How deep must a building sewer be in Mesa? Plumbing Code 305.4.1

Mesa's amendments to the 2024 International Plumbing Code are in City Code 4-5-2. Section 305.4.1, Sewer Depth, is rewritten in two sentences:

• At a private system. "Building sewers that connect to private sewage disposal systems shall be installed not less than 12 inches (304.8 mm) below finished grade at the point of septic tank connection."
• Everywhere else. "Building sewers shall be installed not less than 12 inches (304.8 mm) below grade."

The residential code has the matching rule. Mesa's amendment to IRC P2603.5.1 in City Code 4-3-2 says building sewers that connect to private sewage disposal systems "shall meet the requirements of the Maricopa County Environmental Services Department" and that building sewers "shall not be less than 12 inches (305 mm) below grade."

• Building sewer. The drain pipe from the building to the public sewer or private disposal system, outside the building walls.
• Below grade. Measured down from the ground surface, which for the tank connection is the finished grade.
• Finished grade. The final ground level after landscaping or paving.
• Point of septic tank connection. Where the building sewer enters the tank, which is where the depth is measured for private systems.

The code treats depth as a design number, not a judgment call, so a reviewer or inspector can check it against a tape measure. Mesa also put the same minimum in its residential amendments, which means a contractor working on a house and a contractor working on a store are held to the same 12 inches. Comparing the wording across cities shows how little varies: Glendale uses the same sentence, and Scottsdale keeps 12 inches but hands the private system detail to the county.

Contractors installing a new sewer, plumbers replacing a failed line, and owners planning landscaping, grading or paving over an existing one. Because the rule applies to any building sewer, it applies to a repair that replaces pipe, not only to a first installation.

A replacement in a back yard. A plumber trenches out a failed sewer between the house and the alley. The finished pipe needs at least 12 inches of cover, so the trench depth is set from grade, not from where the old pipe happened to sit.

A grading change. An owner regrades a yard and lowers the surface over an existing sewer. A pipe that once had enough cover may now have less, so the new grade should be checked against the 12 inch minimum before the work is done.

The edge case. A septic system. The depth is measured at the tank connection, and the county's requirements can add to the city's minimum, since the residential rule points to Maricopa County Environmental Services for private systems.

1. 1Locate the building sewer and its cleanouts before any digging.
2. 2Set the trench profile so the pipe has at least 12 inches of cover at every point, and at the tank connection if there is one.
3. 3Pull the plumbing permit for the work before the pipe goes in. See 8-4-8 if the work touches the public sewer connection.
4. 4Ask the city when the plumbing inspection can be scheduled, and keep the trench open until it is done.
5. 5Confirm final grade and any planned paving before backfill so the depth is measured against the finished surface.

• "12 inches is generous." It is a minimum measured from grade, and the pipe still has to be laid to the rest of the plumbing code.
• "The old pipe depth is the standard." The new pipe is measured against the code, not the old line.
• "The rule is the same for every city." Glendale matches Mesa. Scottsdale routes private system sewers to the county.
• "Septic tanks are exempt." They have their own measurement point, at the tank connection.