Gravity drains work by fall: the pipe drops steadily from start to finish, so water and solids flow downhill. Too little fall and solids settle and block the pipe. Too much and water can run ahead and leave the solids behind, especially on long runs. Getting the gradient right, and keeping it consistent, is the heart of drainage work.
Ways of stating a gradient
- As a ratio: 1:40 means a drop of 1 unit for every 40 units of length. It's the standard way of stating gradients in Australia.
- As a drop per metre: 25 mm per metre is the same as 1:40.
- As a percentage: drop ÷ length × 100. 1:40 is 2.5%.
Typical gradients
| Pipe | Typical fall |
|---|---|
| DN65 and smaller | 1:40 minimum |
| DN80 and DN100 drains | 1:60 minimum |
| DN150 drains | 1:100 minimum |
These are the AS/NZS 3500.2 minimum grades for sanitary drains. Confirm against the standard and your state's plumbing rules: sanitary drainage must be laid by a licensed plumber or drainer.
Setting out
- Fix the outlet level, where the pipe ends (the point of connection to the sewer, shown on the water authority's sewer diagram).
- Fix the inlet level, where it starts. The pipe must be deep enough there to pick up the fittings it serves.
- Total fall = run length ÷ gradient ratio. Check the fall you have between the two levels is at least that much.
- If it isn't, you need a pump, a different route, or a different connection point.
- Set the pipe bed at the right fall with a level and spacer block, a laser, or sight rails and a traveller on long runs.
A worked example
A 12 m run of DN100 drain at the 1:60 minimum needs 200 mm of fall. If the house end invert is 450 mm deep, the outlet end must be at least 650 mm deep. Laid at 1:40 instead, it would need 300 mm.
Bedding and backfill
Pipes need even support along their whole length, or they sag and hold water. Lay them on a bed of granular material, surround them as the pipe maker specifies, and backfill in layers. Protect shallow pipes under driveways from traffic loads. Keep rock and large stones away from the pipe.
Access and inspection
Drains need access for clearing blockages: inspection openings at junctions, changes of direction, the base of stacks and near the point of connection, brought up to the surface where required. Plan them in when you set out the run, not afterwards.
Testing
New drains are normally tested before they're covered, usually with an air or water test, and inspected where your state's plumbing regulator requires it, with the work certified by the licensed plumber. Price the testing time, and don't backfill until the test is passed.
Common mistakes
- Not checking the outlet level before digging
- Inconsistent fall: dips and humps along the run
- Too steep a fall on long runs
- Poor bedding, leading to sagging joints
- Missing access points
The drain fall calculator gives the total drop and the fall per metre for any run, and the slope calculator converts between ratios, percentages and degrees.
Surface water and land drainage
Falls matter above ground too. Patios and paths should fall away from buildings, and the NCC requires the ground next to a house to fall at least 50 mm over the first metre (25 mm for paving in low-rainfall areas), then about 1:60 to 1:100 beyond, and driveways need a fall toward a channel or permeable edge. Ag drains in backyards are usually laid at a gentle fall, about 1:100 to 1:200, in a gravel-filled trench to a stormwater pit or the legal point of discharge. The same setting-out method applies: fix the outlet level first, then work back up the run.
Common questions
Can a drain be too steep?
On long runs, very steep falls can let water run ahead of solids, which then settle. If the ground falls steeply, drop the pipe down in steps with a backdrop or ramp at a chamber, rather than laying it at a steep angle all the way.
What if there isn't enough fall?
Look for a lower connection point, re-route the pipe, or use a pumping station. Laying a drain flatter than the minimum invites blockages.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.