Full "S" Traps to the “Oh Snap” – It’s Prohibited in Mass! Why S Traps Aren’t Allowed

Did you know that full S traps, or just “S traps” for short, are not allowed in homes in Massachusetts?

It’s true!

If you have one or more S traps in your plumbing, they need to be replaced with a more modern “P trap” or a similar plumbing solution.

Why can’t S traps be used in Massachusetts, and what is an S trap anyway?

We’re glad you asked! Let’s teach you everything you need to know about S traps, how they work, and why they need to be replaced.

What Is an S Trap?

An S trap is a type of “plumbing trap.” All plumbing traps are meant to do the same thing. They consist of some kind of bend in the pipe, which traps some liquid in the pipe.

This liquid blocks gases, such as sewer gases, from entering your home. The gas cannot pass by the trap, since the liquid is blocking its path out of the drain, tap, or another fixture.

An S trap consists of two “U” shaped traps. One of the “U” traps is installed normally, while another is flipped upside down and attached to it, creating the shape of an “S” on its side – hence the name!

Why Are S Traps Not Allowed in Massachusetts Plumbing Codes?

S traps may not seem dangerous, but they are! They are now banned by many plumbing codes, including Massachusetts, Minnesota, and others.

The reason for this is that the “S” shape of the trap, combined with the fact that it does not have a vent, means that water can be siphoned out of the trap.

In other words, if enough water flows into the S trap, it will all get sucked out of the trap – which completely eliminates the fluid barrier that prevents sewer gas from escaping into your home.

That can stink up your home – but more importantly, sewer gases can be dangerous to your health when breathed in, or can even explode in some cases.

For these reasons, S traps are not allowed in Massachusetts.

What Should I Do If I Have an S Trap Sink or Find an S Trap in My House?

Don’t panic! You can contact a plumber in your area to assess the situation. There are a few different fixes you can look into.

Your plumber may recommend replacing the S trap in your plumbing with a P trap, which is allowed by the Massachusetts code, and may also recommend the installation of an Air Admittance Valve (AAV), which will allow the pipe to vent properly.


In some cases, however, it may be necessary to cut a hole into the wall and the ceiling to add the new P trap to your plumbing stack vent system.

This depends on the specifics of your situation.

Contact A Plumber Today If You Have A Full S Trap

If you have one or more S traps in your plumbing and aren’t sure what to do, contact an experienced plumber in Massachusetts right away, such as G&C Plumbing and Heating.

We can provide you with the guidance you need, and ensure that the recommended fix meets all Massachusetts plumbing requirements. Contact us online or call now at 508-541-8783 to get started.

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