Every sink, basin and laundry tub needs a trap, but grabbing the wrong one off the van is how a ten-minute fit-off turns into a second trip. This guide covers the four trap types you'll reach for most on residential and light commercial work – the P trap, the S trap, the S&P trap and the combination trap – and how to pick between them on the job.
What every trap does
Whatever the shape, a trap holds a water seal between the fixture and the drain so sewer gases can't come back into the room. The differences between trap types come down to two things: where the outlet goes (into the wall or down through the floor) and what else needs to connect (a dishwasher or washing machine hose, a second bowl, or nothing at all).
P trap: outlet into the wall
A P trap's outlet leaves horizontally, so it suits any fixture where the waste point comes out of the wall. It's the default for most kitchen sinks, vanities and laundry tubs in newer builds. Use a 40mm P trap for a basic basin or tub connection with no appliance waste.
S trap: outlet through the floor
An S trap's outlet drops vertically, for fixtures where the waste comes up through the floor – common in older houses, island benches and freestanding tubs. See the DWV 40mm x 40/32mm S trap or the Allproof compact waste trap range.
S&P trap: one trap, either outlet
An S&P trap converts between a wall and floor outlet, which is why most plumbers carry them instead of separate P and S traps. If venting is a concern on the job, an adjustable S&P trap with an air admittance valve (also in 50mm) is an option – check that an AAV is acceptable for the installation under AS/NZS 3500.2 and your local authority before fitting one.
Combination trap: when an appliance hose needs to connect
A combination trap (or combo trap) is a trap with one or more hose nipples moulded into the body, so a dishwasher or washing machine waste hose can discharge through the same trap as the sink or tub. It saves running a separate standpipe and is the standard fit for kitchen sinks with a dishwasher and laundry tubs with a washing machine alongside.
The weak point of a standard combo trap is that the nipples are fixed. If the angle they leave the factory at doesn't suit the cabinet, the hose route or the appliance position, you end up adding elbows or kinking the hose. That's the problem the Twister trap combination was designed to fix: both nipples rotate a full 360°, so you twist them to suit the job instead of working around them.
Low-level combination traps
A low-level combo trap has a shorter overall height. You need one when there's less room than usual between the fixture outlet and the waste point – for example where the wall waste has been set high, or where under-sink drawers or a shallow cabinet leave limited depth. Rather than carrying a separate low-level trap, the Twister converts on the spot: remove the extension piece and it becomes a low-level combo trap.
Quick comparison
| Trap type | Outlet | Appliance hose? | Typical use | Options at PSA |
|---|---|---|---|---|
| P trap | Wall | No | Basins, sinks and tubs with a wall waste | P trap 40mm (TTP40) |
| S trap | Floor | No | Floor wastes, older homes, island benches | DWV S trap, Allproof waste trap |
| S&P trap | Wall or floor | No | General fit-off stock | Adjustable S&P with AAV |
| Combination trap (fixed nipple) | Wall or floor | Yes, fixed angle | Sink + dishwasher where the hose lines up | ACP S&P combination DN40 / DN50 |
| Twister combination trap | Wall or floor | Yes, 2 × 360° rotating | Tight cabinets, awkward hose routes, low-level installs | Twister trap 40mm & 50mm |
40mm or 50mm?
Match the trap to the fixture's waste outlet and the size of the drain it's connecting to. 40mm is the common size for basins, laundry tubs and many kitchen sinks; 50mm suits larger sinks, double bowls and anywhere the drawing or the existing drainage calls for it. For a double bowl sink, pair the trap with a double bowl sink connector, and if you're short on reach, a solvent weld trap extension saves remaking the line.
Fit-off checks before you leave the job
- Use WaterMark-approved traps and fittings, installed by a licensed plumber in line with AS/NZS 3500.2.
- Follow the appliance manufacturer's instructions for hose height and loop so the dishwasher or washing machine can't siphon.
- Cap any unused hose nipple.
- Fill the fixture and run the appliance through a drain cycle, then check every union and the hose connection for leaks.
FAQs
What is a combination trap?
A trap with built-in hose nipples so a dishwasher or washing machine can discharge through the sink or tub trap. Most combination traps can also be set up with either a wall (P) or floor (S) outlet.
What's the difference between a P trap and an S trap?
A P trap's outlet goes horizontally into the wall; an S trap's outlet drops vertically through the floor. An S&P trap can be set up either way.
When do I need a low-level combo trap?
When there isn't enough height between the fixture outlet and the waste point for a standard-height trap. The Twister covers both standard and low-level installs by removing the extension piece.
Why use a Twister trap instead of a standard combo trap?
The Twister's two nipples rotate 360°, so the dishwasher or washing machine hose can come off the trap at whatever angle suits the cabinet. A standard combo trap's nipples are fixed, which often means extra fittings or a kinked hose.
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