Your house can get flooded when you least expect it and even when the skies are clear. For instance, a sudden drop in the water pressure toward your home can create a siphon effect that draws contaminated water back into your potable water supply. With no more room for toilet water to go down, it's forced to come back up. Congratulations. Your bathroom is now flooded.To prevent sewer backups from happening, plumbers install backwater valves. The valve is a simple construction that houses a flapper that can be opened and closed to control the flow of water. The valve is usually installed beneath the house in the private main, complete with surface access. Interestingly, the California Plumbing Code does not require backwater valves in homes, namely the California Plumbing Code. As per Section 710.1 of the Code, authorities only require these valves when a fixture on ground level is placed lower than the closest uphill manhole cover outside.