A blocked P-trap under your laundry tub is one of the more satisfying DIY plumbing jobs — it's usually caused by a build-up of lint, soap scum, or debris, and most homeowners can clear it in under an hour with basic tools. Here's how to do it safely and correctly.
What Is a P-Trap and Why Does It Block?
The P-trap is the curved section of pipe beneath your laundry tub — named for its P-like shape when viewed from the side. It holds a small amount of water at all times, which acts as a seal to stop sewer gases from entering your home (an important requirement under NZ Building Code clause G13). Because it curves, it's also a natural collection point for lint from washing machine hoses, soap residue, hair, and other debris. Over time, this build-up restricts flow until the tub drains slowly or not at all.
Tools and Materials You'll Need
Step-by-Step: Clearing the Blocked P-Trap
1. Prepare the Area
Place your bucket directly under the P-trap. Lay old towels around the base of the cabinet to catch any spills. Put on your rubber gloves and safety glasses — the water sitting in the trap can be unpleasant and may contain mild cleaning chemicals.
2. Loosen the Slip Nuts
The P-trap is held in place by two slip nuts — one where the trap connects to the drainpipe from the tub, and one where it connects to the waste pipe in the wall or floor. Turn these nuts anti-clockwise by hand if possible, or use adjustable pliers if they're tight. Take care not to overtighten or crack plastic fittings.
3. Remove the P-Trap
Once both nuts are loose, carefully slide the P-trap free. The water inside will drain into your bucket — this is normal. Have your bucket positioned well before you do this.
4. Clean Out the Trap
Tip the contents of the trap into the bucket. Use your bottle brush or flexible drain brush to scrub the inside of the trap thoroughly under running water (use a laundry sink or outdoor tap). Remove all lint, soap build-up, and debris. If the trap is cracked, warped, or badly corroded, replace it — new plastic P-traps are inexpensive and widely available at hardware stores.
5. Check the Drain Pipes
While the trap is off, shine a torch into both the pipe coming from the tub and the waste pipe in the wall. If you can see further blockage, use a flexible drain brush or a small hand drain snake to clear it. If the blockage seems deep in the wall pipe, this may require a drain cleaning professional.
6. Reassemble and Test
Refit the P-trap, hand-tightening the slip nuts first, then snugging them up with pliers — do not overtighten plastic fittings. If the threads look worn or the joints leaked before, wrap them with PTFE (plumber's) tape before reassembling — though note that PTFE tape is not always appropriate for slip-joint fittings; check whether replacement washers are the correct solution for your fitting type. Run the tub and check carefully for leaks at both connections. Let it run for a minute or two to be sure.
Preventing Future Blockages
- Fit a lint trap or filter on your washing machine outlet hose — these are cheap and catch the bulk of lint before it reaches the drain.
- Use a drain strainer in the laundry tub to catch debris.
- Pour boiling water down the drain monthly to help dissolve soap build-up (only suitable for metal or newer PVC pipes — check your pipe type first).
- Avoid pouring grease, paint, or heavy chemicals down the laundry drain.
Is This Restricted Plumbing Work?
Removing and cleaning a P-trap that is above ground and accessible is generally considered maintenance work that a homeowner can carry out themselves — it does not involve connecting or altering sanitary drain pipework. However, if you need to cut into pipes, alter the drain layout, or work on any pipework inside walls or underground, this becomes restricted sanitary plumbing work under the Plumbers, Gasfitters and Drainlayers Act 2006. This work must be carried out by a licensed tradesperson registered with the Plumbers Gasfitters & Drainlayers Board (PGDB). Always check with your local council if you're unsure whether your job requires a building consent.
When to Call a Professional
Call a licensed drain cleaner or plumber if:
- The blockage doesn't clear after cleaning the trap and nearby pipes
- Water is backing up into other fixtures (e.g., the toilet or shower) — this suggests a deeper drain blockage
- You notice bad smells even after reassembly, which could indicate a broken trap seal or venting issue
- The waste pipe in the wall appears damaged or corroded
- You're not comfortable working with plumbing fittings
A qualified drain cleaner can use a drain snake or hydro-jet to clear stubborn blockages further down the line. You can find licensed local drain cleaners and plumbers on tradedirectory.co.nz.
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