CIPP Lining Pre-Survey: Why Grade Accuracy Matters Before Pipe Lining

Cured-in-place pipe (CIPP) lining is one of the most common sewer rehabilitation methods. It's fast, trenchless, and cost-effective. But it's also unforgiving of preparation errors. A CIPP liner installed over a pipe segment with a reverse grade, insufficient slope, or unverified invert elevation will fail — and the repair is expensive.

The CIPP pre-survey is the field work that prevents those failures. Here's what it involves and why grade accuracy is central to a successful lining project.

What Is a CIPP Pre-Survey?

A CIPP pre-survey is a combination of CCTV inspection and field elevation work performed on the host pipe before a lining contractor installs the liner. Its purpose is to:

1. Document existing pipe condition — cracks, root intrusion, joint offsets, service connections, and any structural defects the liner may not be able to address
2. Verify invert elevations along the pipe run to confirm the host pipe has adequate grade for gravity flow after lining
3. Identify problem areas — sags, reverse grades, and flat sections that could trap water or affect liner installation

Most CIPP contracts require the pre-survey to be completed and submitted to the engineer before the lining contractor can proceed. It's a precondition, not an option.

What Data Does the Pre-Survey Collect?

CCTV Inspection Data

A CCTV camera is pulled through the pipe from manhole to manhole. The operator logs:

• Distance from the upstream manhole to each defect, service connection, and notable condition
• Defect type and severity codes (per NASSCO PACP protocol)
• Pipe material, diameter, and observed condition at joints
• Evidence of infiltration, inflow, or grease/debris buildup

Invert Elevation Data

Separate from the CCTV work, a survey crew shoots invert elevations at the upstream and downstream manholes, plus at intermediate stations. Standard intervals are 25 feet. This gives the engineer a grade profile of the actual pipe, not just the plan grades from original construction.

Use the Pipe Run Calculator to calculate grade percentage and fall-per-100-feet between any two measured inverts. The Elevation Calculator helps you establish all intermediate station elevations from your benchmark shots.

Why Grade Accuracy Is Critical for CIPP

CIPP does not change the host pipe's grade. The liner follows the existing pipe profile exactly. If the host pipe has a sag, reverse grade, or section with inadequate slope, the liner will have the same condition.

The consequences of inadequate grade

Ponding and solids accumulation. If a section has insufficient slope for self-cleansing velocity (typically 2 ft/s at full flow), solids will drop out of suspension and accumulate on the liner invert. This negates the lining investment and leads to maintenance calls within a few years.

Liner installation problems. The liner installation process requires consistent flow conditions during cure. Sections with reverse grades can trap water and steam during steam-curing, creating voids or delamination in the finished liner.

Warranty rejection. Most CIPP manufacturers and contractors specify a minimum grade for successful installation. A pre-survey that identifies inadequate grade must go back to the engineer — the segment may need point repairs or bypass before lining proceeds.

The Pre-Survey Workflow

Step 1: Access and safety. Set up traffic control, confined space entry procedures, and bypass pumping if required. Bypass pumping diverts active sewer flow around the work segment during inspection and lining.

Step 2: Manhole-to-manhole CCTV. Pull the CCTV camera from the upstream manhole to downstream. Document all conditions. Mark service connections for reinstatement after lining.

Step 3: Invert elevation survey. Set up your level at a benchmark, establish HI, and shoot the upstream manhole invert. Then work downstream, shooting the pipe invert through the manhole at 25-foot intervals if accessible. At minimum, shoot both manhole inverts and calculate grade using the Pipe Run Calculator.

Step 4: Compare to plan. Compare your measured inverts to the original plan elevations. Document any discrepancies greater than 0.10 feet. Identify all segments with grade below the project minimum (typically 0.4–0.5% for 8-inch pipe).

Step 5: Report. Submit a pre-survey report with the CCTV footage, invert elevation data, grade calculations, and any recommendations for pre-lining repairs.

What Happens When Grade Is Insufficient

If the pre-survey identifies a section with inadequate grade — a sag, reverse grade, or flat section — the engineer has several options:

• Point repair and re-grade: Excavate the problem section, correct the grade, and relay the pipe before lining
• Segment bypass: Skip the problem segment from the lining contract scope and address it separately
• Acceptance with conditions: For minor grade issues (slightly below minimum but still positive flow), the engineer may accept the segment with additional post-lining inspection requirements

The pre-survey finding is not a project killer — it's a project saver. Finding grade issues before the liner goes in is always better than finding them after.

Documenting for Approval

The pre-survey package submitted to the engineer should include:

• Manhole-to-manhole CCTV video files, indexed by segment
• Defect log with NASSCO codes, distances, and photos
• Invert elevation table with benchmark, measurements, and calculated grade per segment
• Grade profile drawing or table showing design vs. measured inverts
• Recommendations for any pre-lining repairs required

Sitemark's MH-to-MH pipe run tool (available on the Field Supervisor plan) lets you log invert elevations, calculate grade automatically, and export a formatted table for the engineer — saving the office work that usually follows a day of field measurements.

The pre-survey is the most important work on a CIPP project. The liner installation is fast. Getting the preconditions right takes more time — but it's where the project is won or lost.

Minimum Grade Requirements for CIPP Lining Projects

CIPP does not add structural slope to a pipe. The liner follows the existing pipe profile exactly. This makes minimum grade verification a critical pre-survey task, not an optional one.

For gravity sewer mains, most municipal CIPP specifications cite minimum slopes by pipe diameter:

• 6-inch pipe: 0.5% minimum (0.06 inches per foot)
• 8-inch pipe: 0.4% minimum (0.048 inches per foot)
• 10-inch pipe: 0.28% minimum (0.034 inches per foot)
• 12-inch and larger: Engineering design, typically 0.2–0.4%

These minimums reflect the self-cleansing velocity requirement — the flow rate needed to keep solids in suspension and prevent buildup on the pipe invert. A CIPP liner installed in a pipe segment that's already borderline on slope will underperform over time, generating maintenance calls that reflect poorly on the rehab contractor.

Use the Pipe Run Calculator to calculate grade percentage and total fall between any two measured inverts. Enter the upstream invert elevation, downstream invert elevation, and run length — it calculates grade percentage and flags whether it meets the minimum for standard gravity sewer. The Elevation Calculator handles the HI and rod shot math to get you those invert elevations from your level setup.

What Happens When a Segment Fails Pre-Survey

Finding a problem during the pre-survey is not a project failure — it's the system working correctly. The purpose of the pre-survey is to surface problems before the liner is committed. Here's how different findings are typically handled:

Reverse grade (negative slope). The segment must be excluded from the lining scope or point-repaired and re-graded before lining. Lining over a reverse grade creates a permanent pond inside the pipe. No engineer will accept this.

Flat section below minimum slope. The engineer has discretion. On minor flat sections (e.g., a 20-foot section at 0.3% on an otherwise adequate run), the engineer may accept lining with a post-lining inspection requirement. On longer flat sections, point repair is more likely.

Sag (low point that recovers downstream). A sag creates a permanent pooling location. If the sag depth is significant (more than 0.1 feet below the surrounding grade), the segment usually requires point repair before lining. If the sag is minor and the downstream grade recovers adequately, the engineer may accept lining with documentation of the known condition.

Grade below minimum but positive. Document the segment with the exact grade measurement and flag it for engineer review. Most engineers will accept lining on segments that are slightly below the published minimum if CCTV shows the pipe is otherwise in acceptable condition and there's no evidence of current solids accumulation.

In every case, the pre-survey finding — and the engineer's decision — should be documented in writing before lining begins. This protects the lining contractor from disputes about the pipe condition at the time of installation.

Common Pre-Survey Mistakes That Cause Delays

Not tying to a benchmark. Invert elevations that aren't tied to a recognized datum can't be compared to design drawings or to future as-built surveys. Every invert elevation set in a pre-survey must reference a documented benchmark.

Measuring rim elevation instead of invert. The rim elevation is easy to get with a rod shot from outside the manhole. The invert requires measuring the rim-to-invert depth (with a tape or rod inside the manhole) and subtracting. Missing this step produces rim elevations that the engineer can't use for grade verification.

Skipping intermediate manholes. On a multi-manhole run, shooting only the endpoints doesn't give the engineer the segment-by-segment grade data they need. Shoot every manhole in the run.

No benchmark loop closure. For longer survey runs, close your level loop back to the starting benchmark at the end of the day. A loop closure check within ±0.05 feet confirms your data is reliable. If it's outside that tolerance, find the error before you submit the survey.

Late submittal. The pre-lining survey is often a required submittal before the lining contractor can mobilize. Submitting a complete, clean report 48–72 hours after fieldwork keeps the project schedule moving. Submitting an incomplete package that generates RFIs costs more time than the survey itself.