There is no single fall or slope that suits every sewer pipe. The required grade depends on the pipe diameter, drainage load, layout, approved design and applicable plumbing requirements. A designer or licensed plumber must confirm the grade for the actual installation.
A 1% drainage slope is a useful calculation example. It means a vertical drop of one unit for every 100 units of horizontal distance, which is the same as a 1 in 100 gradient or 10 mm fall per metre. It is not automatically the compliant grade for a particular pipe.
How to calculate a 1% fall
Multiply the horizontal run by 0.01. When the run is measured in metres and the fall is needed in millimetres, multiply the run by 10.
| Horizontal run | Fall at 1% |
|---|---|
| 1 m | 10 mm |
| 3 m | 30 mm |
| 5 m | 50 mm |
| 6 m | 60 mm |
| 10 m | 100 mm |
For example, a 6 m horizontal run at 1% has a total fall of 60 mm in the intended direction of flow. Measure the horizontal distance between endpoints, not the slightly longer distance along the sloping pipe.
If the approved design specifies a steeper grade, check how a 2% slope compares rather than assuming the 1% example applies.
How is the correct sewer grade confirmed?
The required grade should be taken from the approved design and checked against the pipe size, fixtures served, direction changes, access points and current plumbing requirements. Existing ground levels, connection depth and obstructions can affect what is practical, but they do not justify a non-compliant installation.
Grade is only one part of a sound drain. Alignment, support, joints and post-installation drainage testing also matter before pipework is concealed or put into service.
Do not excavate or alter sewer or stormwater pipework yourself. Underground utilities, unstable excavations and sewage exposure create serious risks, and regulated drainage work must be completed by an appropriately licensed person.
Under AS/NZS 3500.2 (the national sanitary plumbing and drainage standard, which establishes mandatory minimum falls—such as 1:60 for 65 mm pipes and 1:100 for 100 mm drains—so solids and liquids flow together without depositing waste or causing chronic blockages), pipe gradients must be verified precisely before trenches are backfilled. Contact our licensed Sydney plumbing team to verify the specified grade and assess your private drainage system before work proceeds. Fixed Today is the reliable family-owned plumbing service Sydney residents have counted on across decades for fully compliant drainage solutions. Informing our dispatch team whether you have an older earthenware line, sandstone foundation constraints, or suspected tree root intrusions helps our technicians bring the right high-precision digital levels and CCTV diagnostic equipment to your property.