If a drain is subject to back flow or back pressure, the drain shall be equipped with a ______ valve.

A drain that is subject to backflow or back pressure must have a backwater valve installed to prevent sewage or wastewater from flowing back into the drainage system. Backwater valves are specifically designed to allow flow in one direction while blocking any reverse flow that could occur during conditions such as flooding or blockages in the public sewer system.

The presence of a backwater valve protects the integrity of the plumbing system and prevents contamination within the building, which could lead to health hazards, unpleasant odors, or property damage. These valves are typically placed in the floor drain lines of basements or lower levels where backflow risks are higher.

While a check valve is also used to prevent backflow, it is more commonly found in pressurized systems rather than standard drainage applications. Thus, the backwater valve is the appropriate choice in this scenario. Drain valves and relief valves serve different purposes: drain valves are used to empty systems and relieve valves manage system pressure but do not prevent backflow in the context of drainage.