Materials that cause serious drain clogs include cements and plasters that chemically harden even in the presence of water. Rinsing out a container used to mix concrete or Plaster of Paris and sending the residue down the drain can cause a clog. The material still hardens in water. Items not meant to be flushed down the toilet also contribute to serious drain clogs. Debris that backwashes into an underground sewer main during a severe rain storm can also be a culprit behind a serious clog.
Storm debris backing into a sewer line from the sewer main at the street is a rare occurrence. Most routine clogs are caused by fats and fibers, and most serious clogs are caused by tree roots or a collapsed terracotta pipe. The majority of routine clogs can be prevented, simply: