If a drain is slow, or a section has never quite performed the way it should, one of the questions worth answering before any repair is booked is whether the pipe actually has enough fall. A pipe with correct gradient but a structural defect is a relining job. A pipe that’s lost its fall, or was never laid with enough of it, is a different problem entirely, and relining alone won’t fix it. The good news is that gradient can be measured accurately without digging a single hole, using the same access points already available for a standard CCTV inspection.
Why fall testing matters before you commit to a repair
Every gravity drain relies on a minimum slope to keep waste and stormwater moving at a “self-cleansing” velocity, fast enough that solids don’t settle out along the way. NSW plumbing standards (AS/NZS 3500.2) specify these minimums, and they get flatter as pipe diameter increases, a 100mm residential sewer line needs roughly 1:60, while a 225mm line can run at a flatter 1:150. A pipe below its minimum gradient collects debris at the low points regardless of how clean and structurally sound the rest of the run is. Jetting will clear it temporarily, but the accumulation returns, sometimes within weeks, because the underlying hydraulics haven’t changed.
This is why gradient testing sits alongside, rather than instead of, a standard defect inspection. A pipe can pass every structural check, no cracks, no root intrusion, sound joints, and still block repeatedly because a section of it runs flat or has developed a reverse fall. Conversely, a pipe with real structural defects but adequate gradient throughout is usually a straightforward relining candidate. Knowing which situation applies changes the recommendation completely, and it’s not something you can tell from a fixture backing up or a wet patch on the lawn.
How gradient is measured without excavation
Inclinometer CCTV
The most common and most practical method for residential and light commercial work is a CCTV camera fitted with a built-in inclinometer. As the camera travels through the pipe, following its actual path rather than a straight line, the inclinometer records the pitch of the camera head continuously. This data is converted into a fall reading, expressed as a ratio or percentage, for each section of the run. Because this happens as part of a standard inspection, there’s no separate mobilisation required: the same camera pass that identifies root intrusion, cracking or joint displacement also produces a section-by-section gradient profile.
The output is most reliable over longer runs. On a pipe of 5 metres or more, the cumulative rise and fall gives a solid average gradient reading. On very short runs, under 3 metres, the angular change the inclinometer is measuring becomes small enough that reading confidence drops, and a surface-level method is a better cross-check.
Sonde-based depth comparison
Where a camera includes a sonde transmitter, the inclinometer data can be paired with an independent depth check taken from above ground. By locating the sonde’s position at two or more points along the pipe run and recording the depth reading at each, we get a second, ground-truthed measurement of the fall between those points that isn’t dependent on the camera’s continuous pitch data alone. This is particularly useful for confirming a suspected belly (a low, sagged section) or a reverse-fall section, since the depth readings at each end of the suspect section will show whether the pipe is actually deeper in the middle than at either end. Our drain sonde locating guide covers how this equipment works in more general terms.
Surface-level and access-point methods
For very short runs, or where an inclinometer reading needs independent confirmation, a laser level or dumpy level set up at an access point (a cleanout or inspection opening) can measure the invert height at two points directly, giving a straightforward fall calculation over that distance. This traditional method is precise but limited to points where you already have access; it can’t tell you what’s happening at every metre along a long run the way a continuous inclinometer pass can.
Water/flow observation
During a CCTV pass, standing water at a low point is a direct visual indicator of insufficient or reversed gradient, water that should be flowing continues to pool at the same spot regardless of how recently the pipe was jetted. Combined with the inclinometer reading at that section, this confirms whether the pooling is a genuine gradient problem or simply residual debris that jetting hasn’t fully cleared yet.
What a gradient test result actually tells you
A gradient report, whether generated from a standalone assessment or as part of a broader CCTV inspection, typically flags results in one of three categories:
- Adequate gradient throughout. The pipe meets or exceeds the minimum fall for its diameter across the full inspected length. Any structural defects found are candidates for relining or point repair without a hydraulic complication.
- Marginal gradient in isolated short sections. Common on Central Coast properties with sandy or reactive clay soils where minor settlement has occurred. Depending on length and severity, these sections may still be relinable, since a liner doesn’t correct gradient but also doesn’t necessarily prevent adequate performance if the shortfall is small.
- Significant belly or reverse fall. A section of pipe that has genuinely lost its slope, sagging low enough that water pools rather than continuing downhill. This can’t be fixed by relining. The section needs to be excavated and re-laid at the correct fall before any lining (if still wanted for the rest of the run) makes sense.
Our guide to relining slope and gradient requirements covers the compliance standards and what happens once a gradient problem is confirmed in more detail; this page is specifically about how that gradient gets measured in the first place.
Why this matters for the blocked-drain-vs-broken-pipe question
Gradient testing is one of the tools that helps answer a question we cover in our guide to telling a blocked drain from a broken pipe: is a recurring blockage a structural issue (roots, cracks, open joints) or a hydraulic one (insufficient fall)? The two aren’t mutually exclusive; a pipe can have both a marginal gradient section and separate root intrusion elsewhere in the same run. Distinguishing between them, and locating exactly where each problem sits, is what a combined CCTV and gradient assessment is for.
What it costs
Gradient testing via inclinometer CCTV is generally included as part of a standard residential inspection where the camera equipment has the built-in capability, so there’s usually no separate line item for it. Where a dedicated fall assessment is the specific purpose of the visit, for example confirming suspected belly sections ahead of a relining decision, this is typically quoted within the same range as a standard CCTV inspection.
| Service | Approximate Cost |
|---|---|
| Standard CCTV inspection with inclinometer gradient reading | $250–$450 |
| Sonde depth cross-check (included with sonde-equipped inspections) | Included |
| Dedicated gradient/belly assessment | $250–$500 |
| Excavation and re-lay for confirmed belly section | Quote required, depends on depth and access |
Frequently asked questions
How do you know if a low reading is a genuine belly or just measurement noise? We look for consistency, does the camera show the pipe angling clearly downward, reaching a low point, and then angling back up, with standing water or silt visible at the low point? A single marginal reading over a very short section without visual confirmation of pooling is treated cautiously and, where useful, cross-checked with a sonde depth reading or surface-level measurement.
Can gradient be measured accurately in an older terracotta pipe with rough internal walls? Yes. The inclinometer measures the pitch of the camera head as it travels through the pipe, which is independent of the internal wall texture. Rough walls can affect flow velocity and self-cleansing performance, but they don’t affect the accuracy of a gradient reading.
If our pipe has a minor gradient shortfall, does that rule out relining altogether? Not necessarily. A minor, short shortfall may still perform acceptably after relining, particularly if the structural defects being addressed (root intrusion, cracking) were the primary blockage driver. A genuine belly or reverse-fall section is the case that specifically rules out relining for that section.
Is gradient testing something we need to specifically ask for, or is it automatic? If the inspection is being carried out with sonde/inclinometer-equipped equipment, gradient data is typically captured as standard. It’s worth confirming with your contractor what their reporting includes, since not every basic CCTV pass records or reports fall data explicitly.
Does a short pipe run (like a single fixture branch) need gradient testing? Less commonly. Short branch runs under 3 metres are harder to measure accurately with an inclinometer and are less prone to belly formation given their length. Gradient testing is most relevant for main sewer or stormwater runs of meaningful length.