Checking your drain pipe has the correct fall before backfilling is one of the most important steps in any drainage job — get it wrong and you risk blocked drains, sewage backups, and costly dig-ups later. In New Zealand, drain gradients are governed by the NZ Building Code (specifically Clause G13 – Foul Water) and must meet minimum standards before any inspection or backfilling takes place. Here's how to check it properly.
Why Correct Fall Matters
Drain pipes rely on gravity to carry waste away efficiently. Too little fall and solids settle, causing blockages. Too much fall and the water races ahead of the solids, leaving them behind — also causing blockages. The sweet spot is a consistent, measured gradient along the full length of the pipe.
Under MBIE Building Performance guidelines and NZ Building Code Clause G13, the generally accepted minimum fall for a 100 mm diameter drain is 1:60 (roughly 17 mm drop per metre). For 150 mm pipes, the minimum is typically 1:100. Always confirm the specific requirements with your local council and the licensed drainlayer overseeing the work, as site conditions and pipe material can affect what's acceptable.
Tools You'll Need
- Builder's level (spirit level) or laser level
- Tape measure or folding rule
- String line and line level (for longer runs)
- Straight edge (for shorter sections)
- Calculator (or a gradient chart)
- Pegs and stakes for marking
A laser level makes this job significantly easier and more accurate, especially over longer runs. You can hire one from most tool hire shops if you don't own one.
How to Calculate the Required Drop
Before you start measuring, work out how much drop you need over your pipe run.
Formula: Drop (mm) = Length of pipe run (m) ÷ Gradient ratio
For example, for a 1:60 gradient over a 6-metre run:
- 6 ÷ 60 = 0.1 m (100 mm) total drop
Keep this target figure handy as your benchmark.
Step-by-Step: Checking the Fall
1. Set a Reference Datum
Drive a peg at the high end of the pipe run (usually near the building or gully trap) and set it to a known height — this is your datum point. Mark the top of the pipe at this end.
2. Run a String Line
Stretch a string line from your datum peg to a peg at the low end of the run (near the inspection chamber or connection point). Use a line level or laser level to get the string perfectly horizontal.
3. Measure the Drop at the Low End
At the low end, measure the vertical distance from the string line down to the top of the pipe. This is your actual drop. Compare it to your calculated required drop.
- If the actual drop equals or slightly exceeds your required drop — you're on track.
- If the drop is less than required — the pipe is too flat and needs re-laying.
- If the drop is far greater than required — the fall may be too steep; check against maximum gradient limits (typically 1:10 for 100 mm pipes, but confirm with your drainlayer).
4. Check Intermediate Points
Don't just check the ends. Measure the drop at several points along the run to confirm the fall is consistent — there should be no dips, humps, or flat spots. Dips are particularly problematic as they trap solids and water.
5. Use a Spirit Level for Short Sections
For runs under 2–3 metres, a long spirit level (1.2 m or longer) laid directly on the pipe can give a quick check. Use a packing shim of the correct thickness under the uphill end to represent the required gradient, then check the bubble is level.
Pipe Bedding and Support
Correct fall is only maintained if the pipe is properly bedded. Pipes must be laid on compacted granular bedding (typically 10 mm pea gravel or approved bedding material) to the depth specified in your consent documents. Poorly bedded pipes can shift after backfilling, destroying the gradient you've carefully set. Check BRANZ guidance and your consent conditions for bedding specifications.
Getting a Pre-Backfill Inspection
In most cases, drainage work requires a building consent and a pre-backfill inspection by your local council building inspector before you cover the pipes. Do not backfill until this inspection has been passed and signed off. Contact your council early to book the inspection — inspectors are often busy and you may need to give several days' notice.
Some minor drainage work may fall under Schedule 1 exemptions of the Building Act 2004, but always confirm with your council before assuming no consent is needed.
When to Call a Professional
All drainlaying work that connects to the public sewer or stormwater system is restricted work under the Plumbers, Gasfitters, and Drainlayers Act 2006. It must be carried out by — or under the supervision of — a licensed drainlayer. Even if you do preparatory work yourself, the connection and sign-off must involve a licensed professional registered with the Plumbers Gasfitters & Drainlayers Board.
If you're unsure about any aspect of the gradient check, pipe bedding, or inspection process, it's well worth getting a licensed drainlayer involved early. You can find qualified local drainlayers on tradedirectory.co.nz.
Safety note: Open trenches are a serious hazard. Never enter an unsupported trench that poses a risk of collapse, and follow, and follow WorkSafe NZ guidelines for trench safety at all times.
Certifying Drainlayer
