A non-return flap valve (also called a backflow flap or tide flap) on a stormwater outlet is a simple but important device that stops water — and unwanted visitors like rats — from flowing back into your drainage system. Whether you can install one yourself in New Zealand depends on where the valve sits in your drainage system, your local council's rules, and what NZ law says about restricted drainlaying work.
What Does a Non-Return Flap Valve Do?
A non-return flap valve is a one-way valve fitted to the end of a stormwater pipe where it discharges — typically into a kerb, stream, drain, or coastal outlet. When stormwater flows out, the flap opens. When water levels rise outside (during heavy rain or tidal surges), the flap closes and prevents backflow into your property's drainage system. They're especially useful in low-lying areas of New Zealand prone to flooding or tidal influence.
Is This Restricted Drainlaying Work in NZ?
This is the key question. Under the Plumbers, Gasfitters, and Drainlayers Act 2006 (confirm the current Act title and year with the PGDB), drainlaying is restricted work in New Zealand. This means that connecting to, altering, or extending a drain — including work on stormwater pipes — generally must be carried out by, or under the supervision of, a licensed drainlayer registered with the Plumbers, Gasfitters and Drainlayers Board (PGDB).
However, there is an important distinction:
- Fitting a valve to the end of a pipe that discharges openly (i.e., the pipe end is not buried and you are not cutting into or joining any underground pipework) may be considered minor maintenance rather than drainlaying, depending on your council's interpretation.
- Any work that involves cutting into, joining, or altering underground stormwater pipework is almost certainly restricted drainlaying work and must be done by a licensed professional.
Because interpretations can vary between councils, always check with your local council before starting work. Some councils also require a building consent for drainage alterations, though many minor works fall under Schedule 1 exemptions of the Building Act 2004 — again, confirm this with your council rather than assuming.
What a Homeowner Can Typically Do
If the flap valve simply clips, bolts, or screws onto the exposed end of an existing pipe outlet — with no cutting, gluing, or joining of underground pipework involved — many homeowners carry out this work themselves. Typical steps include:
- Measuring the pipe diameter accurately (common sizes are 100 mm, 150 mm, and 225 mm in NZ residential drainage).
- Selecting the correct valve — flap valves are available at trade suppliers and some hardware stores. Check that the product suits your pipe material (PVC, concrete, or corrugated HDPE).
- Cleaning the pipe end thoroughly before fitting.
- Securing the valve as per the manufacturer's instructions, using stainless steel fixings where possible to resist corrosion, especially in coastal environments.
Always check the manufacturer's installation guide and confirm the product meets any relevant standards. BRANZ is a useful resource for understanding NZ building and drainage best practice.
When You Must Use a Licensed Drainlayer
You must engage a licensed drainlayer if the installation involves any of the following:
- Cutting into or joining underground stormwater pipework
- Relocating or extending a drain to accommodate the valve
- Work on a public stormwater network (council infrastructure) — this is always council or contractor territory
- Any situation where your council or the PGDB considers the work restricted drainlaying
If in doubt, the safest and legally correct approach is to hire a qualified drainlayer. You can find licensed local drainlayers on tradedirectory.co.nz.
NZ Building Code and Compliance
Stormwater drainage in New Zealand is governed by NZ Building Code Clause E1 (Surface Water). Your drainage system must be designed and maintained so that stormwater does not cause damage to your property or neighbouring properties. A correctly installed non-return valve can be part of meeting this obligation, but the overall system must still comply. For detailed guidance, refer to MBIE Building Performance, which publishes acceptable solutions and verification methods for the Building Code.
Practical Tips for NZ Conditions
- Coastal and low-lying areas: Tidal backflow is a real issue in many NZ coastal towns. A marine-grade stainless steel or rubber flap valve will last far longer than a standard plastic one in salt-air environments.
- Maintenance: Flap valves can jam open with debris (leaves, sediment, sticks). Inspect and clean yours at least twice a year, especially before winter.
- Rats and pests: A well-seating flap valve also acts as a pest barrier — an added bonus in many NZ urban areas.
- Flooding-prone properties: If your property regularly floods, a flap valve alone may not be sufficient. Talk to a licensed drainlayer or your council about a more comprehensive solution.
When to Call a Professional
Call a licensed drainlayer if:
- You need to cut into or alter any underground pipework
- You are unsure whether your planned work is restricted under the PGDB Act
- Your council requires a building consent for the work
- The existing pipe end is damaged, misaligned, or in poor condition
- You are connecting to or near a public stormwater main
A licensed drainlayer will also be able to certify the work, which protects you legally and is important if you ever sell your home. Find a qualified local professional at tradedirectory.co.nz.
Safety Note
Working around stormwater drains can involve confined spaces, slippery surfaces, and exposure to contaminated water. Always wear appropriate PPE (gloves, eye protection, waterproof footwear) and never enter a confined drainage structure without proper training and equipment. Refer to WorkSafe NZ for guidance on safe working practices.
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