Storm drain installation requires verifying inlet and outlet invert elevations before connecting any pipe run. The design invert at each catch basin, inlet, and headwall is shown on the storm drain profile. Verify the as-built elevation at each structure with a level shot before setting pipe grade — structure inverts in the field routinely differ from plans by 0.05–0.20 ft, and an undetected discrepancy sends the grade wrong for the entire run.
Storm Drain vs Sanitary Sewer: Key Differences
Storm drain and sanitary sewer are both gravity pipe systems, but with different slope minimums and different inspection requirements. Storm drain typically uses larger-diameter pipe (12-inch and up on most road projects), has shallower minimum slopes (because storm flows are intermittent and high-velocity, not continuous like sewage), and is generally less tightly toleranced than sanitary sewer. But the installation verification procedure is the same: check every invert before backfill.
Typical Storm Drain Minimum Slopes
| Pipe Size | Min Slope (typical) | Notes |
|---|---|---|
| 12" | 0.15%–0.20% | Varies by municipality; verify with plans |
| 15" | 0.10%–0.15% | — |
| 18" | 0.08%–0.12% | — |
| 24" | 0.05%–0.08% | — |
| All sizes | Per project plans | Project specification always controls |
Verifying Invert Elevations Before Connecting Pipe
Before connecting any pipe run:
- Shoot the as-built invert at the upstream structure (catch basin, inlet, or existing manhole) with a level and rod.
- Compare to the design invert on the storm drain profile sheet.
- If the as-built invert differs from design by more than 0.10 ft, notify the engineer before setting pipe grade — a small adjustment to the design may be needed to maintain proper flow direction.
- Calculate all intermediate design inverts using the verified upstream invert and the design grade.
Setting Pipe Grade
Use the same methods as for sanitary sewer: a pipe laser at the upstream structure for active installation, or a level and rod for verification. See How to Calculate and Set Pipe Grade and Pipe Grade Calculator.
Catch Basin and Inlet Installation
When setting precast catch basins or inlets:
- Verify the as-built sump or outlet invert elevation before placing the structure
- Set the top of grate to match the finished pavement elevation (typically flush for road inlets, or per the grading plan for area drains)
- Confirm the pipe stub-out at the back of the inlet is at the design invert elevation before connecting the trunk line
Documentation
Log all invert elevations in Gradelog: design invert, measured invert, deviation, and pass/fail at every structure and at intermediate stations. The completed record is the storm drain as-built for the project file. Use the Pipe Grade Calculator and Drain Slope Calculator to verify minimum slope compliance before backfill.