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Why Is An “S” Trap Illegal?

By August 14, 2020March 10th, 2021Plumbing
why is an s trap illegal

You may have heard about the “S” trap drain line or its cousin, the P Trap. But why is the S trap illegal?

The “S” trap is prohibited under the Uniform Plumbing Code throughout the United States. This is because the “S” trap will siphon or suck water out from the trap which will end up releasing methane (sewer) gases into the home.

So, it is a code violation and if you are doing remodeling, you will have to replace it.

What Is An “S” Trap?

An “S” trap is just like P trap, but the S trap has an extra curve that goes up and back down. (see the featured image of the post).

The final curve of the S trap points the drain line down rather than straight out compared to an example of a P trap below.

Example of a “P” Trap

p trap versus s trap
P Trap Versus an S Trap

When Were S Traps Installed?

S traps were commonly installed on older homes. You may still find S Traps today that have not been repaired, or were installed by homeowners/handy-people.

Can You Still Buy S Traps?

Why yes, you can!

You can still buy S traps from any home improvement store or even off of Amazon.

This does not mean you should use them however!

Do I Need To Replace My S Trap?

As long as you don’t mind the smell of sewer gases, then you don’t really have to replace your S trap.

However, you will have to replace an S trap if you plan to do remodeling that is permitted – as all remodeling should be! 😁

If home inspectors call out an S trap in your inspection, then keep in mind the seller does technically not have to bring the home up to code. So, you may have to budget to do repairs yourself.

How Do You Fix An S Trap?

Replacing an S trap with a P trap is a lot harder than you may think.

It typically takes a licensed plumber to do the job, unless you are handy. The reasoning is because a proper vent stack is not installed. In some cases, depending on code, you might be able to install air admittance valves rather than a vent stack.

But, you will still have to move the connection to the main sewer drain line of your home to be able to a P trap go straight out.

Why You Should Not Use S Traps


An S trap is prohibited under Uniform Plumbing Code. If you have an S Trap, you should consider hiring a licensed plumber to complete repairs so you do not smell sewer gases.

Have a question? Comment below!


  • Allie Jensen says:

    I paid $1000 to a well-known plumbing company in my area to fix the gas smell in the basement tub that a handyman had installed incorrectly. They had to dig through the concrete to get to the tub’s drain. One year later, I am having recurring issues with the floor drain in the same basement, and two different plumbers looked at the tub drain as they were trying to troubleshoot, and each said, “an S-trap? – interesting.” I asked the 2nd one who said that, ‘why is that interesting?’ He said because they are illegal.

    What is my course of action to take with the company that installed it? Is there any reason they could have done it legally? I still have the invoice, but it doesn’t go into detail of what type of trap they put in.

    • Aaron Shishilla says:

      Hi Ally,

      My recommendation would be to reach out to your local code enforcement/building official. They are responsible for permitting and ensuring work is completed by code in your area.

      Hope this helps,