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What Are P-Traps?

In a P-trap, the curve in P-trap always retains water. Water is retained at the bottom of the curve — hence its name. This small volume of water prevents sewer gases from entering your bathroom, sparing your nose from nasty odors and protecting your health. P-traps are typically used to stop fumes from backing up through sink drains by venting them above or near the flood level of a sink or shower before they leave the building through a vent stack. P-Traps are often found near showers and bathtubs where wastewater from these fixtures drain into P-Traps for disposal into sanitary sewers. P-Traps are also utilized for other drain fixtures such as washers, washing machines, and dishwashers. P-Traps are used to keep gases from coming up into your home.

The P-Trap design was invented by Joseph Adam Hele in 1775 as part of his “Concealed Waste Preventer” waste plumbing system proposal for British Parliament which became known as the S-trap. P-Traps were not widely used until the 1880s. P-Traps are named as such because they “trap” a bit of water in the P-shaped section under the drain.

P-Traps work by filling with water and this small quantity of water stays put even when there is no flow going through it at all, preventing fumes from entering the room/home via air pressure. P-Traps prevent odors from getting into a home or building but can also be a source of odors themselves if they dry out and become stagnant

How Do P-Traps Work?

Lavatories, sinks and tubs utilize P-Traps to help prevent sewer gas from entering the home through drain openings. P-Traps are not meant to hold water under any pressure, unlike a u trap which can be pressurized by hot or cold water running or waste discharge. A P-Trap is made of two separate pieces of pipe that form an inverted P opening at the bottom, with each leg of the P pointing down and meeting underneath the drain opening so there is no room for wastewater to escape between them. The drain opening on most fixtures will have a horizontal section before connecting with vertical plumbing that will connect to the P-Trap. P-Traps are designed to hold a small amount of water by gravity, preventing sewer gas from passing into the home through drain openings. P-Traps are usually found under lavatories, showers and bathtubs where wastewater will drain away from anything that could pressurize the P-Trap.

Are P-Traps Still Required?

P-Traps are still required in most areas by plumbing codes because they work so well at what they do–keeping sewer gases out of your house! Some homes in dry climates don’t seem to have any problems with sewer gases unless there is an active bathroom for multiple uses in which case an air admittance valve can be installed between P-Traps to allow air into the system that would otherwise be restricted by P-Traps. P-Traps are so effective at keeping sewer gas from entering your home that some manufacturers have opted to eliminate P-Traps altogether, but the downside of these is that they can be noisy during operation and drain water will not remain in them without a P-Trap. If you want to install one of these P-Trap eliminators, make sure you know what you’re getting into first–it might end up being more trouble than it’s worth trying to figure out why they stopped working or stopped keeping the smells out!


P-Traps should never ever dry out causing a bad smell in your bathroom. P-Traps must always contain a small amount of water preventing all those foul odors from wafting through the vents and spreading throughout your home. P-Traps are the most effective way to ensure that sewer gases don’t end up in your bathroom, kitchen, or laundry room–and eventually in your living rooms and bedrooms. P-Traps are an often-overlooked component of a plumbing system that works hard to help maintain healthier indoor air quality.

Do P-Traps Require Maintenance?

Under normal circumstances, P-Traps do not require much maintenance unless you have had problems with drains backing up into your sink(s) or overflowing onto the floor below due to drain clogs of some sort. P-Traps do work hard at what they’re built for–keeping icky odors out! If you live in an area with water where deposits can build up, P-Traps can become clogged just like the drains they’re attached to. P-Traps are easy to clean out by gently lifting up on the drain plug of your sink, tub or shower and then pour a steady stream of hot water into the P-Trap until all debris is washed down and away.

Water that sits in P-Traps for extended periods of time between uses (vacations, long-term guests, etc.) can give off an odor so it’s suggested to empty P-Traps before any extended absence. Make sure to call a sewer line plumber to handle any of your P-Trap repair needs.

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