Replacing an existing floor waste grate in your garage can be a straightforward DIY task — but only if you're swapping like-for-like and not touching the drainage pipework beneath. The moment you start modifying, extending, or connecting new drainage, you're into restricted drainlaying work under New Zealand law, and you'll need a licensed professional.
What Counts as DIY vs. Restricted Work?
This is the key question, and the answer comes down to what exactly you're doing:
- Lifting and replacing the grate itself (the metal or plastic cover sitting in an existing frame) — generally fine as a DIY task, as you're not touching any pipework.
- Replacing the grate and its body/frame within an existing, already-connected drain — this is a grey area. If you're unsetting and resetting the drain body without modifying pipework, some homeowners do this themselves, but it's worth checking with your local council first.
- Installing a brand-new floor waste where none existed before — this involves connecting to the stormwater or wastewater system and is restricted drainlaying work. You must use a licensed drainlayer.
- Relocating an existing floor waste — also restricted work.
Under the Plumbers, Gasfitters and Drainlayers Act 2006, drainlaying work on sanitary or stormwater drainage must be carried out (or supervised) by a person licensed by the Plumbers, Gasfitters and Drainlayers Board (PGDB). Doing this work unlicensed can result in fines and may void your home insurance.
Garage Floor Wastes and the NZ Building Code
Garage drainage in New Zealand must comply with the NZ Building Code, particularly Clause G13 (Foul Water) and Clause E1 (Surface Water). Which system your floor waste connects to matters enormously:
- Stormwater system — generally used for garages where only rainwater or clean wash-down water is expected.
- Wastewater/sanitary system — required if the garage is used for vehicle maintenance where oils, fuels, or chemicals could enter the drain. In this case, a grease trap or interceptor may also be required.
Connecting a garage floor waste to the wrong system is a Building Code breach. Your local council can advise which system applies to your property and whether a building consent is required. Many simple drainage connections fall under Schedule 1 exemptions to the Building Act, but you should always confirm with your council — don't assume.
Choosing the Right Grate
If you are replacing a grate like-for-like, here's what to look for:
| Feature | What to Check |
|---|---|
| Size | Measure the existing frame opening (common sizes: 100 mm, 150 mm) |
| Load rating | Garage floors need a grate rated for vehicle traffic (look for Class B or C ratings) |
| Material | Stainless steel or cast iron for durability; plastic for light-duty use |
| Outlet size | Match the existing pipe diameter (typically 100 mm in NZ residential) |
| Trap type | Check whether your existing drain body has a built-in trap — required to prevent sewer gases |
Grates and drain bodies are available from Bunnings, Mitre 10, and PlaceMakers. Always buy a product that meets the relevant NZ standard — ask the store staff if you're unsure.
Step-by-Step: Replacing a Grate Only (Like-for-Like)
If you've confirmed you're simply swapping the grate cover in an existing frame, here's how to approach it:
- Lift the old grate — most unclip or lift straight out. Some are screwed down; use a flathead screwdriver.
- Clean the frame — remove any debris, sediment, or rust from the channel and frame.
- Check the drain body and trap — look down into the drain. If you can see standing water in a trap, that's good. If it's dry or the trap looks damaged, call a drainlayer before proceeding.
- Drop in the new grate — ensure it sits flush with the floor surface and is secure.
- Test with water — pour a bucket of water in and confirm it drains freely.
Safety note: If your garage drain connects to the wastewater system, sewer gases (including hydrogen sulphide) can be present. Work in a ventilated space and avoid prolonged exposure to any odours coming from the drain.
Do You Need a Building Consent?
For a straightforward grate-for-grate swap with no pipework changes, a building consent is very unlikely to be required. However, for any new drainage installation or modification, a consent may be needed depending on your council and the scope of work. Always check with your local territorial authority before starting. The MBIE Building Performance website has useful guidance on when consents apply.
When to Call a Professional
You should engage a licensed drainlayer if:
- You're installing a floor waste where there isn't one already.
- You need to connect to or modify any underground pipework.
- You're unsure which drainage system (stormwater vs. wastewater) the drain connects to.
- The existing drain body or trap is cracked, blocked, or damaged.
- Your garage is used for vehicle servicing and you need to check interceptor requirements.
- Your council requires inspections or a Certificate of Acceptance.
A licensed drainlayer can also ensure the work is signed off correctly, protecting you if you ever sell the property. You can find qualified local drainlayers on tradedirectory.co.nz.
Certifying Drainlayer
