S-traps vs P-traps – Differences and Code Requirements

ShapeResemble the letter “S”Resemble the letter “P”
Common UsageLess common in modern plumbingCommonly used in modern plumbing
FunctionTrap and retain water to prevent sewer gases from entering the buildingTrap and retain water to prevent sewer gases from entering the building
DesignTypically have a shallow bend followed by a vertical outlet pipeHave a deeper bend with a horizontal outlet pipe followed by a vertical drop
InstallationUsually installed when there’s limited vertical space or in older plumbing systemsStandard choice for most plumbing applications
MaintenanceMore prone to siphonage and may lose the water seal if not properly vented or used infrequentlyLess prone to siphonage and typically maintain the water seal effectively
VentingMay require additional venting to prevent siphonageLess susceptible to siphonage and often doesn’t require additional venting
Code ComplianceMay not meet current plumbing codes in some regionsGenerally meets plumbing codes and standards
Space EfficiencyTakes up less horizontal spaceTakes up more horizontal space, making it less suitable for tight spaces
Common ApplicationsOlder homes, specific situations where space is limitedStandard choice for sinks, showers, and most fixtures in new construction and renovations

A P-trap is connected to a horizontal pipe while an S-trap is connected to a vertical pipe. As such, the horizontal pipe allows air to flow to the P-trap while draining out water/waste, while the vertical pipe in an S-trap doesn’t, resulting in siphoning out of water from the trap.

The main reason S-traps are illegal is because they cause siphoning/sucking of water out of the trap during draining. This allows sewer gases to flow out of the drain instead of the plumbing vent and hence a sewage smell in the house.

Before going on any further, let us look at how S-traps and P-traps work and why you need them in the first place.

You house has several fixtures (sinks, tubs, showers, washing machines etc.) each with its own drainpipe. All these drainpipes are farther down the line connected to the main house drainpipe which then drains to the city’s sewer lines or septic tank.

For efficient draining, you house needs to have a plumbing vent. This is the pipe connected to the main house drainpipe and runs through the roof of the house.

A plumbing vent has 2 functions. It removes sewer gases/smells from the drainpipe and also allows in air into the drainpipe. This is why a bottle of water will drain faster if you make a small hole at the top.

Slow draining fixtures are usually a caused by a clogged plumbing vent but could also be caused by a clogged P-trap or drainpipe.

How they Work

As I have mentioned above, the plumbing vents admits air into the drainpipes, which prevents the creation of a vacuum. But what is wrong in a having a vacuum in the drainpipes?

If you look at a drain trap, you will notice that it is designed with a U-bend. Firstly, as its name implies, a drain trap traps potential drain clogs, preventing them from clogging the drainpipe farther away, where unclogging it would be way harder.

Secondly and most important, a drain trap holds a little amount water at all times thanks to its shape. This is the water you see at the bottom of the toilet bowl.

The water in the drain trap acts a barrier preventing sewer gases from coming up through the drain. Sewer gases should only flow out through the plumbing vent.

I had asked a rhetorical question above. Why do we need to avoid a vacuum in the drainpipes by installing a plumbing vent?

As you know, it is very hard for a vacuum to exist.  What will happen is that the water in the drain traps will be siphoned out/sucked into the drainpipe, breaking the barrier it had created. Sewer gases will then flow out of the drains unrestricted.

Why are S-traps Illegal?

In houses built a long time ago, it is not unusual to find sinks with S-traps but you will rarely see S-traps in modern or remodeled houses. As a matter of fact, S-traps are banned by building codes.

Why is this though? What makes P-traps good to use but not S-traps?

If you look at a P-trap, you will notice that it has a short arm and a long arm. The long arm is connected to the drain’s tailpiece while the short arm is connected to a horizontal wall on the wall via an elbow.


As water drains out of a P-trap, air is able to flow towards the trap at the same time, preventing the creation of a vacuum. This ensures that the P-trap is always full of water and that sewer gases are restricted from flowing through it.

An S-trap however is made of 2 P-traps. The first P-trap’s long arm is connected to the sink’s tailpiece while the second P-trap’s long arm is connected to a vertical drainpipe. The short arms are joined together.


Why is that a problem though?

If say you are draining a sink full of water, the vertical drainpipe will be full of water and as such there will be no air admittance into the trap. As a result, a vacuum will be created in the drainpipe which will suck water out of the S-trap.

That is what happens when you are flushing a toilet or siphoning water out of a tank using a hose pipe. The good thing with a toilet is that the toilet tank sends a little water back to the bowl after the initial large dump which makes sure there is water at all times at the bottom of the toilet.

Without water in the trap, sewer gases/smells will come up through the drain without any restriction. This is the reason S-traps are illegal almost everywhere in the world.

What to Do if you Have an S-trap

It is not easy to replace an S-trap with a P-trap. Remember that a P-trap has a horizontal pipe drainpipe close to it while an Strap doesn’t, so it means you needs to do a lot of repiping in the house, which is not cheap.

I have seen folks try to convert an S-trap into a P-trap but all they end up doing is creating an extended S-trap like in this video. So, what can you do if you have an S-trap in your sink and want to be compliant with the code without having to pay and arm and leg for it?

The easiest and cheapest thing you can do is install an air admittance valve (AAV). An air admittance valve is a short pipe with a valve at the top that is connected straight on the S-trap (or on P-traps in non-vented sink drainpipes).

Air admittance valves allow air to flow into the drain trap to avoid the creation of a vacuum, but sewer gases cannot flow out through it. It works like a water/gas check valve.

These valves are short enough to fit under the bathroom or kitchen sink so you do not need to create a hole through the wall.

Check out air admittance valves here on Amazon

You really don’t need to replace an S-trap with P-trap if you don’t mind the sewer smell. However, if you intend on remodeling your house you will have to.  Plumbers are mandated by code to do so.

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