When a drain backs up in your house, it most often results from a clog at that drain. This is usually the case when a kitchen or bathroom sink backs up. However, in other instances, the clog does not originate at the spot of the backup but further down the system, such as a main line clog.
When thinking about a clog, it is helpful to recognize that water finds its own level.
A clogged pipe will cause water to back up at the lowest point above the clog. So, the height of the drains on the floor where the backup occurred can help diagnose the location of the clog.
Consider, for example, a bathroom on the first floor of a residence with a shower and a sink. If water backs up in the sink, it could be due to a clogged sink or a clog further down the plumbing system. Since the shower drain is lower than the sink’s drain, the shower should also be clogged if the clog is further down the system. If you run the shower briefly and it backs up, you likely have a main line clog. If the shower drains appropriately, the clog is most likely due to hair or other materials stopping the drainage in the bathroom sink.
