Getting the slope of a drain pipe right is one of the most important — and most misunderstood — parts of any drainage job. Too little fall and solids settle out, causing blockages; too much fall and the water races ahead of the solids, leaving them stranded. The good news is that New Zealand has clear standards to guide you.
Why Slope (Gradient) Matters
Drain pipes rely on gravity to carry a mix of water and solid waste. The flow needs to be fast enough to keep solids suspended and moving — what engineers call self-cleansing velocity — but not so fast that the liquid separates from the solids. Getting this balance right is the whole point of specifying a correct gradient.
In New Zealand, residential drainage is governed by NZS 3604 (for timber-framed buildings) and, more directly for drainage, by NZS 4404 and the NZ Building Code clauses G13 (Foul Water) and E1 (Surface Water). Your local council may also have its own drainage bylaws, so always confirm requirements with them before you start.
The Standard Gradient Rules
The most widely used rule of thumb in NZ residential drainage is:
| Pipe Diameter | Minimum Gradient | Maximum Gradient |
|---|---|---|
| 100 mm (standard sewer) | 1:60 (approx. 17 mm per metre) | 1:20 |
| 150 mm | 1:150 (approx. 7 mm per metre) | 1:40 |
| 75 mm (branch drains) | 1:40 (approx. 25 mm per metre) | 1:20 |
Note: These are general guides based on common NZ practice. Always verify against the specific NZS standard and your council's requirements, as conditions vary.
The most common residential drain is 100 mm diameter, and the go-to minimum gradient is 1:60 — that's a fall of roughly 17 mm for every metre of pipe run. A gradient of 1:40 (25 mm per metre) is often considered ideal for self-cleansing in a 100 mm pipe.
How to Calculate and Set the Fall
Step 1 – Measure the pipe run
Measure the total horizontal length of the drain from start (e.g., the toilet or gully trap) to the point it connects to the main drain or public sewer.
Step 2 – Calculate the required fall
Multiply the pipe length (in metres) by the gradient fraction:
- For a 1:60 gradient over 6 m: 6 ÷ 60 = 0.1 m (100 mm) of fall
- For a 1:40 gradient over 6 m: 6 ÷ 40 = 0.15 m (150 mm) of fall
Step 3 – Set out with a level and string line
Use a builder's level or a laser level to establish a reference line, then set your string line at the calculated fall. A long spirit level with a gradient vial (sometimes called a drain level or slope level) is handy for checking as you lay each pipe section.
Step 4 – Bed and haunch the pipe correctly
Lay pipes on a compacted granular bedding (typically 100 mm of pea gravel or crusher dust) to maintain the gradient. Haunching — packing material up the sides of the pipe — prevents the pipe from shifting after backfill.
Common Mistakes to Avoid
- Too flat: A gradient less than 1:100 in a 100 mm pipe is almost certain to block over time.
- Too steep: Gradients steeper than 1:20 can cause the liquid to outrun the solids.
- Bellying: Low spots in the pipe run trap solids and water — always check with a level as you go.
- Incorrect pipe joints: Poorly made joints can allow root ingress or joint displacement, which changes your gradient over time.
- Ignoring inspection access: NZ Building Code clause G13 requires inspection openings at specified intervals. Don't skip these.
Drainage and Building Consents
Most new drainage work — including laying new drain pipes, altering existing drains, or connecting to the public sewer — requires a building consent in New Zealand. Some minor work may fall under Schedule 1 exemptions, but this is a grey area. Always check with your local council before starting. You can find guidance on consent requirements at MBIE Building Performance.
When to Call a Professional
Drainlaying is restricted work in New Zealand. Under the Plumbers, Gasfitters, and Drainlayers Act 2006, connecting to or working on public or private drains generally must be carried out by — or under the supervision of — a licensed drainlayer. This is not just a recommendation; doing restricted drainlaying work without a licence is illegal and can result in significant fines.
Check that anyone you hire holds a current licence with the Plumbers Gasfitters & Drainlayers Board. You can find qualified, licensed drainlayers in your area right here on tradedirectory.co.nz.
Homeowners may carry out minor maintenance (such as clearing a blocked drain with a plunger or drain snake) on their own property, but laying, altering, or connecting drain pipes is best left to a professional — both for legal compliance and to ensure the work passes inspection.
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