There is no single compliant fall for every sanitary drain.
The correct grade depends on pipe size, wastewater load, layout, connection levels, support and the adopted design pathway. A licensed designer or drainer must confirm it for the section.
What does pipe fall mean?
Fall is the vertical change over a horizontal pipe run. It may be written as a percentage, a ratio or a drop per metre.
A 1% grade means one unit of fall over 100 horizontal units. That equals 10 millimetres per horizontal metre, but it is only an arithmetic conversion.
Do not treat that example as the required grade for every drain. The compliant value comes from the approved design and the requirements applying to the actual pipe.
How do you calculate total fall?
Once the design grade is known, multiply the horizontal run by the percentage written as a decimal. Keep the measurement units consistent.
| Stated grade | Horizontal run | Arithmetic fall |
|---|---|---|
| 1% | 1 metre | 10 millimetres |
| 1% | 3 metres | 30 millimetres |
| 1% | 6 metres | 60 millimetres |
For example, six metres multiplied by 0.01 gives 0.06 metres, or 60 millimetres. Measure the horizontal run, not the longer distance along the sloping pipe.
For the related general plumbing issue, see Does a drainage pipe need gravel underneath?
Why is there no universal sewer grade?
Householders often call several different pipes a sewer pipe. A private sanitary drain, utility sewer main, stormwater line, pumped discharge and on-site wastewater pipe follow different design controls.
The pipe diameter and expected wastewater flow also affect the design. Direction changes, branches, inspection points, ground movement and the available connection depth can alter the layout.
Business Queensland identifies NCC Volume Three and Queensland-specific laws and codes as part of the state's plumbing framework. Detailed sanitary-drain design refers to AS/NZS 3500.2.
What can go wrong with the fall?
- Too little fall: wastewater can move poorly and leave material within the line.
- Reverse fall: part of the pipe rises against the intended direction of flow and holds water.
- A local sag: unsupported or moved pipe creates a low point despite correct endpoint levels.
- An unsuitable steep layout: the design can create hydraulic, connection or support problems.
A single blockage does not prove the grade is wrong. Obstructions, roots, damage, poor joints and fixture or venting faults can produce similar symptoms.
The NCC sanitary-drainage requirements address safe sewage transfer, blockage, uncontrolled discharge, contamination and system damage. Grade is one part of that wider performance.
How is grade checked during installation?
- Confirm the design: identify the pipe section, nominal diameter, wastewater load, grade and connection levels.
- Set out the endpoints: establish the intended invert levels, which are the levels at the pipe's internal base.
- Check intermediate points: verify the run does not contain a sag, hump or local reverse fall.
- Inspect support and alignment: confirm bedding, joints and side support will preserve the intended line and grade.
- Test before cover: complete the applicable drainage test, inspection and records before concealment or use.
Correct endpoint arithmetic cannot reveal every low point. Learn how plumbers identify pipe size before applying the design to a section.
Can CCTV measure drain slope?
A camera can show accessible standing water, joints, deformation, roots and obstructions. It does not replace surveyed levels or prove continuous grade by itself.
Water can remain because of a sag, obstruction, deformation or access condition. The licensed assessor must compare the internal view with levels, pipe location and other evidence.
For an existing problem, read when household drain inspection is useful. Do not diagnose reverse fall from one camera frame.
Who can alter sanitary drainage in Queensland?
QBCC says regulated drainage work requires an appropriately licensed plumber or drainer. The permit, notifiable or minor-work pathway depends on the project.
Do not excavate, pack, cut, relay or reconnect a sanitary drain as a household test. Buried services, unstable trenches and sewage exposure add separate hazards.
When can the pipe be covered?
New work should remain visible until the required test and inspection path is complete. The exact process depends on its Queensland work category and local authority.
Business Queensland says permit work is generally inspected before cover. Brisbane also requires permit plumbing to be visible and under test for the booked Council inspection.
Grade checking and pressure testing answer different questions. Review how newly laid drainage is tested before the responsible practitioner clears the cover stage.
What information should you ask for?
- the pipe's function, nominal diameter and design grade
- the start, finish and connection invert levels
- the approved plan and applicable work category
- intermediate level checks and any recorded low points
- testing, inspection and as-constructed records.
If the designed fall cannot be achieved within the available depth, stop before changing the grade. Ask the responsible designer or appropriately licensed drainer for help resolving the constraint with the relevant authority.