The Short Answer

Both Trap Water — but the Pipe Leaves in a Different Direction.

The purpose of both designs is essentially the same. A bend in the pipe retains water after the fixture has drained, creating a barrier between the room and the foul drainage system.

A P trap has a U-shaped bend followed by a more horizontal outlet, commonly connecting towards a wall or into appropriately arranged waste pipework.

An S trap continues downward after the bend, producing the characteristic S-shaped profile and historically allowing waste to discharge directly towards a floor connection.

The important practical difference is that traditional S trap arrangements can be more vulnerable to siphoning, which may draw water out of the trap and weaken the protective seal.

P Trap Explained

What Is a P Trap?

A P trap is a plumbing trap designed to maintain a water seal between a fixture and the drainage system.

Its name comes from the profile created by the curved trap and the adjoining waste pipe, which resembles the letter P when viewed from the side.

The curved section retains a small quantity of water after the fixture has emptied. This standing water prevents sewer gases and foul odours from travelling back through the plughole.

P traps are commonly associated with modern sink, basin and other sanitary plumbing arrangements because the trap seal can be protected through appropriate pipework and ventilation.

How It Works

How Does a P Trap Stop Drain Smells?

Whenever a sink, basin or shower is used, wastewater travels through the curved trap and into the drainage system.

Not all of the water leaves. A small amount remains within the lowest section of the trap.

That retained water forms the trap seal. Sewer gases cannot pass freely through the water, so the room remains separated from the foul drainage system.

Each time the fixture is used, new wastewater passes through and replaces the water sitting within the trap.

S Trap Explained

What Is an S Trap?

An S trap performs the same basic job as a P trap: it holds water within a curved section of pipe to create a barrier against foul drainage gases.

The difference is what happens after the trap bend. Rather than continuing predominantly horizontally, the pipe turns downwards again, creating an S-like profile.

This type of arrangement was historically common where a fixture discharged into pipework passing through the floor.

Traditional S traps are associated more often with older plumbing systems and can be vulnerable to self-siphoning if the drainage arrangement does not adequately protect the water seal.

The Main Weakness

Why Can an S Trap Lose Its Water Seal?

When a large volume of water flows through a traditional S-shaped trap and immediately downwards, the moving column of water can create a siphoning effect.

Instead of leaving the normal amount of water sitting inside the trap, the flow can draw too much water out with it.

If the remaining seal becomes too shallow — or disappears completely — the trap can no longer reliably stop foul air travelling back towards the fixture.

That vulnerability is one reason modern plumbing systems place so much emphasis on appropriate trapping, ventilation and pipe configuration.

Why P Traps Are Commonly Preferred

A More Reliable Water Seal.

The strongest advantage of the P-trap style arrangement is not simply its shape — it is the ability of the wider drainage installation to maintain the water seal more reliably.

Correctly configured ventilation helps balance pressure within the drainage system and reduces the likelihood of wastewater flow pulling water completely out of the trap.

This makes properly installed P-trap arrangements well suited to modern plumbing systems where reliable odour prevention and trap-seal protection are important.

If a sink or basin repeatedly develops unpleasant drain smells despite regular use, the trap itself, ventilation or downstream drainage may need investigation.

Problems With Traps

What If a P Trap Still Smells or Drains Slowly?

A trap does not prevent every plumbing problem. It can itself become restricted by debris, grease, hair or other material passing through the fixture.

Kitchen sink traps can collect food residue and grease, while bathroom traps commonly accumulate hair, soap and other deposits.

If the trap is clear but the fixture remains slow, the blockage may sit further into the waste or drainage pipe. Our guide explains more about how to unblock a sink, while shower problems are covered in our blocked shower drain guide.

Persistent problems affecting several fixtures can indicate a wider blocked drain rather than an isolated trap issue.

Other Trap Types

Are P Traps and S Traps the Only Options?

No. Plumbing uses a variety of trap designs depending on the fixture, available space and drainage arrangement.

Bottle traps, for example, are often used beneath basins where a compact and visually tidy installation is desirable.

Baths and showers can use lower-profile traps where there is limited space beneath the tray or bath.

Whichever design is used, the underlying requirement remains the same: wastewater must flow away while an effective water seal is retained between the room and the drainage system.

Drain Smells or Slow Waste?

Think the Problem Goes Beyond the Trap?

A blocked or dry trap can create local problems, but persistent smells, gurgling and several slow-running fixtures may indicate an issue further into the drainage system.

KD Drainage can investigate and clear domestic and commercial drainage problems where the cause is not obvious at the fixture itself.