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.