A slow-draining gully trap is one of the most common drainage headaches for NZ homeowners — and in many cases, you can sort it out yourself without calling a drainlayer. Gully traps collect wastewater from laundry tubs, showers, and similar fixtures before it enters the underground drain, so they're prone to collecting lint, hair, grease, leaves, and silt over time. With a bit of patience and the right approach, clearing a blocked or sluggish gully trap is a straightforward DIY job.
What Is a Gully Trap and Why Does It Block?
A gully trap is a small, ground-level drainage fitting — usually a square or round plastic or concrete box with a removable grate — found outside your home near laundry or bathroom outlets. It holds a small amount of water to act as a seal against sewer gases (similar to a P-trap under a sink).
Common causes of slow drainage include:
- Accumulated silt and sediment washing in from the yard
- Lint and hair from laundry and bathroom outlets
- Grease and soap scum building up on the trap walls
- Leaf litter and garden debris blocking the grate
- Tree root intrusion in older clay or concrete pipes (this is a job for a professional)
Tools and Materials You'll Need
Before you start, gather the following:
- Rubber gloves (heavy-duty)
- Old clothes or a waterproof apron
- A stiff-bristled brush or old toilet brush
- A garden trowel or small scoop
- A bucket
- A garden hose with good water pressure
- Drain cleaning solution (optional — see below)
- Safety glasses
Step-by-Step: How to Clear a Slow Gully Trap
Step 1 — Remove the Grate
Lift off the grate or cover. Many simply lift out; others may be clipped or screwed down. Give it a good clean with your brush — a blocked grate alone can slow drainage significantly.
Step 2 — Scoop Out Debris
Using your gloved hand or a trowel, remove any visible silt, leaves, hair, or sludge from inside the trap body. Place the waste into your bucket for disposal — don't hose it further down the drain at this stage.
Step 3 — Scrub the Trap Walls
Use your stiff brush to scrub the inside walls of the gully trap, loosening any grease or soap build-up. This is the step most people skip, but it makes a real difference to long-term flow.
Step 4 — Flush with Water
Run your garden hose into the trap at full pressure. Watch how quickly the water drains away. If it drains freely now, you're done. If it's still sluggish, move to the next step.
Step 5 — Try a Drain Cleaner (Optional)
For grease or soap build-up further down the pipe, a caustic or enzymatic drain cleaner can help. Always follow the manufacturer's instructions and wear eye protection and gloves. Avoid mixing products. Note that caustic cleaners are not suitable for all pipe types — check the label.
Step 6 — Check the Water Seal
Once the trap is draining well, make sure there is still a small amount of water sitting in the bottom of the trap. This water seal is important — it stops foul sewer gases from entering your home. If the trap seems dry or cracked, it may need replacing.
Step 7 — Replace the Grate and Clean Up
Refit the grate securely, rinse the surrounding area, and dispose of the collected debris in your rubbish (not your compost).
Maintenance Tips to Prevent Future Blockages
| Frequency | Task |
|---|---|
| Monthly | Remove and rinse the grate |
| Every 3 months | Scoop out silt and scrub the trap walls |
| Annually | Flush with hose and inspect for cracks or root intrusion |
Keeping up with regular maintenance is the best way to avoid a full blockage. If your gully trap is near established trees, check it more frequently — tree roots are a leading cause of serious drain blockages in NZ.
What You're Legally Allowed to Do
Under NZ law, clearing and cleaning an existing gully trap is considered routine maintenance and does not require a licensed drainlayer. However, any work that involves connecting, altering, or replacing drainpipes or fittings that are part of the sanitary drainage system is restricted work and must be carried out by a licensed drainlayer. You can verify licensing requirements through the Plumbers Gasfitters & Drainlayers Board.
For guidance on what work requires building consent, check with your local council or visit MBIE Building Performance.
When to Call a Professional
Some drainage problems go beyond a DIY clean-out. Call a licensed drainlayer if:
- The trap drains slowly again within a few days of cleaning
- You can see tree roots inside the trap or pipe
- The trap body is cracked or broken
- Water is backing up from multiple fixtures at once (this may indicate a blocked main drain)
- There is a persistent sewage smell even after cleaning
- The underground pipe appears to have collapsed or subsided
These situations may require a drain camera inspection, high-pressure water jetting, or pipe replacement — all work for a qualified professional. You can find licensed local drainlayers on tradedirectory.co.nz.
Safety note: Always wear gloves and eye protection when working with drain waste or cleaning chemicals. Wastewater contains harmful bacteria. Wash your hands thoroughly when finished.
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