A French drain is one of the most effective ways to tackle soggy ground, pooling water, or a waterlogged fence line on a NZ property. The good news is that a basic surface-water French drain is generally something a capable homeowner can tackle themselves — but there are important rules around what counts as restricted drainlaying work, so read on before you pick up a spade.
What Is a French Drain?
A French drain (also called an agricultural drain or ag drain) is a gravel-filled trench containing a perforated pipe that collects excess water and channels it to a safe discharge point. Along a fence line, it intercepts water before it saturates your lawn, garden, or foundations.
Is This DIY-Legal in New Zealand?
This is the most important question to answer first. Under NZ law, drainlaying is restricted work in many situations. Work that connects to the public stormwater network, or that involves sanitary drainage, must be carried out by (or under the supervision of) a Licensed Drainlayer registered with the Plumbers Gasfitters & Drainlayers Board.
However, a simple French drain that discharges to your own land (for example, to a soakage area, a rain garden, or a legal discharge point entirely within your property boundary) is generally considered minor drainage work that a homeowner can do themselves. If your drain will connect to a kerb, road culvert, or the council's stormwater system, stop and contact your local council and a licensed drainlayer first. Always confirm with your council whether a building consent is required — some drainage work falls under Schedule 1 exemptions of the Building Act, but this varies by council and situation.
Tools and Materials You'll Need
- Spade and/or trenching spade
- Wheelbarrow
- String line and pegs
- Spirit level or laser level
- Perforated ag pipe (typically 100 mm diameter for residential use)
- Geotextile filter fabric (drainage sock or wrap)
- Drainage aggregate (clean, washed chip — 20 mm is common)
- Compactable backfill or topsoil for the surface
- End cap and outlet fitting
You'll find ag pipe, drainage aggregate, and geotextile fabric at Bunnings, Mitre 10, or PlaceMakers.
Step-by-Step Installation
1. Plan Your Route and Discharge Point
Mark the trench line along your fence with string. Decide where the water will discharge — a soakage pit, a lower garden area, or a legal outlet. Never direct water onto a neighbour's property — this can create legal liability.
2. Check for Underground Services
Before digging, call Dial Before You Dig (0800 248 344) or visit their website to identify buried power, gas, water, and telecom lines. This is a critical safety step — striking a buried cable or pipe can be fatal or extremely costly.
3. Dig the Trench
Dig your trench to a minimum depth of 300–450 mm (deeper if you need to get below a hardpan layer). Aim for a consistent fall of at least 1:100 (1 cm drop per metre of run) toward your outlet — use a spirit level and string line to check this as you go. A wider trench (around 300 mm) allows more aggregate and better performance.
4. Line with Geotextile Fabric
Lay geotextile filter fabric into the trench, draping it up the sides with enough overlap to fold back over the top later. This stops fine soil particles from migrating into the aggregate and blocking the pipe over time.
5. Add a Gravel Bed
Pour a 75–100 mm layer of clean drainage aggregate into the bottom of the trench on top of the fabric.
6. Lay the Ag Pipe
Place your perforated ag pipe on the gravel bed with the perforations facing downward (this is the standard practice — water enters from below and the sides). Connect lengths with push-fit couplings. Fit a solid end cap at the uphill end and a solid outlet pipe at the downhill end leading to your discharge point.
7. Backfill with Aggregate
Fill the trench with drainage aggregate to within about 100–150 mm of the surface, covering the pipe fully.
8. Fold Over the Fabric and Finish
Fold the geotextile fabric flaps back over the top of the aggregate to create a complete wrap. Backfill the remaining depth with topsoil or lawn mix and compact gently. You can re-turf or re-seed over the top.
Key Performance Tips
| Factor | Recommendation |
|---|---|
| Minimum pipe diameter | 100 mm for residential |
| Minimum fall | 1:100 (1 cm per metre) |
| Aggregate size | 20 mm clean chip |
| Fabric type | Non-woven geotextile |
| Outlet protection | Rodent mesh on outlet end |
When to Call a Professional
Call a Licensed Drainlayer (find one on tradedirectory.co.nz) if:
- Your drain needs to connect to the council stormwater network
- You're dealing with groundwater rising from below (this may indicate a more complex issue)
- The area is near your house foundations — poor drainage work here can breach NZ Building Code clauses E1 (Surface Water) or B1 (Structure)
- You're unsure about the correct discharge point or fall
- Your property has clay soils or a high water table that may need a more engineered solution
For guidance on NZ Building Code requirements, MBIE Building Performance is an excellent starting point. BRANZ also publishes practical technical guidance on site drainage.
Sources & Further Reading
- Plumbers Gasfitters & Drainlayers Board — Licensed Drainlayers
- MBIE Building Performance — Building Code and consents
- BRANZ — Building research and technical guidance
- WorkSafe NZ — Safe excavation and underground services
- Bunnings — Drainage materials and ag pipe
- PlaceMakers — Drainage and civil supplies
Certifying Drainlayer
