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From the 2018 UPC Illustrated Training Manual, Chapter 7, SANITARY DRAINAGE

710.1 Backflow Protection. Fixtures installed on a floor level that is lower than the next upstream manhole cover of the public, or private sewer shall be protected from backflow of sewage by installing an approved type of backwater valve. Fixtures on such floor level that are not below the next upstream manhole cover shall not be required to be protected by a backwater valve. Fixtures on floor levels above such elevation shall not discharge through the backwater valve. Cleanouts for drains that pass through a backwater valve shall be clearly identified with a permanent label stating “backwater valve downstream.”

In this situation, the flow of the drainage piping is still accomplished by gravity but some of the fixtures within the building have an elevation that is below the upstream manhole cover. This means that if the sewer becomes plugged or overloaded and backs up, then the effluent would back up into the fixtures below the elevation of the top of the upstream manhole and overflow into the building. In order to protect the occupants of the building and prevent the backflow of sewage or liquid waste, an approved-type backwater valve must be installed (see Figure 710.1).

Backwater valves shall be installed only in that branch or section of the drainage system that receives the discharge from fixtures installed on a floor level that is below the next upstream manhole. The waste from the fixtures located above that level is prohibited from flowing through the backwater valve. This is to provide an unobstructed path to the atmosphere for venting purposes. If all the waste flowed through the backwater valve flapper, which is normally shut, it would prevent the free flow of air within the system as required in Chapter 9.

The 2018 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.

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