Installing a small retaining or drainage channel at your garage door threshold is a popular and practical project — it keeps rainwater, surface runoff, and driveway wash from flooding into your garage. In many situations a homeowner can handle the physical installation themselves, but how and where the channel drains is where NZ regulations come into play.
What Is a Threshold Retaining Channel?
A threshold retaining channel (sometimes called a garage door drain or linear drain) is a narrow, grated channel set into the concrete or paving just outside — or just inside — your garage door. It intercepts water before it crosses the threshold. Common options include:
- Pre-made polymer concrete channels (e.g., ACO or similar brands stocked at Bunnings or Mitre 10)
- Galvanised or stainless steel grated channels
- Simple rubber or aluminium threshold seals (not a drain, but a barrier — no drainage connection needed)
If you're only installing a rubber or aluminium threshold seal that simply blocks water rather than draining it away, this is straightforward DIY with no drainage compliance concerns.
The Key Question: Where Does the Water Go?
This is where things get important under NZ law. The NZ Building Code (particularly Clause E1 – Surface Water and Clause G13 – Foul Water) requires that surface water is managed so it doesn't cause damage or nuisance, and that drainage connections are made correctly.
- Discharging to a soakage pit or garden bed on your own property: Generally lower-risk and may not require a consent, but check with your local council — some areas have restrictions, especially on clay soils or near boundaries.
- Connecting to a stormwater drain: This is restricted drainlaying work under the Plumbers, Gasfitters and Drainlayers Act 2006. Any connection to a public or private stormwater system must be carried out (or supervised) by a licensed drainlayer.
- Connecting to a foul sewer: This is absolutely prohibited for surface water — never connect a surface drain to the sewer system.
What Can a Homeowner Do Themselves?
Under the Plumbers, Gasfitters and Drainlayers Act, homeowners are permitted to do certain drainage work on their own home (owner-occupiers only, not rental properties). However, the rules are specific and limited. As a general guide:
- You can excavate and bed the channel in concrete yourself.
- You can install the channel body and grate.
- You may be able to connect a short run of pipe to a soakage system on your own property in some circumstances — but confirm this with your council and the PGDB before proceeding.
- You cannot make connections to the public stormwater network yourself — this requires a licensed drainlayer.
When in doubt, it's always safer (and often not expensive) to have a licensed drainlayer do the connection while you handle the prep work.
Does This Work Need a Building Consent?
Many small threshold channel installations fall under Schedule 1 of the Building Act 2004 as exempt building work — particularly if they are minor drainage works not connected to the public system. However, you must confirm this with your local council before starting, as exemptions depend on the specific scope of work, your site, and your council's interpretation. Check MBIE Building Performance for general guidance on consent exemptions.
Step-by-Step: Installing the Channel
1. Plan the Location and Fall
The channel should sit at or just below the threshold level. Ensure it has a minimum fall of 1:100 toward the outlet so water drains freely and doesn't pool.
2. Mark and Cut the Concrete
Use a diamond blade angle grinder or concrete saw to cut the slot. Wear appropriate PPE — eye protection, hearing protection, and a P2 dust mask. Refer to WorkSafe NZ guidelines for safe concrete cutting, as silica dust is a serious health hazard.
3. Excavate and Bed the Channel
Remove the concrete and excavate to the required depth. Bed the channel on a fresh concrete mix (typically a 3:1 sand:cement mix or a bagged concrete product), checking level and fall as you go.
4. Connect the Outlet (Licensed Work if to Stormwater)
If connecting to stormwater drainage, engage a licensed drainlayer at this stage. If discharging to a soakage pit, confirm this is acceptable with your council first.
5. Backfill and Finish
Once the channel is set and the outlet is connected, backfill around the channel with concrete, finish flush with the surrounding surface, and allow adequate curing time before use.
Choosing the Right Channel
| Feature | What to Look For |
|---|---|
| Load rating | At least Class A (pedestrian) for foot traffic; Class C or D if vehicles will drive over it |
| Grate material | Galvanised steel or stainless steel for durability |
| Channel length | Measure your door opening width and allow for end caps |
| Outlet size | Match to your pipe size (typically 90mm or 100mm) |
When to Call a Professional
You should engage a licensed drainlayer (find one at tradedirectory.co.nz) if:
- The channel outlet needs to connect to the stormwater or any underground drainage system
- You are unsure where your existing drains run
- Your property has a high water table, clay soils, or poor natural drainage
- The work may require a building consent
- You are a landlord or property investor (owner-occupier DIY exemptions do not apply)
A licensed drainlayer can also advise on whether your proposed soakage solution complies with BRANZ good practice and your local council's requirements.
Safety Note
Concrete cutting produces respirable crystalline silica dust, which can cause serious lung disease. Always use wet-cutting methods where possible, wear a P2 (or higher) respirator, and work in a well-ventilated area. See WorkSafe NZ for full guidance.
Certifying Drainlayer
