How a Main Sewer Line Is Installed: Slope, Connections & Cleanouts

01Start with elevations, not a trench

The private building sewer connects the building drain to an approved sewer connection or disposal system. Before excavation, the contractor identifies the upstream and downstream invert elevations—the level of the inside bottom of the pipe—and checks whether gravity drainage is feasible. The public utility controls its own main and connection requirements.

Have utilities located through 811 before digging and arrange private utility locating where needed. Confirm property lines, easements, tree conflicts, connection permits and who is responsible for the lateral. A camera survey of an existing line can help identify its condition; it does not replace a grade survey or a utility locate.

02Understand slope without guessing

A common planning reference for horizontal drainage is 1/4 inch of fall per foot of run, about a 2 percent grade. For example, 20 feet at 1/4 inch per foot requires 5 inches of fall. Measure consistently from the same part of the pipe and check intermediate points, not just the ends.

Do not assume an internet chart allowing 1/8 inch per foot on every 3-inch pipe applies in California. The UPC-based framework treats reduced slope as a size- and condition-dependent exception requiring approval. Ask the designer and local authority to confirm the permitted grade for this line. A sag or reverse grade cannot be corrected by making another part steeper.

03Choose drainage fittings and support the whole run

A pressure-water elbow is not a substitute for a drainage-pattern fitting. Horizontal branches typically use a wye or combination fitting oriented with flow; a sanitary tee has a different purpose and orientation. The approved layout determines the fitting at every transition, especially where a vertical stack turns horizontal.

Provide stable, continuous bedding compatible with the pipe and soil. Rocks, isolated blocks and unsupported joints create stress and low spots. Transitions between materials need listed couplings for the materials and burial conditions—not a convenient sleeve that merely fits. Keep the planned cleanouts accessible after landscaping or paving.

• Approved diameter, material and connection point
• Uniform grade with no bellies or reverse slope
• Correct fitting orientation and listed transitions
• Accessible cleanouts and documented elevations

04Inspection comes before backfill

The inspector needs to see the work and the required test before the trench is covered. Confirm test method, cleanout locations, separation from water services and any utility-specific lateral requirements. Protect the pipe during staged backfill and compaction so a passing installation does not move afterward.

Keep photos of the open trench, connections and cleanouts, plus the inspection record and an as-built route. A water-tightness test checks leakage; it does not independently prove that every foot has the right slope or that the line is correctly sized.