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What is a P-trap?

Friday, October 19, 2018

A P-trap is that U-shaped loop under each of the sinks in your home, between the vertical pipe from the bottom of the sink and the horizontal pipe that runs to the wall. It is only appears P-shaped when you turn your head sideways and is an often misunderstood plumbing requirement. But a P-trap serves important purpose: it acts as a liquid seal that keeps sewer gas stink from rising out of the drains.

   The depth of this water seal should be enough to keep sewer gas at bay, but not so deep that it impedes drainage from the sink. The translates to a minimum of 2-inches and a maximum of 4-inches of depth as shown below.

  Sometimes, when a homeowner or handyman does a sink replacement, they don't quite connect with the concept. The result is a configuration like the one in the photo below, where drain water sits in all the piping below the red lines all the time—which makes for a very sluggish drain. The amateur plumber also used a Sanitary Tee at the arrow, which does not direct the fluid flow around the 90º elbow as required.

    Another example is the photo below. Two traps are not better than one and create an easily clogged drain. This problem is compounded by the accordion type pipe that is used for the drain section that is directly under the sink drain, called a tailpiece. Although sold in hardware stores, it is not approved by any plumbing codes because the internal recesses collect debris that is not washed away by draining water as it would in a pipe with a smooth interior surface. 

   Next is an example of someone else who did quite understand P-traps. The arm from the right side sink drains directly into the side of the trap, resulting in standing water in the arm. Also, the trap seal exceeds the maximum 4-inch height because it is installed backwards.

   And, finally, the pipe from the trap to the wall below is called the trap arm. This configuration has a trap arm running uphill, which makes most its length become part of the trap and filled with standing water. Again, a recipe for a sluggish drain. 

   Although TV home-improvement shows sometimes make installing your own plumbing look easy, doing it right requires both training and years of experience. Repairs should always be performed by a licensed plumber. The plumbing inside your home is so critical that every building department in the United States requires plumbers to be licensed. 

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Here’s links to a collection of our other blog posts about PLUMBING PIPES:

How can I protect my pipes to keep them from bursting during a hard winter freeze in North Florida?

Can galvanized steel pipe still be used for new water lines in a house? 

How can I tell if I have cast iron pipes in my house? 

Why can't a sanitary tee be used for a horizontal-to-horizontal drain pipe connection? 

Is a washing machine drain hose required to be secured at the standpipe?

What are the abandoned pipes sticking out of the wall in my house?  

What are the code requirements for plumbing vent terminations?

What are the code requirements for layout of drain piping under sinks?

What causes a gurgling sound when a bathtub or sink drains? 

What is a "combination waste and vent" in a plumbing system? 

What is a building trap?  

What is a galvanized nipple?

What are the pipes sticking out near my water valves?

How do you accurately find a broken water pipe leak under the floor slab?

What is the difference between water pipe and sewage (waste) pipe? 

Are plastic pipes (PVC, CPVC, and PEX) safe for drinking water? 

Is a hot water faucet handle required to be on the left? 

What is a dielectric union? 

What's that powdery crust on the pipe connections at the water heater? 

If all the plumbing drains have water in them and you can still smell sewer gas, what's causing the problem?  

How can I tell what type of plumbing pipe I have?

Why is there a flexible accordion pipe under the sink? 

• What is the difference between PVC and ABS plumbing pipe?

     Visit our PLUMBING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles. 

Illustration by Code Check.

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