Why S traps are illegal in drains

Are s traps illegal? Why? You may have heard S traps are illegal, without explanation. Or maybe you’ve wondered why you have to make a P trap under your drain when an S trap would be easier. Here’s why S traps are illegal in drains.

S traps are illegal because they cause a health and a safety hazard. But the conditions under whether you have to replace them do vary.

Are s traps legal?

why are s traps illegal
An S trap under your drain can be dangerous, so that is why they are illegal in most jurisdictions now.

The trap is the bend in the pipe under your drains. They resemble the letter P or the letter S, so that’s why they’re called P traps and S traps. A trap’s job is to keep toxic and flammable sewer gases out of your house while letting your wastewater through. And today we understand that S traps just aren’t as good at that as P traps. Having a poorly functioning S trap in your house is a lot like having a leaky natural gas fitting. It can cause a fire or an explosion.

Today, P traps are the most common type of trap. S traps are illegal in most states and countries. So the answer to the question of whether s traps are legal is no.

That doesn’t necessarily mean you can’t buy one. You can buy flexible accordion drain pipe and flexible p traps too, but that doesn’t mean an inspector will give you a pass if they see one. It’s odd that you can buy and sell these things legally, but you can’t use them legally. But that’s a quirk of our legal system.

The reason why S traps are illegal

P traps have a horizontal run that goes to your drain and to your vent, in the roof of the building. S traps make a bend and then run straight down. No vent. That’s the main reason why S traps are illegal. It’s impossible to vent the drains.

Why is venting the drains important? Because S traps don’t work as well. They keep most of the sewer gases out by siphoning your wastewater into the sewer system, but a small amount of sewer gas does come into your house because of the siphoning effect.

The vent prevents any siphoning by equalizing the pressure, allowing waste water to go down the drain while keeping the sewer gases somewhere other than in your house.

Do you have to remove an S trap if you have one?

Now in some jurisdictions, an S trap is okay if it’s old work and you haven’t touched it. That’s the case where I live, in St. Louis. Or at least that was the case in 2011. I had a contractor vent the drain and he did it incorrectly, so I failed inspection. The county required me to bring in a licensed plumber to redo the work correctly. The licensed plumber told me if we’d left the drain alone, we would have been fine.

So it pays to call your jurisdiction’s building inspection department and ask what the local rules are regarding venting drains and an existing, unvented drain. And then have a licensed plumber do the work. There’s a lot of plumbing work that a general contractor or handyman can do, but don’t make the mistake I made. If it involves venting a drain, get a licensed plumber to do it. That way, you don’t end up having a delay and paying to do the work twice.

How will they know if you’ve touched it, you ask? By looking at the materials. Modern building materials have date codes marked on them. Not to mention that building materials have changed over time. Metal pipes were the norm when S traps were legal. So if the inspector sees an S trap made of PVC pipe, they probably won’t even bother looking for the date stamp.

Alternatives to venting to the roof

If venting to the roof is impractical, you can install what’s called an AAV, or air admittance valve. You replace the S trap with a P trap, then put the AAV above the horizontal run and over where the lower leg of the S trap used to be.  But the caveat with an AAV is it has to be six inches above your water faucet, which isn’t how most people use them. Most people install them under the sink.

You can also install a dual loop drain as an alternative to conventional venting. But this requires a permit, so have a licensed plumber pull the permit and do the work.

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