Why S-traps are Illegal – And What to Do About them

Can I Use S-trap for Sink?

Each of the several drains in your house are connected to the sewer line outside your house, and needless to say, there is a sewer smell inside the sewer line. That smell is not pleasant.

To prevent the smell from coming up to your house via the drains, they (the drains) are fitted with a trap which creates a barrier. There are 2 types of drain traps.

s-trap

You could have an S-trap or a P-trap. P-traps are the recommended drain traps while S-traps are banned. So, why are S-traps illegal?

The reason S-traps are banned by the Uniform Plumbing Code is because they create negative air pressure inside drain lines which siphons/sucks water from the trap. That results in sewer gases coming up through the drains and therefore a sewage smell in the house.

You cannot use an S-trap for a sink drain. When the sink will be draining, a vacuum (negative air pressure) will be created inside the drain line which will siphon water out of the trap allowing sewer gases to enter the house.

A P-trap is definitely better than an S-trap. Fixtures with an S-trap will always allow sewer gases into the house while a P-trap will always have a water barrier which will prevent sewer gases from entering the house.

S-traps are not allowed in Texas and everywhere in the United States. The Uniform Plumbing Code prohibits S-traps in new and remodeled buildings therefore no plumber will install an S-trap for you.

The best and easiest way to replace an S-trap or convert an S-trap into a P-trap is by installing an air admittance valve. The valve allows air into the drain line as the fixture is drain to prevent siphoning of water from the trap.

If your sink is always gurgling when draining, you most likely have an S-trap. The gurgling is as a result of the water in the trap being siphoned out into the drain line.

How an S-trap Work – S-trap vs P-trap

So, how does an S-trap work? And how different is it from a P-trap? Let us look at the differences.

Although all fixtures in your house that are connected to the sewer line have a drain trap, only the toilet trap and sink trap are visible. Tubs, showers and washing machine drains all have traps only that you cannot see them.

Drain traps have 2 main functions:

  • They trap potential drain clogs. Usually, it is easier to unclog or remove a clog from a drain trap than have to use a snake to pull it out from several feet away from the fixture. In the case of a toilet you only need to pick it from the bottom of the toilet.
  • Thanks to their shape, drain traps are able to hold water at all times. The water acts as a barrier preventing sewer gases from entering the house. Instead, the gases are forced out through the plumbing vent.

The first difference between a P-trap and an S-trap is in their shape. A P-trap is a U-bend which looks like an inverted P. On the other hand an S-trap is made up of 2 U-bends

p-trap
P-trap

The second difference is that a P-trap is connected to a horizontal drain line while an S-trap is connected to a vertical drain line.

If you keenly look at a sink with a P-trap, you will notice that the P-trap is connected to a horizontal drain (which exits through the nearby wall) with the help of an elbow.

As I mentioned, an S-trap is made up of 2 P-traps or U-bends. The second U-bend connects to a vertical drain line and that is where the problem with S-traps starts.

Why are S-traps Illegal?

s-trap

So, why does it matter that P-traps are connected to horizontal drain lines while S-traps are connected to vertical drain lines? Well, it matters a lot.

The connection of a P-trap to a horizontal drain line means that as the fixture is draining, air is able to flow back to the trap. That however does not happen with an S-trap.

When a fixture has an S-trap, it means that as water drains out of the vertical drain line, there is no air flowing back to the trap. As such, a vacuum (also known as negative air pressure) is created inside the drain.

Since a vacuum cannot easily exist, it will try to pull air from the surrounding. There will however be water in the S-trap between the vacuum and the air in the surrounding.

To access the air, the water inside the S-trap will be sucked/siphoned out. At that time, you will hear a gurgling sound almost like a flushing toilet.

So, where does the air which equalizes at the P-trap come from?

Remember that your drainage system is connected to a plumbing vent which runs through the roof of the house.

Apart from removing sewer gases, the plumbing vent also introduces air into the drainage system. The air in the drain lines needs to be at atmospheric pressure levels. And that is where a plumbing vent comes in.

When your drains are not vented or when the vent is clogged, your fixtures will drain slowly, toilets will flush weakly, water from traps will be sucked out and ultimately you will have a sewage smell in the house.

How to Replace S-traps

Since it will be in violation of the code, no plumber will install an S-trap if you are remodeling your house/bathroom. Also, it is not easy to convert an S-trap into a P-trap since as I said, with a P-trap, you need a vertical drain line exiting the wall next to the trap.

With an S-trap however, you will have you drain line at the bottom. It can be done but it involves a lot of work and definitely needs to be done by a professional plumber.

What I have seen some folks do is try to convert an S-trap into a P-trap only to end with 2 S-traps in series. Not only will that not help but you will end up with a drain line that will clog more often.

If you already have an S-trap in your house and it is not bothering you, you do not need to remove it. What you can do to prevent the sewer gases is to pour down a little amount of water every time after draining the sink/tub.

Little amounts of water will refill the trap but will not be enough to create the siphon effect as when a fixture full of water is draining.

If you however want to deal with the problem, the most practical and easiest way of eliminating the effects of an S-trap is by installing an air admittance valve.

An air admittance valve (Amazon) is a short type of vent that fits under the sink (or even just outside the wall). As the sink is draining, the negative air pressure will be created and just before the water in the trap is siphoned out the valve will open and let in air into the drain line.

That will prevent the S-trap water from being siphoned out and as a result no sewer gases will enter the house. Air admittance valves are one-way like a check valve.

Air can get inside the drain line via the valve but sewer gases cannot get out. These valves are also cheap and easy to install as a DIY.

And basically that is why S-traps are illegal. I hope you found this post helpful.

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