Sewer Pipe Liners: Felt vs. Fiberglass, Epoxy vs. Polyester, Coating vs. Structural Liner

Three choices that define a liner

Every sewer pipe lining job uses a product called a liner, and every liner is three things decided together: the tube it is built on, the resin that saturates it, and the cure that hardens it. Those decisions, more than the brand on the box, determine how thick the finished pipe is, what flow it tolerates, how fast it goes in, and what it costs. Owners rarely hear them explained; specifications written for the municipal market list them in detail. The sections below cover each in turn, and the last section covers the product that is often sold as a liner but is not one.

Felt vs. fiberglass tubes

Needled polyester felt

The original cured-in-place liner, and still the most common, is a tube of needle-punched polyester felt with a thin polyurethane or polyethylene coating on one face. The felt is a sponge for resin, which is what makes wet-out simple, and it is flexible enough to invert, to turn a 45- or 90-degree sweep, to follow a four-to-six-inch transition and to press into the bells of an old clay line. Cured felt-and-resin is the material the ASTM F1216 minimums were written for: a flexural modulus of at least 250,000 psi and a flexural strength of at least 4,500 psi, measured on cured samples by ASTM D790. Those are modest numbers, so felt liners are relatively thick, and in large diameters they need long cures with hot water or steam.

Glass-reinforced liners

A glass liner is built from layers of fiberglass fabric, often with a thin felt layer at the surface, factory-impregnated with a light-sensitive polyester or vinyl ester resin and shipped in a light-tight sleeve. It is pulled into the pipe rather than inverted, inflated with air, and cured by a train of ultraviolet lamps drawn through it at a controlled speed, under ASTM F2019. The glass raises the flexural modulus several-fold, into the range of 1,000,000 psi and above, so the liner is thinner for the same design load, loses less of the pipe's diameter, cures in a fraction of the time, and is not affected by groundwater temperature or weather. Glass liners are stiffer, which limits them in tight bends and most residential laterals, and the UV equipment is a larger investment, so the market has settled where you would expect: glass and UV dominate 8- to 48-inch municipal and storm drain lining work; felt dominates house laterals and everything with bends. Some manufacturers now make felt-glass hybrids and small-diameter UV liners that are moving into the lateral market.

Epoxy vs. polyester vs. vinyl ester resin

All three are thermosetting resins: a liquid resin plus a hardener or catalyst that cross-link under heat or light into a solid that does not soften again. The material standard that covers them for sewer use is ASTM D5813. They differ in cost, shrinkage, odor, chemical resistance and the cure they need.

• Epoxy is the residential standard and the usual choice for small-diameter and under-slab work. It shrinks very little as it cures, so the liner stays tight against the host and the annular gap that lets water track behind a liner is minimal. It has no styrene and so no odor to drift up a dry trap. It bonds to the host pipe better than the others. It costs more per gallon than polyester, and its working time after mixing is fixed, so the crew sizes each batch for the run. Ambient and hot-water cures are typical.
• Unsaturated polyester is the lowest-cost resin and the workhorse of municipal sanitary mains cured with hot water or steam, and of many UV-cured glass liners. It shrinks more than epoxy, so it is less often chosen where a tight bond to the host matters, and it contains styrene, which has a sharp odor during cure and is managed with ventilation and by capping laterals. Its chemical resistance covers ordinary domestic sewage comfortably.
• Vinyl ester sits between polyester and epoxy in cost and above both in chemical and temperature resistance. It is specified for industrial pipe lining where the flow is hot, acidic, caustic or solvent-laden, for some pressure-pipe liners, and in sewer mains where hydrogen sulfide corrosion is severe. It also carries styrene.

Which resin a bid names tells you something about the installer. A residential quote for a polyester liner in a lateral under a house deserves a question about odor and shrinkage; an industrial quote that proposes ordinary polyester for a process drain deserves a question about the effluent.

Hot water, steam, ambient and UV cure

The resin decides the cure, and the cure decides the day. Ambient-cure epoxy hardens on its own in a few hours and is common on short laterals. Hot-water cure circulates heated water through an inverted liner for several hours, which is gentle and even but slow and uses a lot of water. Steam cure is faster and the usual municipal choice for felt liners, with the risk that an uncontrolled cool-down leaves stresses in the liner. Ultraviolet cure is the fastest and the most repeatable, with the light train's speed logged as a record of the cure, but it only works with glass liners and light-activated resins. The boilers, steam generators and light trains are described on the sewer pipe lining equipment page.

Coating vs. structural liner

Two different products are both sold as "epoxy pipe lining," and the difference matters more than any choice above.

A structural cured-in-place liner is a pipe. Its wall is designed under ASTM F1216 to carry the groundwater and soil loads on its own, on the assumption that the host pipe may eventually fail entirely. It fixes open joints, cracks, root intrusion, missing pipe wall and an eroded invert, because it does not depend on the host for anything but shape.

A sprayed or brush-cast epoxy coating, sometimes called spray-in-place pipe or SIPP, is a layer of epoxy a few millimeters thick at most applied to the inside of a pipe that is structurally sound. It seals pinholes, stops corrosion, smooths a scaled bore and keeps a cast iron drain line from getting worse. It is the right product for two-inch and smaller branch lines that no tube can be inverted through, for cast iron under a slab with pitting but no breaks, and for potable and HVAC pipe. It is the wrong product for a lateral with separated joints, a crack that opens with ground movement, or any pipe that needs its strength back. A coating applied over an open joint seals it until the ground moves, and then the joint opens again under the coating.

Ask which one is being proposed. If the answer is a coating, ask why the pipe does not need a structural liner. The sewer pipe lining pros and cons page lists the defects each one addresses.

Drain pipe liners for small diameters

The two- and three-inch drains under a slab, kitchen, laundry and bathroom branches, were long the gap in trenchless work: too small for a standard felt inversion, too far under concrete to dig. Small-diameter liners built for the purpose, felt or glass tubes from 1.5 inches up with low-shrink epoxy, now cover most of that range, installed with small inversion units that fit in a bathroom. Where the drain is too small or too twisted even for those, a coating is the fallback. Whether the branch lines are worth lining at all is usually decided by the camera and the slab: a two-inch kitchen line lined for $7,800 against a $13,000 slab cut, as one job on the sewer pipe lining cost page shows, is an easy call.

Reading a liner specification

A written proposal or a municipal specification should name the tube material, the resin type, the cure method, the design standard (F1216 for inversion, F1743 for pull-in, F2019 for UV glass), the design thickness and how it was derived, the testing of cured samples to D790, and the inspection video before and after. If a proposal names none of that, it is a price, not a specification. The municipal pipe lining page covers how public-works specifications build on these standards, and the clay sewer pipe lining guide covers which liner suits which host pipe.

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