You have probably heard that drain relining involves putting a “liner” inside your drain. But what is that liner made of, how does it get inside a pipe without digging, and how does it actually bond to the old host pipe to create a structural new pipe wall? This guide explains the technology in plain terms.
Quick answer (BLUF)
A pipe liner is a flexible tube made from felt or fibreglass fabric, saturated with a liquid epoxy resin. It is installed inside the existing host pipe, inflated to press against the pipe wall, and then cured, the resin hardens through a chemical reaction into a rigid, structural tube. The bond comes from the cured resin mechanically locking into the surface texture of the host pipe, creating a new smooth pipe within the old one.
The anatomy of a pipe liner
A finished, installed pipe liner has three functional layers:
1. The carrier fabric (outer layer)
The carrier fabric is the tube that gives the liner its shape and handles the structural loads during and after installation. Two main fabric types are used:
- Needled felt: A non-woven polyester felt mat, commonly used in residential relining. It is flexible, conforms well to non-circular host pipes (deformed terracotta sections), and provides good resin holding capacity.
- Fibreglass fabric: A woven or stitched fibreglass mat. Stiffer than felt, higher tensile strength, preferred for larger-diameter pipes, structural relining of heavily deteriorated hosts, and applications requiring specific chemical resistance.
2. The resin (the structural component)
The resin is the liquid component that impregnates the carrier fabric and cures hard. The resin provides all the structural strength of the finished liner, the fabric is the template, the resin is the material.
The two most common resin types:
- Epoxy resin: Two-component (base and hardener), mixed immediately before impregnation. Excellent long-term chemical resistance to sewer gases, fatty acids, mild alkaline and acid environments. Longer working time (pot life). The preferred system for residential sewer relining.
- Vinyl ester resin: Faster cure, very good chemical resistance to harsh industrial chemicals. Used in commercial and industrial drain relining where the waste stream contains more aggressive chemicals than typical domestic sewage.
Less commonly used: polyester resin (cheaper, lower chemical resistance, less favoured in Australian practice).
3. An internal film layer
Most liner systems include an inner plastic film (typically polyethylene or polyurethane) that serves as the bladder during inversion installation. After cure, this film forms the internal surface of the relined pipe, it is smooth, non-porous and provides the hydraulic flow surface.
How the liner gets inside the pipe
There are two installation methods, both achieving the same result:
Inversion installation
The liner tube is folded on itself and fed into the pipe through the inspection opening. Using water pressure or compressed air, the liner is turned inside-out (inverted) as it progresses through the host pipe. As it inverts, it moves forward and presses outward against the host pipe wall. This is the most common method for residential relining.
The inversion process is elegant: the liner never slides against the pipe wall (which would risk damage and contamination), instead, the inversion point is always at the leading edge, where the clean inner face turns outward.
Pull-in-place installation
The liner is attached to a cable and pulled through the host pipe to the correct position. A separate internal bladder is then inserted and inflated, pressing the liner against the host pipe wall. Pull-in-place is used where inversion is impractical, very long runs, vertical installations, or access configurations that do not suit inversion.
The curing chemistry: how the resin hardens
The resin impregnated into the carrier fabric is a two-part system that begins curing (cross-linking, hardening) once the two components are mixed. The contractor controls the cure time by the resin formulation selected and the cure method applied.
Ambient cure: The mixed resin cures at room temperature through a chemical exothermic reaction. At Central Coast ambient temperatures (15-35°C year-round), ambient cure takes 4-8 hours. On hot summer days in Gosford, the cure accelerates.
UV cure: The liner contains a photo-initiator component. A UV light train is pulled slowly through the installed, inflated liner. UV light triggers rapid polymerisation, the resin cures in the UV beam’s wake. Cure time: 30-90 minutes. This method requires specialist UV equipment but provides a much faster return to service.
Steam cure: Hot steam is injected to raise the liner temperature, accelerating the cure reaction. Used for rapid cure on commercial jobs. Less common in residential applications.
How the liner bonds to the host pipe
The bond between the cured liner and the host pipe is primarily mechanical adhesion, not chemical bonding. Here is what happens during installation:
- Hydro-jet cleaning roughens the host pipe surface and removes all loose material, deposits and biofilm
- The soft, resin-saturated liner is pressed against this clean, slightly textured surface under pressure
- Liquid resin flows into the microscopic surface texture of the host pipe, the pits, scratches, joint mortar texture, or terracotta granule surface
- As the resin cures, it locks mechanically into these surface features
- The result is a liner that is effectively keyed to the host pipe, it cannot pull away from the wall without fracturing
This mechanical adhesion is why surface preparation (hydro-jetting) is so critical. Dirty, greasy or wet host pipe surfaces dramatically reduce bond strength. A liner installed over a poorly prepared surface may delaminate from the host pipe, creating a gap where groundwater can pool.
In structural terms, the cured liner is self-supporting, it does not rely on the host pipe for ongoing structural integrity once cured. The host pipe becomes the permanent formwork and the liner is the new pipe within it.
How thick is the liner and does it reduce flow?
Liner wall thickness is specified based on pipe diameter, pipe condition (how much structural support the host pipe provides) and the hydraulic requirements of the application.
For standard residential sewer relining (100 mm pipe):
- Liner wall thickness: typically 4-6 mm
- Internal diameter of finished liner: approximately 88-92 mm (from original 100 mm)
- Flow area reduction: approximately 12-16%
However, flow capacity is not just a function of diameter, it also depends on the roughness of the pipe’s internal surface. Terracotta and AC cement have rougher internal surfaces than cured epoxy liner. The Hazen-Williams roughness coefficient (a flow calculation factor) for cured-in-place liner is significantly better than for old clay or cement pipe. In most cases, the finished relined pipe has equivalent or better flow capacity than the original unlined pipe at the same gradient.
FAQs
Can a liner fail by delaminating from the host pipe?
Yes, in cases of poor surface preparation, resin contamination, or incorrectly mixed resin systems. This is why the pre-installation jetting and the post-installation CCTV inspection are essential, the inspection can identify any sections where the liner has not bonded correctly before the job is completed and the warranty clock starts.
Is the liner material safe for drinking water systems?
Epoxy liners used for sewer and stormwater are not the same specification as liners used for potable (drinking) water pipes. Drinking water drain relining requires a specific food-grade or potable-water-approved epoxy formulation. This distinction matters for property owners with private water supply infrastructure.
Does the liner degrade over time?
The cured epoxy matrix is chemically inert in normal sewer and stormwater environments. It does not soften, corrode, oxidise or support biological growth. The 50-year structural warranty is supported by accelerated ageing tests and decades of in-service performance data from installations in Europe and North America.
What happens to the liner when roots eventually re-enter?
Roots cannot enter through the liner material itself, it is smooth, continuous and has no joints. The only vulnerability points are the liner ends and the lateral reinstatements. If root ingress occurs after relining, it is almost always at these transition points. Correct installation of end seals and robotic lateral reinstatement eliminates this risk.