A drum trap is a cylindrical, canister-style trap commonly found under bathtubs in older New Zealand homes — particularly those built before the 1980s. Unlike modern P-traps, drum traps have a removable lid that allows access for cleaning, but they can be tricky to deal with if they haven't been opened in years. Here's how to tackle the job safely and effectively.
What Is a Drum Trap and Why Does It Block?
A drum trap is a cast-iron or lead canister, usually around 100–150 mm in diameter, installed below floor level beneath the bathtub. Water enters near the bottom and exits near the top, trapping debris and maintaining a water seal. Over time, hair, soap scum, and sediment accumulate inside, causing slow drainage or a complete blockage.
Because drum traps don't self-flush the way P-traps do, they need periodic manual cleaning — sometimes every year or two in a busy household.
Tools and Materials You'll Need
- Adjustable pipe wrench or large slip-joint pliers
- Old towels and a bucket
- Rubber gloves and eye protection
- A stiff bottle brush or drain cleaning brush
- Plumber's grease or thread sealant (e.g. PTFE tape)
- A torch/headlamp
- Drain snake or flexible cleaning rod (optional, for stubborn blockages)
Step-by-Step: Cleaning the Drum Trap
1. Locate the Access Lid
The drum trap is usually accessible from a small hatch in the floor, from a cupboard beside the bath, or directly under a removable bath panel. Shine your torch around to find the cylindrical canister with a threaded lid on top.
2. Prepare the Area
Lay old towels around the trap and position your bucket directly underneath. The trap holds water and trapped debris — expect a smelly spill when you open it.
3. Remove the Lid
Using your pipe wrench, turn the lid anti-clockwise to loosen it. Old cast-iron lids can be seized with corrosion, so apply steady, even pressure. Avoid using excessive force, which can crack old cast-iron fittings. If the lid won't budge, penetrating oil (left for 30 minutes) can help.
Safety note: Wear rubber gloves and eye protection throughout. Drain water can contain harmful bacteria. Ensure the area is ventilated — sewer gases can accumulate in confined spaces.
4. Scoop Out the Debris
Once the lid is off, use a gloved hand or a small scoop to remove the accumulated hair and sludge from inside the drum. Place it directly into a rubbish bag — don't flush it back down the drain.
5. Scrub the Interior
Use your bottle brush to scrub the inside walls of the drum trap thoroughly. Flush with warm water from a jug to rinse it out. For stubborn build-up, a drain cleaning brush on a flexible rod works well.
6. Check the Inlet and Outlet Pipes
While the lid is off, use your torch to check that both the inlet (lower) and outlet (upper) pipes are clear. If the outlet pipe is blocked further along, you may need a drain snake to clear it.
7. Reseal and Replace the Lid
Clean the threads on both the lid and the drum body. Apply fresh PTFE tape or plumber's grease to the threads, then hand-tighten the lid before snugging it up with the wrench — don't overtighten, especially on old cast-iron, as the threads can strip or the body can crack.
8. Test the Drain
Run the bath and watch for leaks around the lid. Check that the bath drains freely. If it's still slow, the blockage may be further down the drain line.
Common Problems With Old Drum Traps
| Problem | Likely Cause | Action |
|---|---|---|
| Lid won't unscrew | Corrosion/seizure | Penetrating oil, steady pressure |
| Cracked drum body | Age, overtightening | Replace trap — call a plumber |
| Persistent slow drain | Blockage past the trap | Drain snake or call a plumber |
| Sewer smell after cleaning | Poor lid seal | Re-tape threads, retighten lid |
| Lead trap identified | Pre-1970s plumbing | Consider upgrading — see below |
A Note on Lead Drum Traps
Some very old NZ homes have lead drum traps. These are soft, dull grey, and easily dented. If you suspect your trap is lead, take care not to damage it — and seriously consider having a licensed plumber replace it with a modern fitting. Lead plumbing components are no longer acceptable under the NZ Building Code for new work, and replacement is strongly advisable for health reasons.
Should You Replace the Drum Trap Entirely?
Many plumbers recommend replacing old drum traps with a modern P-trap or bottle trap when the opportunity arises. Modern traps are easier to maintain, less prone to blockage, and fully compliant with current standards. Replacing a trap involves cutting into the drain line and is restricted plumbing work under the Plumbers, Gasfitters and Drainlayers Act — it must be carried out by a licensed plumber. Check the Plumbers Gasfitters & Drainlayers Board to verify a tradesperson's licence.
When to Call a Professional
Call a licensed drain cleaner or plumber if:
- The drum trap lid is cracked, corroded through, or the body is damaged
- You suspect a lead trap that needs replacing
- The blockage persists after cleaning the trap
- You find the drain line is broken or root-invaded
- You're uncomfortable working in a confined space or around old plumbing
A qualified local drain cleaner or plumber can be found quickly on tradedirectory.co.nz. For guidance on what plumbing work requires a licensed professional, see the PGDB website.
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