How do P traps work?

Asked By: Xueqin Hapov | Last Updated: 7th January, 2020
Category: home and garden home appliances
4.3/5 (52 Views . 22 Votes)
The p-trap traps water in its dip, sealing off the pipe. As water flows down the drain, the water level rises and flows down the drain pipe with accompanying waste to the sewage system. This water eliminates space for any air or gases to flow back up through the drain.

Click to see full answer


Similarly, are P traps required?

(1) Traps required. Each plumbing fixture, except listed toilets, shall be separately trapped by approved water seal “Ptraps. The “Ptrap must be installed at the center fixture when three such fixtures are installed. (3) Prohibited traps.

One may also ask, what's the difference between an S trap and P trap? The only difference between a P-trap and an S-trap is the horizontal length of pipe on the outflow side which allows you to connect the vent. You can turn an S-trap into a P-trap by adding this pipe -- it should be at least 4 inches long -- and connecting a vent to it.

Then, how far below drain should P trap be?

According to the International Residential Code, the maximum vertical distance between the sink drain and the entrance to the p-trap is 24 inches.

What does the P stand for in P trap?

A P-trap gets its name because it combines two 90 degree joints with a horizontal overflow pipe and gives the entire unit the shape of the letter “P.” One of the 90 degree joints exits the drain of the sink and then is joined to another which contains a water seal system that allows for water to flow into the overflow

34 Related Question Answers Found

Does every P trap need a vent?

3 Answers. All traps need venting in all cases. Without a vent, the water seal in the trap gets sucked out and sewer gases can enter the room.

Why is an S trap illegal?

The purpose of a trap is to prevent sewer gases, and possibly vermin, from coming in to the home. Back to “Straps – The reason “Straps aren't allowed is because they have the potential to suck, or 'siphon', water out of the trap as the water flows down the drain.

Do all showers have P traps?

According to regulations, all shower drains are required to have P-traps. This includes all fixtures connected to a building's main plumbing system. P-traps are curved pipes installed under showers and drains of all kinds.

Are all P traps the same size?

When replacing a P-trap, you should use the same type of trap you remove. Traps are made from ABS (black), PVC (white) or brass (either chrome-plated or natural colored). Traps come in 1 1/4 inch (standard bathroom sink) or 1 1/2 inch (standard kitchen sink) inside diameter sizes.

Can P trap be lower than drain pipe?


You attach the p-trap directly to the drainage and manuever the p-traps exits into you existing drain. It is not ideal to have the p-trap below the the exit drain because water gravity has to force the water out instead of it flowing downwards naturally.

Where are P traps required?

Sewer gas may not only have an offensive odor, it may include methane, which is potentially combustible. P traps are built into toilets and are required in plumbing drains under State building code. If you smell sewer gas inside a home or other building, a faulty or dry P trap may be the cause.

How far can vent be from P trap?

909.1 Distance of Trap From Vent
SIZE OF TRAP (inches) SLOPE (inch per foot) DISTANCE FROM TRAP (feet)
11/4 1/4 5
11/2 1/4 6
2 1/4 8
3 1/8 12

Can P trap installed backwards?

Take for example this P-trap connection at a Garbage disposal. At first glance it looks almost normal, when actually it is installed backwards. Well eventually the trap will clog up with debris and water will just not drain properly.

Are flexible P traps good?


P-traps take on and maintain the shape of a letter p to keep sewer gases out of your house. When you use a flexible p trap, you're more likely to get the shape wrong, and you're more likely to accidentally misshape it when you bump the pipe.

What size is a shower P trap?

The usual size for a shower drain trap is 2 inches, which is the diameter of the trap. This size is the minimum size that is allowed, which means that you cannot install a trap with a smaller diameter, or the trap is not within code.

How long can a sink drain pipe be?

The total fall in a fixture drain resulting from pipe slope shall not exceed one pipe diameter, nor shall the vent pipe connection to a fixture drain, except for water closets, be below the weir of the trap. So if you're using 2" PVC, the drain must be less than 8' long horizontally.

Can two sinks share a trap?

Speaking from personal experience, only use one trap in a dual sink bathroom. With two traps, you can get air pockets blocking one or the other's drain. Normally, these double tees are used for adjoining bathrooms, so you can have a sink on each side of the wall, and share the same stack.

Does every drain need a trap?

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.

What is a trap in a toilet?


A toilet trap, or trapway, is a curving channel inside the base of a toilet that leads from the hole at the bottom of the bowl to the drainpipe. When a toilet backs up and the clog did not occur in the drainpipe, the stoppage usually exists in the trap.

Do all drains need a trap?

This water creates an air seal that prevents sewer gas from passing from the drain pipes back into the building. Essentially all plumbing fixtures including sinks, bathtubs, and showers must be equipped with either an internal or external trap. Toilets almost always have an internal trap.

What is S trap used for?

The mechanical purpose of the trap is to hold water to prevent sewer gases containing viruses and bacteria from entering the home through the drain. With an “Strap drain, the trap is easily siphoned dry with each use and the water seal no longer exists.