One of the most common questions from Central Coast homeowners and property managers is whether their specific pipe can actually be relined. Pipe size is a fundamental constraint, not every diameter can be addressed with the same technique, and access requirements vary with pipe size. This guide explains the practicalities.
Quick answer (BLUF)
Most residential sewer and stormwater pipes on the Central Coast can be relined. The practical lower limit for inversion-liner relining is around 75-100 mm diameter. Smaller pipes (50-65 mm, common in older kitchen and bathroom branch drains) are typically addressed with point repair products. At the upper end, relining equipment is available for pipes up to 900 mm and beyond. Access requirements, not just diameter, are the other constraint.
Common pipe diameters on Central Coast properties
To understand the relining options available to you, it helps to know what pipe diameters are typical in different parts of a residential drainage system:
| Location | Typical diameter | Pipe type (older homes) |
|---|---|---|
| Kitchen/laundry branch drain | 40-65 mm | PVC or cast iron |
| Bathroom waste | 40-65 mm | PVC or cast iron |
| Toilet connection | 100 mm | Terracotta or PVC |
| Main house sewer drain | 100 mm | Terracotta |
| Stormwater downpipe connection | 90-100 mm | AC cement or PVC |
| Main stormwater line | 100-150 mm | AC cement or terracotta |
| Sewer main to boundary trap | 100 mm | Terracotta |
| Boundary trap to public main | 100-150 mm | Terracotta |
In older Gosford, Wyong and Woy Woy homes, the 100 mm terracotta main sewer drain is the most commonly relined pipe, it is the right size for inversion-liner relining and the most likely to have root infiltration and joint failures.
Minimum pipe size: what can and cannot be relined
Below 75 mm diameter: Standard inversion liner relining is not practical below about 75 mm. The liner material and the inversion/pull-in-place installation equipment cannot effectively navigate these smaller-diameter pipes. For defects in 40-65 mm branch drains (kitchen wastes, bathroom connections), the options are:
- Point repair using epoxy fill or short patch: A resin is injected and cured in place. This works for specific defects but not for full-length re-lining.
- Pipe replacement: For small-diameter pipe defects, selective excavation and replacement is often more practical and cost-effective than any form of relining.
- Nu-Flow or similar coating systems: A specialist liner system using blown-in epoxy coating can rehabilitate pipes as small as 12 mm diameter. These are used more commonly in commercial building internal plumbing than in residential drain systems.
75-100 mm diameter: This is the sweet spot for residential relining on the Central Coast. Most relining contractors carry liners in this range as standard stock. Both inversion and pull-in-place installation methods work well.
100-150 mm diameter: Standard for both sewer and stormwater relining. Most well-equipped contractors work comfortably in this range.
150-300 mm diameter: Larger residential and commercial stormwater pipes, industrial drains and some older public infrastructure mains. Relining is fully feasible but requires more specialist equipment and larger liner stock. Not all Central Coast contractors work in this range.
300-900 mm diameter: Infrastructure-scale relining, typically for council stormwater mains or commercial sewer mains. This is specialist contractor territory with dedicated large-diameter liner systems.
Access requirements by pipe size
Pipe size and access requirements are closely linked. The liner has to reach the defect somehow, and the equipment has to fit through the access point.
Standard inspection opening (150 mm shaft): Allows CCTV camera access and small-diameter liner installation for 75-100 mm pipes. Most residential properties have a 150 mm inspection shaft somewhere along the drain run.
150 mm to 225 mm access for 100-150 mm pipes: If the only access point is a 100 mm cleanout, a 100 mm liner can be installed through it with the right equipment, but the process is more limited than working through a proper inspection shaft.
No inspection shaft present: In some older pre-1950s properties and in some rural areas of the Central Coast, there is no accessible inspection opening at all. In this case, the crew must establish access before relining can begin, typically by exposing the pipe at a strategic point and installing a new inspection opening. This adds $400, $1,200 to the project cost but is a one-time expense.
Vertical stacks (multi-storey buildings): Vertical soil stacks require access from either roof level (through a stack vent) or basement level (through a floor-level cleanout). The liner inversion can be done vertically, gravity assists in some inversion methods. Access through the correct floor level is important to ensure the liner reaches the full vertical extent of the defect.
When a pipe is too small for relining: alternatives
If your problem pipe is too small for conventional relining, discuss these alternatives with your contractor:
- Full replacement with a larger-diameter modern PVC pipe (sometimes sensible if the smaller drain is an older undersized installation anyway)
- Spray lining / coating: An epoxy resin is sprayed or blown through the pipe, coating the interior wall. This is used for small-diameter pipes and requires specialist equipment
- Pipe bursting: An expanding head is pulled through the pipe, shattering it outward and pulling in a new pipe behind it. This is available for some pipe sizes and avoids full-trench excavation
FAQs
What diameter is the typical house sewer drain in Gosford or Wyong?
The standard residential sewer drain main (the run from the house to the public main) has been 100 mm diameter since the mid-20th century. Older pre-war properties occasionally have 75 mm or even 65 mm mains, but 100 mm is the standard for post-1950 residential construction. This diameter is perfectly suited to standard inversion-liner relining.
Can a 150 mm stormwater pipe be relined without excavation?
Yes, 150 mm is a standard diameter for residential stormwater relining. Access through an existing inspection opening or stormwater pit is usually sufficient. If the pit is at the wrong end of the run for access, a small excavation to establish a new access point may be needed.
Does relining reduce the pipe’s internal diameter?
Slightly, yes. A standard liner adds approximately 6-10 mm of wall thickness to the pipe interior (3-5 mm per side). For a 100 mm pipe, this leaves an internal bore of 90-94 mm. Flow hydraulic calculations show that the smoother internal surface of the cured liner compensates for the slight bore reduction, flow capacity is typically equivalent to or better than the original pipe at the same gradient.
I have been told my pipe is too small to reline. What should I do?
Get a second opinion, and ask the contractor specifically what diameter the pipe is and what the minimum diameter is for their equipment. If the pipe genuinely is below 75 mm, relining may not be the best option, but confirm the measurement. Some contractors use “too small to reline” as a shortcut to recommending full replacement, which is more profitable.