When a full-length CIPP liner is installed through a sewer or stormwater run, it doesn’t stop at the main pipe, it also passes over every branch connection along the way: the kitchen waste, the laundry line, the second bathroom, any junction where a lateral joins the main run. That’s necessary for the liner to seal the whole length in one continuous piece, but it leaves every one of those junctions closed off once the resin cures. Robotic cutting is the step that reopens them, precisely and from inside the pipe, without digging anything up.
Why junctions get covered over in the first place
A CIPP liner works by inverting or pulling a resin-saturated tube through the full length of the host pipe, from one access point to another, and inflating it against the pipe wall for the cure. The liner doesn’t know or care that a branch line joins the main pipe partway along the run, it presses against the entire internal circumference, including the opening where that branch connects. Once the resin has cured, the liner is a rigid, continuous structural pipe with no seams, which is exactly the point of the process, but it also means every lateral connection that existed before the reline is now sealed shut behind a smooth wall of cured liner.
For a single straight run with no branch connections, this isn’t an issue. But most residential sewer laterals have at least one junction along their length, and many have several. Left unaddressed, a covered junction means wastewater from that branch has nowhere to go, which obviously isn’t an acceptable outcome for a working drainage system.
What the robotic cutter actually does
A robotic cutting unit is a remote-controlled device, fitted with a camera and a rotating cutting head, that’s fed into the cured liner from an access point after the reline is complete. The operator watches a live feed from the unit’s onboard camera and uses the distance and clock-position data recorded during the original CCTV inspection to locate each junction precisely, then steers the cutter to that exact spot along the pipe wall.
Once positioned, the cutting head is engaged and used to cut an opening in the liner at the junction, matched to the size and shape of the original connection. The cutter removes only the liner material directly over the opening, it doesn’t disturb the liner bond elsewhere along the run, and the operator can adjust the cut in real time based on what the camera shows, widening it slightly or refining the edges until the branch line drains cleanly into the main pipe again.
The result is a fully open junction where the connection itself is still sealed around its perimeter by the liner’s bond to the host pipe. There’s no gap, no unlined lip at the edge of the cut, and critically, no seam or joint left for tree roots to find their way back in through. This is a meaningfully different outcome from the original junction, which in an older terracotta or AC cement system was typically formed with a separate mortared or cemented fitting, an inherent weak point that robotic reinstatement doesn’t reintroduce.
How this differs from a top-hat junction patch
It’s worth being clear about the distinction, because the two are sometimes confused. Robotic junction reinstatement is the step that follows full-length relining specifically to reopen connections the main liner has closed over as a side effect of covering the whole run. A top-hat (or junction patch) liner is a different, targeted repair used when the defect itself is at the junction, root intrusion or joint failure isolated to the branch connection while the rest of the pipe run is otherwise sound. One reopens a connection that a liner has sealed; the other seals a connection that was already the problem. Our guide to junction patches and top-hat repairs covers that separate technique in detail, including when it’s the better choice instead of a full reline.
Equipment and precision
The cutting units used for this work are purpose-built for CIPP applications, sized for the pipe diameter being worked on (commonly 100-150mm for residential sewer mains, with smaller units available for tighter branch geometries) and fitted with cutting heads suited to the resin and carrier material used in the liner. Because the operator is working from a live camera feed rather than direct sightline, and cutting into a structural pipe wall that has to remain sound everywhere except the junction itself, this is a skilled, deliberate process rather than a quick drill-through. Rushing a junction cut risks an incomplete opening (restricting flow from the branch line) or, if the cutting head strays off the junction, unnecessary damage to the liner elsewhere along the run.
This is also why the original CCTV inspection matters well beyond the initial diagnosis. The distance and clock-position data recorded before relining is what lets the robotic cutter operator find each junction accurately after the liner has cured and the pipe interior looks uniformly smooth, with no visible clue from the surface of the liner itself as to where a branch connection actually sits.
Post-cutting inspection
After every junction along a relined run has been reinstated, a further CCTV pass confirms the work: that each opening is fully clear, that the branch line drains without restriction, and that the liner bond around each cut edge is intact with no bridging or gaps. This footage becomes part of the job record alongside the initial diagnostic inspection and the post-cure inspection of the main liner, so there’s a complete documented history of the pipe from diagnosis through to the finished, functioning system.
What happens if a junction isn’t reinstated properly
An incompletely opened junction restricts flow from that branch line, which typically shows up as a slow-draining fixture on that specific line (a laundry tub or second bathroom, for example) rather than a problem with the main sewer run itself. This is exactly the kind of defect the post-cutting CCTV inspection is designed to catch before the job is signed off, rather than something discovered later when a specific fixture starts draining poorly. It’s one of the reasons a reputable relining job always includes a documented post-cutting inspection as standard, not as an optional extra.
Frequently asked questions
Does robotic cutting weaken the liner at the junction? No, when done correctly. The cutter removes only the liner material directly over the branch opening; the surrounding liner bond to the host pipe is undisturbed, and the junction itself is left with a clean, resin-sealed edge rather than an unsealed gap.
How does the operator know exactly where to cut once the liner has cured? From the distance measurements and clock-face position data recorded during the original CCTV inspection, before the reline was installed. That record tells the operator precisely how far along the run each junction sits and its orientation, so it can be located accurately even though the cured liner looks uniform from the inside.
Can every junction on a relined run be reinstated in the same visit? Yes, this is standard practice. All junctions covered by a full-length liner are typically reinstated in the same visit as part of the overall relining job, with a post-cutting CCTV inspection confirming each one before the job is complete.
What if a junction was already defective before the reline, does robotic cutting fix that too? Robotic cutting reopens the connection, it doesn’t repair a pre-existing defect at the junction itself, because at that point the junction is fully sealed by the main liner regardless of its prior condition. If CCTV showed a specific defect at a junction before relining, that’s generally addressed as part of the overall job scope, either by the main liner (if the defect was within the relined section) or with a separate top-hat patch if it needed a more targeted approach.
Is robotic junction reinstatement included in the cost of a full reline, or charged separately? It’s generally included as part of the overall relining job scope rather than itemised separately, since reinstating every junction covered by the liner is a standard part of completing the work, not an optional add-on.
Want more detail on where this step fits into the full relining sequence? See our how drain relining works guide for the complete CIPP process from CCTV inspection through to post-cure inspection, and our resin types comparison for how liner material choice affects the finished result.