Getting the fall (or gradient) right on a waste pipe is one of the most important — and most misunderstood — parts of any plumbing job. Too little fall and waste sits in the pipe, causing blockages and foul smells. Too much fall and the water races ahead, leaving solids behind. Here's what every NZ homeowner should understand about waste pipe gradients.
What Is "Fall" in Plumbing?
Fall refers to the slope of a pipe — how much it drops vertically over a given horizontal distance. It's usually expressed as a ratio (e.g. 1:60) or as a percentage. A ratio of 1:60 means the pipe drops 1 mm for every 60 mm of horizontal run. Getting this ratio right ensures waste flows consistently and the pipe self-cleans as it drains.
The Recommended Gradient for NZ Waste Pipes
In New Zealand, waste pipe gradients are guided by NZS 3604 and the broader NZ Building Code, particularly clauses relating to sanitary plumbing (G13). While your licensed plumber will work to the specific standard that applies to your situation, the general rule of thumb widely used in NZ practice is:
- Branch waste pipes (e.g. from a basin, bath, or laundry tub): typically 1:40 to 1:60 gradient
- Larger diameter pipes (e.g. 100 mm soil/drain lines): often 1:60 to 1:80
- Minimum accepted fall in most situations: 1:80 (below this, self-cleansing velocity is rarely achieved)
- Maximum practical fall for smaller pipes: around 1:20 to 1:25 before you risk the water outrunning the solids
As a rough guide, a 1:60 fall means a pipe drops about 16–17 mm per metre of run — a gentle but effective slope.
Why the Right Fall Matters
| Too Little Fall | Correct Fall | Too Much Fall |
|---|---|---|
| Waste sits and settles | Solids and liquids move together | Water races ahead of solids |
| Blockages and odours | Pipe self-cleans effectively | Solids left behind, blockages form |
| Potential backflow risk | Consistent drainage | Gurgling, trap siphoning |
A pipe that's too flat is a common cause of recurring blocked drains in older NZ homes, especially where pipes have been extended or rerouted over the years without proper planning.
Pipe Size and Fall — They Work Together
The correct fall also depends on the diameter of the pipe. Larger pipes need less fall to achieve the same self-cleansing velocity because they carry a greater volume of water. Common NZ waste pipe sizes and typical uses:
- 32–40 mm — hand basins, laundry tubs
- 50 mm — baths, showers, kitchen sinks
- 100 mm — toilets, main drain lines
Your plumber will size the pipe and set the fall together as part of a compliant installation.
Venting and Traps — Don't Forget These
Fall alone won't guarantee a well-draining system. Traps (the U-bend under every fixture) hold water to block sewer gases, and vent pipes allow air into the system so water can flow freely without siphoning the traps dry. If you've ever heard gurgling after flushing or noticed a rotten-egg smell, a venting or trap issue is often the culprit — not just the fall. BRANZ has useful guidance on how these systems work together in NZ residential construction.
Wet Areas and the NZ Building Code
If your waste pipe runs through a wet area (bathroom, laundry, or kitchen), the installation must also comply with Building Code clause E3 (internal moisture) and G13 (foul water). This means proper waterproofing, correct pipe support, and — critically — a compliant gradient. Any building work that affects drainage in these areas may require a building consent, depending on the scope. Check with your local council or visit MBIE Building Performance if you're unsure.
What a Homeowner Can and Cannot Do
In New Zealand, sanitary plumbing and drainlaying is restricted work. This means:
- ✅ You can unblock a drain yourself, replace a tap washer, or swap out a showerhead
- ❌ You cannot legally alter, extend, or install waste pipes, soil pipes, or drain connections without a licensed plumber or drainlayer
All restricted plumbing work must be carried out (or supervised) by a tradesperson registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB). Unlicensed plumbing work can void your home insurance, create liability issues, and — most importantly — put your household's health at risk.
Safety note: Sewer gases including hydrogen sulphide can be hazardous. Never work on drain lines in confined spaces without proper ventilation and awareness of the risks. See WorkSafe NZ for guidance.
When to Call a Professional
Call a licensed plumber if you're experiencing:
- Recurring slow drains that don't clear with standard unblocking
- Gurgling sounds from multiple fixtures at once
- Foul smells coming from drains or under the floor
- Planning a renovation that involves moving or extending any waste or drain pipes
- Building a new bathroom, laundry, or kitchen
A qualified plumber will use a spirit level or laser level to set the correct fall accurately — it's not something to eyeball. You can find licensed local plumbers on tradedirectory.co.nz to get quotes and check credentials.