the line. Draining unsuitable substances through the sewer, such as kitchen fats and greases or sand, clay or mud, can cause a build up and blockage in otherwise properly constructed sewers. However, the proper operation of a sewer line requires that the line is constructed "on grade", that is with a consistent slope. High or low areas along a line will cause small amounts of greases, soap scum and other material to accumulate, eventually causing a blockage. "Clean Outs" provide the homeowner or sewer drain contractor an access point for sewer line maintenance.
The second cause of blockages is the presence of sand or roots entering the line through a break or other damage to the line. Tree roots will seek out sources of water, such as sewer lines, and will enter even the smallest cracks in the line. Roots will inevitably clog a sewer line. Larger cracks will not only allow roots to enter, but will cause sand and dirt to enter the line, blocking the flow. As dirt flows into the line, a small "sink hole" or depression appears in the ground above the leak. These sink holes almost always indicate a problem with a sanitary sewer line or a storm drain, and therefore should be investigated or reported.