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

Updated: Jan 19

Closeup of a sink S Trap made of white PVC Plastic
Closeup of a sink S Trap made of white PVC Plastic

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