WHAT IS SEWER BACKFLOW AND WHY DOES IT HAPPEN?
Under certain conditions, stoppages in the sanitary sewer in your neighborhood could
cause sewage waste water to back up into your home, a condition called backflow.
When there is a stoppage in the sewer main in the street, the sewer system upstream of
the stoppage becomes flooded. The water level in the sewer will continue to rise until it
reaches an overflow point, which usually is the sewer manhole above the stoppage. But
in certain situations, the overflow point could be the flood rim level (the point where the
fixture cannot hold any more water) of plumbing fixtures in your home. This could occur
if the fixture flood rim level is lower in elevation than the next upstream manhole.
HOW CAN I TELL IF MY PROPERTY IS AT RISK?
A visual comparison of the manhole elevation to the flood level elevation of the lowest
plumbing fixture (usually a shower, bathtub or toilet) in your home will indicate the
potential for spillage of waste water into your home. The use of a builder’s level optical
instrument can help determine the relative elevations to see if your home requires
backwater valve protection. A licensed plumbing contractor can determine these
elevations.
The location of plumbing fixtures in a basement does not automatically trigger the need
for backwater valves. In many instances, fixtures in a basement are pumped vertically to
a building drain above, and the pump outlet is already provided with a check valve that
protects against backflow. If the basement fixtures drain by gravity and aren’t pumped
out, backflow protection may be required.