UPC Section 710: When Phoenix Code Requires a Backwater Valve

The rule compares two heights. The first is the flood-level rim of a plumbing fixture, the point where water would spill out if the fixture overflowed. The second is the manhole cover of the next upstream manhole in the public sewer. Where a fixture's rim sits below that manhole cover, Section 710 requires a backwater valve. That valve goes in the building drain or the horizontal branch serving the fixture. In practical terms, this describes:

Fixtures on a floor with a finished floor elevation below the elevation of the manhole cover of the next upstream manhole in the public sewer shall be protected by a backwater valve installed in the building drain, or horizontal branch serving such fixtures.

A backwater valve is a one-way flap. Waste flows out through it normally. But if pressure builds from the sewer side, the flap swings shut and blocks it from coming back in. Picture a basement bathroom or a laundry sink installed below street level in an older home. If the public main surcharges, that low fixture is the first spot sewage would resurface inside the house. The valve is what stands between the main and the living space.

This section matters most for a narrower group of Phoenix homes and additions: any space with drainage below the surrounding street or yard grade. Think of a converted basement, a below-grade utility room, or a laundry or bath added in a sunken part of a slab. Most single-story Phoenix homes built on a standard slab sit entirely above the relevant manhole elevation. They never trigger this rule at all. That is one reason backwater valves come up less often here than in cities with more basements.

The rule also runs the other way, and this half gets missed almost as often as the first. Fixtures above the manhole cover elevation must not drain through a backwater valve. A valve's closing flap partly restricts flow even during normal use. Tying an upper-level fixture into it can cause slow drains or backups that have nothing to do with the city sewer at all. The valve exists to guard the low fixtures specifically, not to sit in the path of every drain in the house.

If a remodel or addition puts a drain, sink, or bathroom below the surrounding grade, plan on a backwater valve as part of that scope. Do not treat it as an afterthought added only if an inspector flags it. The valve has to stay accessible for maintenance and testing after installation, since nobody can service a valve they cannot reach. In Phoenix, the risk peaks during monsoon storms that can surcharge the main, so a low fixture is worth protecting before that season starts. See how to prevent sewer backup before monsoon and the warning signs of a sewer backup. Not sure whether a fixture in an older Phoenix home falls below the relevant elevation? That call depends on where the nearest upstream manhole actually sits, not just how the house looks from the street, so a plumbing inspector or a licensed contractor pulling the permit is who should make it.

UPC Section 710 is part of the Uniform Plumbing Code. IAPMO publishes and holds the copyright on it. The excerpt above reflects the section as adopted. Phoenix enforces the 2024 UPC with local amendments as part of its Building Construction Code, effective August 1, 2024. Read the current section directly on UpCodes, or check the City of Phoenix Planning & Development Department for local amendments: phoenix.gov/pdd.