A catch pit (sometimes called a sump or gully trap) is a small underground chamber that collects stormwater runoff and traps silt, leaves, and debris before they block your drainage pipes. Installing a simple one in your garden is a manageable DIY project for many homeowners — but there are important rules around connecting it to the wider drainage system that you need to understand first.
What Is a Catch Pit and Why Do You Need One?
In New Zealand's variable climate — from heavy Auckland downpours to Christchurch nor'westers — surface water management is critical. A catch pit sits at a low point in your garden or driveway and intercepts runoff before it pools or erodes your lawn. It typically consists of a precast or plastic chamber with an inlet, a sediment sump at the base, and an outlet pipe leading to a legal discharge point.
Under the NZ Building Code, stormwater drainage is covered primarily by Clause E1 (Surface Water). Your installation must not direct water onto neighbouring properties or cause nuisance flooding.
Do You Need a Building Consent?
Many simple catch pit installations fall under Schedule 1 exemptions of the Building Act, meaning no consent is required — but this depends on the scope of work and your local council's rules. As a guide, a small garden catch pit that doesn't connect to the public stormwater network and doesn't involve structural work is often exempt. However, always confirm with your local council before starting, as requirements vary by region and some councils have specific stormwater bylaws.
If your catch pit connects to the public stormwater system, a licensed drainlayer must carry out or supervise that connection — more on that below.
Tools and Materials You'll Need
- Spade and/or mini excavator (hire from Bunnings or Mitre 10)
- Precast concrete or plastic catch pit chamber (available from plumbing merchants and some PlaceMakers stores)
- Inlet and outlet pipes (typically 100 mm uPVC drainage pipe)
- Catch pit grate or lid (cast iron or plastic)
- Bedding sand or fine gravel
- Concrete mix for haunching around the chamber
- Pipe fittings, solvent cement, and primer
- Safety gear: gloves, safety glasses, steel-capped boots
Step-by-Step Installation Guide
1. Choose the Right Location
Identify the lowest point where water pools or flows. Make sure you're not digging near underground services — always call Dial Before You Dig (0800 248 344) to locate cables, gas lines, and existing pipes before you break ground.
2. Excavate the Pit
Dig a hole roughly 300–400 mm wider than your chamber on all sides, and deep enough to allow for:
- A 100–150 mm compacted gravel or sand base
- The full depth of the chamber
- The inlet pipe to fall naturally toward the pit (a minimum 1:100 fall is standard practice)
3. Prepare the Base
Compact the base of the excavation and lay a 100 mm layer of bedding sand or fine gravel. This prevents the chamber from settling unevenly over time.
4. Set the Chamber
Lower the catch pit chamber into position. Check it is level using a spirit level. The top of the grate or lid should sit flush with (or just below) the finished ground surface to allow water to flow in freely.
5. Connect the Inlet Pipe
Lay your 100 mm uPVC drainage pipe from the area you want to drain, ensuring a consistent fall toward the catch pit. Use solvent cement and primer on all joints per the manufacturer's instructions. Haunch around the pipe with concrete or compacted gravel to prevent movement.
6. Connect the Outlet Pipe
This is where things get important. The outlet pipe must discharge to a legal point — this could be:
- A soakage trench or soakpit (if your soil permeability allows)
- An existing private stormwater line
- The public stormwater network (restricted work — see below)
The outlet should be set higher than the base of the sump so sediment settles below the outlet and doesn't wash through.
7. Backfill and Reinstate
Backfill around the chamber in layers, compacting as you go to avoid voids. Concrete haunch around the chamber top for stability. Fit the grate or lid and reinstate the surrounding ground surface.
8. Test the System
Run a hose into the inlet and watch that water flows freely through to the outlet without backing up. Check all joints are watertight.
Maintenance Tips
A catch pit only works if it's kept clear. Plan to:
- Clean out the sump at least twice a year (more often in autumn)
- Check the grate isn't blocked after heavy rain
- Inspect pipe joints annually for root intrusion or cracking
When to Call a Professional
Drainlaying is restricted building work in New Zealand. Under the Plumbers, Gasfitters and Drainlayers Board (PGDB) rules, any connection to the public stormwater or wastewater network must be carried out by a licensed drainlayer. Attempting this work unlicensed is illegal and can result in fines, liability for damage, and problems when you sell your property.
You should also call a professional if:
- You're unsure about your soil's drainage capacity
- The catch pit needs to handle large volumes of water (e.g., from a driveway or roof)
- You encounter unexpected underground services or contaminated soil
- Your council requires an inspection or consent
A qualified local drainlayer can assess your site, ensure the work meets MBIE Building Performance requirements, and provide the necessary Certificate of Compliance. You can find licensed drainlayers in your area on tradedirectory.co.nz.
Safety Note
Excavation work carries real risks — trench collapse, striking underground services, and working in confined spaces. Keep excavations as shallow as practical, never work alone, and always call Dial Before You Dig first. Review WorkSafe NZ guidelines for safe excavation practices.
Sources & Further Reading
- MBIE Building Performance – Building Code Clause E1 Surface Water
- Plumbers, Gasfitters and Drainlayers Board – Licensed Drainlayers
- BRANZ – Stormwater and Drainage Guidance
- PlaceMakers – Drainage Products and Materials
- WorkSafe NZ – Excavation Safety
- Standards NZ – NZS 3604 and Related Drainage Standards