For gravity sewer installation: set the pipe laser in the upstream manhole at the design grade percentage and mounting height, align horizontally to the pipe centerline, and verify the laser target stays centered at every joint. Before backfill, take as-built invert shots at both manholes and at least one midpoint — all must be within ±0.05 ft of design before closing the trench.
Why Grade and Alignment Are Both Critical
A pipe installed at the right grade but off horizontal alignment may conflict with existing utilities, encroach on private property, or enter the manhole at the wrong angle. Grade and alignment are verified continuously during installation, not just at the endpoints.
Pre-Installation Checks
- Verify the existing manhole invert. As-built inverts often differ from design by 0.05–0.15 ft. Always field-verify before setting the laser grade.
- Confirm pipe material and OD. Laser mounting height calculation requires outside diameter (bedding height) and inside diameter (invert elevation at pipe barrel).
- Verify trench depth at both manholes. The upstream trench must be deep enough for the pipe invert plus bedding thickness.
Pipe Laser Setup
- Calculate mounting height: Height = (design invert − manhole bottom elevation) + pipe radius. This puts the beam at pipe centerline elevation.
- Install the bracket and mount the laser. Set the grade display to the design grade percentage. Confirm it matches the plans.
- Align horizontally. Use the laser’s built-in sight or a plumb line to point the beam down the pipe centerline. Confirm alignment from both ends on runs over 100 ft.
- Verify beam elevation at the instrument end by holding a target at the laser and confirming it reads the correct mounting height.
Grade Verification at Each Joint
As each pipe section is placed, set the laser receiver in the bell end. Confirm: target shows beam at center, pipe is centered on the horizontal alignment, bell ends mate cleanly. For runs over 150 ft, take an independent level-and-rod grade check at midpoint to verify no accumulated error.
As-Built Verification Before Backfill
Before any lift of backfill, take as-built shots at:
- Both manhole inverts (upstream and downstream)
- At least one intermediate point near mid-run
- Any location where an earlier grade check showed a borderline result
Target rod: 108.25 − 104.63 = 3.62 ft
Actual rod on invert: 3.64 ft → Actual invert: 104.61 ft
Deviation: −0.02 ft ✓ (within ±0.05 ft)
If as-built is outside tolerance, adjust the pipe before backfill — re-laying one joint before backfill takes 30 minutes; after backfill takes a day and full excavation.
Minimum Grade Requirements
| Pipe Size | Min Grade (typical) |
|---|---|
| 6" | 0.60% |
| 8" | 0.40%–0.50% |
| 10" | 0.28% |
| 12" | 0.22% |
See also: Pipe Laser Setup and Grade Transfer, Pipe Grade Calculator, and Minimum Drain Slope Requirements.