How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

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Why is it a problem when a trap under a sink is installed backwards?

Monday, June 18, 2018

The U-shaped pipe configuration under a sink is called a “trap” because it traps and holds enough water after the sink has drained to create a liquid plug that keeps any noxious sewer gas from rising up out of the drain into your home. The pipe from the trap that extends into the wall is called the “trap arm”, and the whole assembly is named a “P-trap”, likely due to its sideways P-shape.

    The height of the water column that keeps the sewer gas out is called the “trap seal”. The bottom is at the “trap dip” and the top is the “weir”, which is the point on a trap where the water begins to flow down the trap arm, as shown above.

    The building codes set the minimum trap seal at 2” and the max at 4”. Less than 2” does not provide adequate protection against any gas pressure penetrating the seal and more than 4” will make the trap more likely to clog. 

    The outlet end of the trap pipe should be lower than the inlet when installed properly. The example shown has it backward, which creates a trap seal more than 4” and makes the drain more likely to clog—and that’s a long explanation for why a backwards trap is a bad thing. Also, no professional plumber would make that mistake, so it indicates  that the plumbing was done by a weekend warrior or handyman.

    But a backwards trap is still better than no trap at all, like in the photo below.

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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? 

What is the difference between green and white sewer drain pipes?

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?

What is the difference between water service pipe and water supply pipe? 

What are the pipes on my roof? 

• How can I find out what type of water pipe runs underground from the water meter to the house (service pipe)?

What is a P-trap?

Why is old galvanized steel water pipe a problem for homebuyers?

What does polybutylene pipe look like? Why is it a problem? 

• Which water pipes are an insurance problem and possibly uninsurable?

• Can you connect CPVC pipe directly to a gas water heater?  

     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. 

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