Drain relining is not just about repairing structural defects, it also needs to preserve adequate drainage flow. A pipe’s gradient (the slope it runs at) is critical: too shallow and solids accumulate, causing blockages. Too steep and the liquid runs away from the solids, also causing blockages. NSW plumbing standards specify minimum gradients for drain pipes, and these apply equally to relined pipes.
The quick answer
NSW plumbing standards (AS/NZS 3500.2) require a minimum gradient of 1:60 (1 unit fall per 60 units of length) for a 100 mm drain. Larger diameters have flatter minimum gradients. A correctly installed liner maintains the existing pipe gradient, it doesn’t change it. If the existing pipe fails the gradient requirement, relining alone can’t fix it; the pipe needs re-grading, which typically requires excavation.
Why gradient matters in drain pipes
A sewer or drain pipe needs to flow fast enough to transport solids (toilet paper, food solids, biological waste) but not so fast that liquid outpaces solids. The “self-cleansing velocity”, the minimum flow speed to keep solids in suspension, is approximately 0.7-0.9 m/s for typical residential pipes.
At insufficient gradient:
- Flow velocity drops below self-cleansing velocity
- Solids settle and accumulate in the pipe
- Grease and fat deposits build up faster
- Blockages become more frequent
At excessive gradient:
- Flow velocity is very high
- Liquid can outrun solids, leaving dry deposits
- Pipe erosion can be a concern at pipe fittings and bends
NSW minimum gradient standards
Under AS/NZS 3500.2 (Plumbing and Drainage, Sanitary plumbing and drainage), the minimum recommended gradients are:
| Pipe diameter (internal) | Minimum gradient | Ratio |
|---|---|---|
| 40 mm (fixture waste) | 1 in 25 | 4.0% |
| 50 mm (fixture waste) | 1 in 35 | 2.9% |
| 80 mm | 1 in 50 | 2.0% |
| 100 mm (most residential sewer mains) | 1 in 60 | 1.7% |
| 150 mm (larger residential / small commercial) | 1 in 100 | 1.0% |
| 225 mm | 1 in 150 | 0.7% |
These are minimum gradients. In practice, most residential installations aim for 1:40 (2.5%) on 100 mm pipe, and steeper is generally better up to about 1:20 (5%) for standard conditions.
How pipe gradient is assessed
During a CCTV inspection, modern camera systems can measure the pipe gradient directly using an inclinometer built into the camera head. This records the pitch of the camera (which follows the pipe) and calculates the fall per metre.
The CCTV report will flag sections where gradient is below minimum standard. This is important because:
- Below-minimum gradient sections are often where blockages concentrate
- A liner installed over a below-minimum gradient section will still have the same gradient problem
Does drain relining change the gradient?
No, drain relining installs a liner inside the existing pipe. The liner conforms to the existing pipe path and gradient. The internal bore decreases very slightly (typically 8-12 mm wall thickness each side on a 100 mm pipe), but the slope of the pipe doesn’t change.
This is both the key benefit and the limitation of relining:
- Benefit: A pipe at good gradient, restored to good structural condition, performs as well as new
- Limitation: A pipe at inadequate gradient, after relining, is still at inadequate gradient
When gradient problems rule out relining
Relining is not the solution when:
- A pipe has settled and lost its fall (pipe sections have moved downward unevenly, creating a belly or reverse fall)
- A pipe was originally installed at insufficient gradient
- Significant length of pipe runs at flat or reverse gradient
In these cases, the pipe needs to be excavated, re-laid at correct gradient, and then (optionally) relined for long-term structural integrity.
Belly pipes: a specific gradient problem
A “belly” in a pipe is a low point where the pipe has settled, creating a sag. Waste flowing down the pipe collects in the belly rather than continuing to flow. Belly pipes are identified on CCTV by pooling water at the sag point and the camera inclining downward then back up as it passes through.
Belly pipes are common on the Central Coast because:
- Sandy coastal soils allow pipe sections to settle unevenly
- Coastal fill soils near Gosford waterfront and the Woy Woy peninsula are particularly susceptible
- Old AC cement and terracotta pipes that have been in the ground for 50+ years have had ample time to settle
A belly pipe cannot be relined to correct the belly. Relining a belly pipe seals the structural defects but leaves the bellied section, which will continue to collect solids. The correct fix for a belly pipe is excavation and re-grade.
Gradient assessment in CCTV reports
When you receive a CCTV report, look for:
- Gradient measurements recorded per section
- Any “negative gradient” or “reverse fall” annotations
- “Pooling” or “standing water” notations at specific chainage points
These annotations indicate gradient problems that may affect the relining decision. A good CCTV report separates structural defects (cracking, root intrusion, joint failure) from hydraulic defects (insufficient gradient, belly, overflow conditions).
Post-relining gradient verification
After relining, a final CCTV should confirm:
- The liner is smooth and adhered
- No wrinkles or folds have created a synthetic belly
- The pipe gradient hasn’t been compromised during installation
On correctly installed liners, post-install gradient should match pre-install gradient within measurement tolerance.
Central Coast specific considerations
On the Central Coast, the following conditions affect gradient:
| Condition | Location | Gradient risk |
|---|---|---|
| Sandy fill soils | Umina, Woy Woy, foreshore areas | High, pipe settlement creates bellies |
| Flat coastal topography | Peninsula suburbs, lake-side areas | Low margin for error, original grades critical |
| Steep hillside blocks | East Gosford, Terrigal hillside | Usually good gradient, steep is typically fine |
| Older housing on clay soils | Wyong township area | Moderate, clay movement can shift pipe grade |
Frequently asked questions
If my CCTV report shows a belly, can it be fixed without digging up the whole pipe? In most cases no, a belly requires the pipe section to be excavated and re-laid at correct grade. However, if the belly is short (under 1 m) and relatively minor, some contractors can sometimes manage the belly during the liner install by careful inflation technique. This is not standard practice and should only be discussed with an experienced contractor who has reviewed the specific CCTV data.
My plumber says my pipe passes gradient requirements. Can I still have flow problems? Yes, a pipe can be at adequate gradient but still have flow problems due to roughness (scale, root debris), partial blockage, or very high flow load. Gradient is necessary but not sufficient for good performance.
Does a relined pipe need re-certification after installation? The installed liner should comply with the relevant standards (AS/NZS 2566 for CIPP). For complex jobs or those requiring a building compliance certificate, the contractor may need to provide certification documentation. Ask your contractor what documentation they provide post-installation.
How steep is too steep for a residential sewer pipe? There’s no formal “too steep” for residential drain pipes, but very steep grades (1:15 or steeper) can create hydraulic issues and erosion at pipe fittings over time. Grades steeper than 1:5 may need special consideration. For most suburban Central Coast properties, this isn’t an issue.
Can CCTV measure gradient accurately in a very short pipe run? Short pipe runs (under 3 m) give less reliable gradient measurements from inclinometer CCTV because the angular change is very small. For short runs, a plumber may use a spirit level or laser level for better accuracy.
Unsure whether your drain passes gradient requirements? Book a CCTV inspection with gradient assessment.