CIPP “Gapping” vs. Robotic Reinstatement: When a Continuous Liner (or, "Gapping") Is the Safer, Cleaner Choice

If you work around cured-in-place pipe (CIPP), you’ve probably heard the debate: should you “robotically reinstate” every connection immediately, or should you keep the liner continuous and address connections with a gapping / connection-sealing approach tailored to the site? The practical answer is that both methods can be appropriate—what matters is the project’s goals (watertightness, debris control, hydraulic performance, downtime), the pipe geometry, and the specification you’re building to.

What tends to get lost is this: CIPP standards and industry specifications put heavy emphasis on liner integrity and leak performance. For example, ASTM International NASSCO commonly describe a “continuous” finished liner in workmanship terms—free from dry spots, lifts, and delaminations—because those are the defects that actually compromise the pipe-within-a-pipe. (ASTM 1743)

WHAT “GAPPING” MEANS IN PRACTICE (AND WHY IT’S NOT “CUTTING CORNERS”)

In CIPP field practice, “gapping” usually refers to keeping the liner continuous through a run and handling service connections deliberately—often by (a) reinstating only confirmed active connections, (b) sealing/rehabilitating main-to-lateral interfaces with connection lining systems (top hat/full wrap, etc.), or (c) using a two-step approach where the lateral is lined/terminated near the mainline and then a connection liner is installed to provide the seal. (NASSCO)

This approach aligns with the core performance objective repeated across many trenchless specifications: a continuous, tight-fitting liner that functions as a jointless, watertight “new pipe within a pipe.”

WHY A CONTINUOUS LINER IS OFTEN SAFER FOR WATERTIGHTNESS

Every time you cut an opening through a newly installed monolithic liner, you are introducing a deliberate discontinuity at a location that can be sensitive to infiltration/inflow (I/I) tracking if details are not executed well. An industry paper on defect coding describes this directly: when lining is intended to reduce I/I, cutting openings can create a pathway for water to travel between the liner and host pipe into the system. (North American Society for Trenchless Technology 2007 No-Dig Conference & Exhibition )

By contrast, a gapping/connection-sealing strategy can preserve “monolithic” continuity where that is the priority, while still delivering sealed connections using purpose-built connection sealing systems. NASSCO’s lateral seal specification guideline, for example, describes systems intended to create a continuous, tight-fitting, watertight seal at the lateral-to-mainline interface to prevent leakage, root intrusion, infiltration, and exfiltration.

Also, where termination fit is the issue (ends, transitions, or cut-back areas), ASTM International’s F1743 text explicitly contemplates sealing the liner circumference at termination points if it does not fit tightly—i.e., sealing details matter, and sealing is part of workmanship. (ASTM 1743)

WHY GAPPING IS OFTEN MORE COST-EFFECTIVE

Robotic reinstatement can be an excellent tool, but it is equipment-intensive and operator-skill dependent. Trenchless Technology notes that robotic cutters are high-precision and fast, but typically require advanced training and come with higher purchase and maintenance costs. (Trenchless Technology)

A gapping/connection-sealing approach can reduce (or better target) cutter time, minimize the number of cut operations, and shift effort toward planned sealing/rehab steps that are directly tied to watertightness and long-term performance—especially on runs with many laterals where “cutting everything” is not the real project goal.

ROBOTIC REINSTATEMENT: WHY IT CAN LEAVE LEDGES (AND WHY THAT MATTERS)

Robotic reinstatement works by mechanically cutting an opening through the cured liner. If the cut is not aligned, not finished, or is impeded by resin buildup, the opening can end up with rough edges, partial obstructions, or a lip/ledge that catches debris.

This is not hypothetical—many project specifications anticipate the risk and require a smooth finish. For example, the City of North Port CIPP specification requires service openings to have smooth edges with no protruding material capable of hindering flow or catching debris. (northportfl.gov)

Industry guidance also flags common field challenges. Trenchless Technology describes “resin slugs” (excess resin hardened in lateral connections) as a reinstatement challenge and notes they can hinder robotic cutters, including situations where cutters may not reach fully up the lateral connection. (Trenchless Technology) When a cutter can’t reach or the cut isn’t properly finished, you’re more likely to be left with an imperfect opening.

Likewise, NASSCO’s CIPP specification guideline contemplates that service connections can be over-cut or under-cut and states those conditions must be properly repaired to meet specification requirements—another acknowledgment that reinstatement quality can vary and needs controls.

This is the core “ledge” issue: a robotic cut is a machining operation in a confined environment. If it’s not executed to a smooth, flush standard, it can become a debris and maintenance problem, even if the liner itself is structurally sound.

WHEN EACH OPTION MAKES SENSE

Gapping / connection-sealing tends to shine when:

• The priority is minimizing I/I pathways and preserving a continuous liner where feasible. (North American Society for Trenchless Technology 2007 No-Dig Conference & Exhibition )
• There are many laterals and the project benefits from a planned, consistent sealing method rather than hundreds of individual cuts.
• You want to reduce cutter time/cost and the associated variability in cut quality. (Trenchless Technology)

Robotic reinstatement tends to shine when:

• Confirmed active laterals must be reopened immediately to restore service. (Trenchless Technology)
• The specification requires a defined reinstatement percentage/shape, with QA/QC and repair requirements for over/under-cut openings.
• The contractor has experienced operators and the pipe conditions are favorable (good access, minimal resin slugs, manageable protrusions). (Trenchless Technology)

The bigger point: reputable industry guidance recognizes that “no two projects are the same,” and that technology selection should be driven by the safest, longest-lasting, and most cost-effective method for the actual conditions. (NASSCO)

BOTTOM LINE: A CONTINUOUS LINER IS A FEATURE, NOT A BUG

If your project’s success is defined by watertight performance and long-term reliability, preserving liner continuity and using a deliberate connection sealing strategy can be a highly effective approach. “Continuous” in workmanship terms is about liner integrity (free from dry spots, lifts, delaminations), and modern specifications repeatedly emphasize leak resistance, sealed interfaces, and quality-controlled finishing—whether you reinstate robotically, gap strategically, or combine methods. (ASTM 1743)