Clean Check extendable backwater valves prevent sewage backup into a residence or building as a result of a plugged sewer system, excess volume in the system or groundwater flooding. They are designed to be installed easily outside a home, business or other structure, without using a costly manhole and are readily accessible for maintenance or cleaning. They are designed to be installed easily outside a home, business or other structure, without using a costly manhole and are readily accessible for maintenance or cleaning. Clean Check valves are available in ABS and PVC and in multiple sizes, to meet the needs of most residential and commercial construction. Vertical offset from upstream to downstream side is: 3 in and 4 in valves - 0.4 in, 6 in valve - 1/2 in.