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  • Joseph & Michael Scaduto


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.