The Purpose of a Drain Trap

If you look below your kitchen or bathroom sink, you will notice a U- or S-shaped curved pipe coming down from the drain opening—this is the drain trap. That may seem like a strange name for a plumbing part until you understand the function of this part. There are different types of drain traps for various purposes, but p-traps are the most common. These are used for plumbing fixtures like sinks and showers

In this guide, you'll learn how a trap works, discover maintenance tips for drain traps, how to deal with a clogged toilet drain trap or sink trap, and learn how to troubleshoot smells stemming from the drain.

How Drain Traps Work 

A plumbing drain trap is designed to retain a small amount of water each time the sink drains. This water standing in the bottom of the curved portion of the trap seals the drain and keeps sewer gasses from escaping the drain and entering your home.

There are traps in every drain because any connection that leads to the drain system is also a possible outlet for sewer gas. Even your toilet has an internal trap shape to its porcelain configuration that serves exactly the same function. 

Drain Trap Odors

Whenever you encounter a strange odor in any room where there is a drain, always make sure that the trap is not dry. If a drain trap is dry, the sewer gas can escape and cause the smell. This is usually a quick fix that can be remedied by running water down the drain and filling the trap back up with water.

Sewer gas is hydrogen sulfide, which is created as organic waste decays. The smell is mostly an annoyance; although it's possible that a very high concentration could cause health problems, it is not very likely.

How to Locate a Drain Trap

Drain traps are located in nearly every plumbing fixture throughout the home, including toilets, showers, sinks, tubs, and washing machines. They're a small but critical piece that keeps these fixtures functioning smoothly.

• Toilet: The standing water in your toilet indicates the presence of a drain trap. If you look to the backside of the toilet, you can see the curved shape of the drain through which the water exits.
• Sink: The drain trap is usually hidden in a cabinet under the sink. When you look at the sink, you cannot see the standing water, but if you follow the drain lines, you can see the required U or S shape where the water is held to block the sewer gas.
• Showers and tubs: Tubs and showers have traps that are harder to get to and either require crawling under the house or cutting a hole behind the tub or shower and digging out the area where the trap is located.
• Washing machine: Washing machine drain traps are mostly in the wall, and to access them you usually have to cut into the wall as well.

Types of Drain Traps

There are several types of drain traps that are used depending on the type of plumbing fixture. Here are a few of the most commonly used types as well as how and why they're used.

• P-trap: A p-trap is used in tubs, showers, and sinks, and can be used to trap solid waste and prevent sewer gas from escaping.
• S-trap: S-traps are most commonly used in older buildings and have been phased out in most applications. In an s-trap, water flow creates suction that prevents sewer gases from escaping. However, if there is a sudden rush of water, that suction is rendered ineffective and the gases can escape.
• Q-trap: A q-trap and a p-trap function in the same way, yet q-traps are typically used only on upper-level floors.
• Bottle trap: Bottle traps are typically not used in modern buildings where they may not be up to code, but they are used with sink pies to filter waste and prevent odors and insects from escaping.
• Intercepting trap: An intercepting trap is a trap that separates the house drain from a street sewer. It serves to intercept any contaminants.

Drain Trap Maintenance

All drains should be used at least once every couple of weeks to keep water in the traps. This includes showers, toilets, tubs, showers, bathroom sinks, kitchen sinks, washing machine drains, and more. It won't hurt anything to let the trap dry out, but you may get some unwanted sewer gas smells creeping into your house.

When going on vacation or traveling away from the house for long periods, some people stuff drain openings with plastic wrap to keep sewer gases from invading the house while the fixtures are not in use. If you choose to go this route, a toilet can be shut off so you can drain your home's plumbing system and the toilet bowl can be loosely stuffed with a plastic bag.

Don't forget floor drains and shower drains; placing a sheet of plastic over the drain and weighing it down will seal the drain if you are gone long enough for the standing water to evaporate out of the trap.