top of page


Updated: Mar 2, 2022

First let's discuss what a plumbing fixture (sinks, tubs, showers, toilets)

trap is suppose to do. And that is simply to prevent nasty sewer gases

(such as methane gas, among other gases) from entering the building and causing harm to the inhabitants. That feat is accomplished by the trap under the plumbing fixture doing its job. Such as seen in the trap under the bathroom sink in the following picture. That type of trap is called a P trap and it is the proper type to use

A plumbing trap under a fixture will have trapped water (hence the word - trap)

at the base of the trap. When the base of a trap is sealed with water sewer gases cannot enter a building A correctly installed proper P-trap, such as the one shown here, will do its job of protecting the homes inhabitants.

Conversely, an incorrectly installed improper trap will not do its job and can cause serious health problems to persons residing in those properties.

Unfortunately for unsuspecting homeowners there are a variety of improper plumbing traps presently still in existence in many homes in Massachusetts.

The most common improper plumbing traps that I very often find on plumbing

fixtures during my home inspections are: S-Traps, Drum Traps and Accordion Traps.

Why the names (S trap) (P trap) and (Accordion trap) ?

The answer is simple - because if you turn a P trap sideways it looks like the letter p, the S trap makes the configuration of the letter s. And the accordion trap looks like the expandable bellows of an accordion.

2 Other improper traps, found mainly in older homes, that I rarely ever come across are a Top Vented Trap. and a Bell Trap.

The following illustration depicts 4 of these improper traps.

All of these plumbing traps are not only improper but are also illegal to be installed in the plumbing system. If found they all should be removed and replaced by a licensed plumber with a proper P Trap.


The reason S-Traps are not allowed is because when a fixture is draining the water in the trap gets sucked out of the trap. This siphoning of the water out of the trap creates an opening for sewer gases to filter back up and out of the plumbing fixture and enter the living spaces of the property. In homes with improper plumbing traps, such as S-traps, the aftermath is often an obnoxious sewer gas odor, along with a host of physical ailments.

The following illustration shows the difference between a proper P- trap and an improper S trap.

S-traps can come in a variety of forms. Some are full S-traps while others are only partial S-traps. It really doesn't matter what shape the S takes it still is illegal and needs to be replaced. The following illustration shows what shapes S-traps can take.

The next illustration shows how water is siphoned out of an S-trap versus how it is not in a proper P-trap.


Drum traps are the 2nd most improper plumbing traps that I find during my home inspections. The reason that they are not allowed to be used and that they are illegal is because they are not self-scouring.

By self-scouring it is meant that due to their design the waste that enters them sits at the base of the trap and over time can accumulate to the point of blocking and preventing that trap to properly drain.

THE ACCORDION TRAP (slinky trap)

While both S-traps and drum traps have been around for a long time the newest edition of an improper plumbing trap is the flexible accordion trap, or as a I like to call them - slinky traps.

These flexible plumbing traps are a major problem for fixture drainage.

Due to their zigzag design they allow for the collection of grime, hair, dirt and other small items to get caught up inside the drain. In addition, compounding the problem is that the zigzag interior of the drain slows down the flow of water.

So if you want to create blockages in your drains - just install these slinky traps.


In addition to those noted improper plumbing traps I also occasionally find in older properties another type of trap which I call a running trap or

a U-trap (because of their u shape)

These old cast iron sewer drain traps are found in the basements of older homes. Because of their inherent poor design they are exceptionally prone to blockages. When inspecting older properties and I find these ancient warriors I always, without any hesitation, recommend that a licensed plumber further evaluate with the likely need for some expensive sewer drain replacement costs.

The following illustration shows the full length of what a running trap (often called a building trap) looks like.

The next trap picture shown below is what I call a "W trap"

Let's say that the "w" can stand for the word worst.

In reality there is no such thing as a W trap. What you are looking at is the creation of some moron that hasn't the foggiest idea of what a plumbing trap should look like and what a trap is suppose to do. What you see in this picture is the double whammy of a twin S-trap combined with an accordion drain.

I ask you this simple question, can anyone possibly conceive of a dumber way to screw something up? The answer is - I don't think so!


Wait a minute - I guess I was wrong when I said that it couldn't get any dumber.

Just take a look at the 2 following pictures.

The top picture shows an S-trap that was installed sideways. If you want to makes things worse than they already are - do this!

The picture under it shows a basement sink with an attached toilet flange connection. Was the installer planning on connecting a toilet to the sink?

Don't ask me why this oddball connection - since I haven't the slightest idea.

No Trap

Although plumbing fixtures by code are required to have a plumbing trap, sometimes we come across a fixture, such as seen in the following picture, that have no trap.

After reading this post you now know what is going on with this particular sink.

And that is sewer gases are wafting up their odious noxious scent into the location of this plumbing fixture, providing the inhabitants with potential illness!

At this juncture in this post you may be asking yourself what's the point to all of this trap talk. Although some casual mention was made of potential health concerns with improper plumbing traps - what's the big deal!

Well the big deal is that sewer gases are composed of highly toxic substances,

including (hydrogen sulfide, ammonia, methane, carbon dioxide and nitrous oxides)

Even at low levels of substances, such as hydrogen sulfide - these toxic fumes are very dangerous. Exposure to these fumes can result in headaches, fatigue, sinus infections, bronchitis, dizziness, memory loss, nausea, difficulty breathing, pneumonia, and most importantly - long term exposure can be fatal!

Hopefully this brief treatise on plumbing traps (Especially Improper Traps)

has enlightened you and now you have a relatively good understanding of this important plumbing component

Finally, when it comes to plumbing traps and all of the other building systems and their components there is no better solution to making sure that your dream home does not have any nightmare plumbing issues, or for that matter, any other problems is by having a qualified, competent licensed home inspector inspect not only the plumbing system but the entire property for any possible unexpected home buying - (pardon the pun) TRAPS!

I'll end this brief excursion into the realm of plumbing traps with a visual reminder of what a proper plumbing trap looks like and what it does.


502 views1 comment

Recent Posts

See All

The Perils of Basement Finishes

Over the decades, I've seen many finished basements - and many problems resulting from those finishes. Many basements are finished with sheetrock covered walls and ceilings - not only making the foun

The Potential PERILS of a Finished Basement

While inspecting a home, the basement is one of the most important areas to inspect. While there are many issues that can occur in basements - moisture, water entry, or flooding are one of the bigge

1 Comment

Rated 0 out of 5 stars.
No ratings yet

Add a rating
Apr 21
Rated 4 out of 5 stars.
bottom of page