A back-water valve is a device that prevents sewage from backing up into your basement. This sewage may be coming from a problem in the street, but more often than not, it is your main sewer pipe that is blocked. When this occurs, flushing toilets, taking showers or doing laundry and dishes can cause a nasty surprise downstairs. Some municipalities have sent letters to residents requesting confirmation that a back-water valve exists, is functioning and is accessible. A simple camera inspection can determine this, and a report made to that effect.