What a Lining Repair Can and Cannot Fix

Why this matters

Trenchless lining gets sold to homeowners as a replacement without the trench, and that phrase is doing a lot of quiet work. A liner is a new pipe wall built inside the old one. It is not a new pipe in a new place at a new grade, which means the elevation and the grade of the old pipe survive the repair intact; the tube bridges a small step and nothing larger. Shops that repeat the marketing line end up owning a repair that solved the defect the customer could see and left the one that was actually causing the calls.

One sentence carries the classification half of this card: a liner replaces the wall and inherits the geometry. The rest is the two costs that sentence does not cover, the bore you lose and the branches you have to reopen, plus what the next tech inherits.

What the process is, and what it is not

The common form is a resin-saturated felt or fiberglass tube pulled or inverted into the host pipe and cured in place, so the finished product is a jointless thermoset tube bonded against the inside of the old pipe. There are two other trenchless families that get called lining in conversation and are not:

• Pipe bursting pulls a bursting head through the old pipe, fragments it outward into the soil, and pulls a new pipe in behind. That one does replace the pipe, and because the head follows the old path, it broadly inherits the old alignment too.
• Slip-lining pushes a smaller new pipe through the old one and grouts the annular space. Simple, and the bore loss is much larger than a cured liner's.

The rest of this card is about the cured-in-place family, which is what almost every residential lateral job is.

What it fixes

The common thread is that each of these is a defect of the wall. The pipe is in the right place doing the right thing and the material has failed.

What it cannot fix

The one that gets ignored is the first. A belly is the single most common reason a residential lateral generates repeat calls, and it is the one defect class that lining is guaranteed not to touch. Every ticket where a lined line kept blocking traces back to somebody classifying a geometry defect as a wall defect.

The two costs nobody quotes

Bore loss. The tube has a wall thickness and it goes inside the old pipe, so the finished inside diameter is smaller by twice that thickness. Whether that matters depends entirely on how rough the host was, and the arithmetic below settles it rather than opinion.

Reinstatement. Every branch connection inside the lined length gets covered over by the tube and has to be reopened afterward with a robotic cutter. A missed reinstatement is a fixture group that stops draining entirely, discovered by the customer after everyone has gone home. Count the connections on the pre-lining camera run, write the count and the footage of each one on the record, and confirm the same count on the post-lining run.

Worked example: the bore loss that was a gain, and the one that was not

A 4 inch host pipe with an inside diameter of about 4.0 inches. Take an illustrative liner wall of 0.15 inch, and treat that as illustrative rather than as a spec, because the finished thickness is the liner manufacturer's design output for that diameter and length, not a number to carry between jobs.

• Finished inside diameter: 4.0 - (2 x 0.15) = 3.70 inches
• Branch connections inside the lined length, counted on the pre-lining run: 3, at 14, 31 and 47 feet. Each is a cut edge in a smooth tube after reinstatement, and a missed one is a fixture group that stops draining entirely, so the count and the footages go on the record before the tube goes in and get matched on the post-lining run.

Area before: pi x 2.00^2 = 12.57 square inches

• Area after: pi x 1.85^2 = 10.75 square inches
• Area loss: 14.5 percent

Area is not the answer, because gravity flow does not scale with area. For open-channel flow at the same relative depth, where hydraulic radius scales with diameter, capacity goes as diameter to the 8/3 power divided by the roughness coefficient. That condition matters: the exponent is derived for geometrically similar flow depth, so it compares a half-full 4.0 inch pipe with a half-full 3.70 inch pipe, not a full one with a half one.

• Geometry alone: (3.70 / 4.00) ^ (8/3) = 0.812, so 18.8 percent less capacity

Now the roughness term, and this is where the two cases split.

Host one: tuberculated cast iron. Published roughness coefficients for old, corroded cast iron drainage run substantially higher than for new smooth thermoplastic, and a factor of about 1.5 between a rough host and a cured liner is a reasonable working figure with real uncertainty on the host's actual value.

• Combined: 1.5 x 0.812 = 1.22, about 22 percent more capacity than before lining

Host two: sound plastic pipe with a cracked section. The host is already smooth, so the roughness ratio is close to 1.0.

• Combined: 1.0 x 0.812 = 0.81, about 19 percent less capacity than before lining

Same liner, same diameter, opposite result, and the variable is the host's roughness rather than anything about the liner. On a rough host, lining buys capacity while removing diameter. On a smooth host it is a straight loss, which is survivable on a lateral running well below its capacity and is not survivable on one already marginal.

Two qualifiers stated above have to be visible in that arithmetic rather than asserted around it. The exponent was applied at the same relative depth in both cases, which is why area loss of 14.5 percent and capacity loss of 18.8 percent are different numbers and the second is the one that matters. And the 1.5 roughness ratio is an estimate of the host's condition rather than a measurement, so the 22 percent gain is directional: it says lining a rough host does not cost you capacity, and it does not say it buys exactly 22 percent.

What the technician inherits afterward

A lined line is a different line to service, and the shop that lined it is often not the shop that services it next.

• Cleaning method limits. The liner manufacturer states permitted cutter types and maximum jetting pressure for its product, and those limits reach you through the manufacturer's documentation and the installer's warranty rather than through any code. A cutter head chosen for cast iron can open a liner.
• Chemical limits. The cured resin has its own chemical resistance, so a chemical drain product used in a lined line is a manufacturer question, and a caustic or acid product is not automatically safe just because the host pipe was.
• Temperature limits. A discharge line carrying repeated hot water from commercial equipment sits against a thermoset tube with a stated temperature rating, and this is worth confirming before lining a line that serves a dishwasher or a laundry.
• The reinstatement edges. Each reopened branch is a cut edge inside a smooth tube, and it is a snag point for a cable that a jointless liner otherwise would not have.

The hazard the cure creates inside the building

This one is specific and it belongs to the group's spine rather than to lining in general. Uncured resin gives off vapor during installation and cure, the route is inhalation rather than contact, and the control is ventilation and exclusion rather than gloves: keep occupants out of the lower level, ventilate the work area to outside air with mechanical exhaust, and where a respirator is used at all it is used under a written program with fit testing per 29 CFR 1910.134 in general industry.

The path that vapor takes into the building is the same path sewer gas takes, which is a trap that has lost its seal. Before a lateral is lined, fill every trap in the building, including floor drains and the trap in an unused basement bathroom, and cap open lines. A dry floor drain in a rarely-used basement is the most likely opening in the entire building, and it is the one nobody checks.

How to verify the repair before you sign it off

Ask for the post-lining camera run and watch it against the pre-lining run rather than on its own. Three things to check, and the third is the one that catches classification errors:

1. Every branch reinstated. Count them and match the pre-lining count. A number written on a record beats a memory of the video.
2. No wrinkles, folds or annular gaps at the ends. A fold is a permanent snag and a flow restriction, and it is easiest to argue about while the crew is still on site.
3. Run water and look at the low points. If a pool sits in the same place it did on the pre-lining run, the geometry defect was never addressed, and the line will do exactly what it did before with a nicer wall. That is not a failed liner, it is a liner installed against the wrong classification, and finding it on day one rather than in four months is the difference between a conversation and a dispute.

References

• Liner manufacturer documentation for wall thickness design, cure procedure, chemical and temperature limits, and permitted cleaning methods, which binds through the product listing and the installer's warranty rather than through code
• 29 CFR 1910.134, respiratory protection program requirements where a respirator is used, general industry
• Published open-channel roughness coefficient tables for drainage piping materials, used here as a range rather than as a measured value
• See related: How to Decide Between Jetting, Cabling and Excavation; Why a Drainage Fault Is Usually a Slope Fault in Disguise
• See related: Why Roots Find a Joint and What That Tells You About the Pipe