P-traps—installing them properly

I don’t go out of my way to be a nit picker as a Seattle Home Inspector—-sometimes it just happens.  I think the kind of nit picker that most agents have a problem with is when those seemingly unimportant items end up in the “Summary of Significant Findings” section of the report.  This is “bad form”—-as Captain Hook would say.  While this kind of information should be mentioned in the context of general “information” about the home, placing it in the Summary gives it “weight” that it doesn’t deserve.

Take for example this P-trap connection at a Garbage disposal.

Improper P-trap

Improper P-trap

At first glance it looks almost normal, when actually it is installed backwards.

With the next couple of pictures, I will attempt to explain what is wrong with this installation.

Traps are engineered to be self scouring (cleaning) by virtue of their shape.  Note how in the first picture below the flow of water enters from the sink (disposal in this case) at Point C and the blue arrow.  The force of the flow of water accelerates when it hits the bottom helping it flow up and over the hill and down the drain at the left blue arrow.

In the bottom picture we can see that when the trap is installed “backwards” the distance labeled “B” is much greater than distance “A”—–a much higher hill for the water to get over.  It also doesn’t have the help of acceleration provided by its being installed the other way.  The flow of water is actually reduced because the flow of water runs into a more vertical wall in the lower installation.

Improper P-trap

What the P-trap should look like

So what will happen if this is not fixed?  Well eventually the trap will clog up with debris and water will just not drain properly.  It isn’t “typically” a difficult fix (although there are instances where it might be fairly difficult)—something any plumber can do when they are at the home for other reasons or something that even a knowledgeable homeowner or other qualified repair person can fix.

It just isn’t the sort of “deal killer” or “safety” issue that warrants placing it on the summary.


Charles Buell, Seattle Home Inspector

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  1. Daniel Rogers says:

    Also the tail ends always go inside the receiving piece in the direction of the flow.
    I put all defects, big or small, in the summary because agents don’t look at or print out the main report. I put significant concerns at the beginning of report

    • Charles Buell says:

      Daniel, with my reports I just don’t do that. It would just make the summary cumbersome for no reason. Only safety issues and defects of “significance” make the summary—along with a carefully worded warning to read the whole report because there “will” be things in the body of the report that the client would also like addressed. I stress that the summary is my opinion and should be added to as they deem necessary.

  2. Great explanation, great photos. I’m going to start linking to this blog post in my inspection reports.

    • Charles Buell says:

      Thanks Reuben. I was just thinking about you. Took a picture on my morning walk of a house with zinc strips—the picture is coming your way soon 🙂

  3. Isaac Toko says:

    Hey Charles,

    Great info. I was wondering if you had any knowledge, since you are a local inspector, of any areas around the Puget Sound that would be susceptible to cases where there were no p-traps installed at all? I’d expect you’d come across a ton of homes that are older that have some interesting plumbling practices. I work with PSE as a emergency responder for natural gas emergencies and am trying to get information for our area where if there was a natural gas odor in the sewer/storm drains if certain houses of certain ages would be more susceptible to infiltration into the house. I’d really appreciate your help.

    • Charles Buell says:

      Isaac, all homes are going to have some means of trapping against sewer gases. Some homes have no trap on the main drain itself from the home. When this is the case any trap that is not used for a long time can dry out and sewer gases from the city sewer can then enter the home when the house is under negative pressure. Quite common really.

  4. Guadalupe Sabillon says:

    I have a question about if a p trap can be placed at 1 m of the sink? It would work the same way? Or is neccesary to be right below the sink?

  5. Actually this configuration is acceptable. Per codes, a trap has to be a minimum of 2 inches in-depth and a maximum of 4 inches. Its not the acceleration of water that makes it overflow into the drain, the water itself seeks level. A riser off the trap into the sink is better off shorter than longer. Any thing over 2 ft, per code, a trap could lose its seals.

    Master plumber

    • Charles Buell says:

      Brian, since “B” dimension in the picture is more than 4″ is is “technically” wrong. Also the engineered shape of the trap does in fact increase the speed of the water through the trap to aid in the trap being “self scouring (cleaning).”

      • Rocket Gal says:

        The near vertical drop allows the waste water draining from the sink to enter the trap with a lot of speed so that the trap self scours. The shape of the p-trap when installed this way takes advantage of the brachistochrone property of the cycloid shape. If you install the p-trap backwards you don’t get the velocity. The trap may/will work but not as designed.

  6. Many thanks for your help. I had a drain problem at my kitchen sink, and your information saved me a plumber call.

  7. Joe Coletta says:

    Because of the location of the sink on the right with a garbage disposal relative to the second sink, a 45o pipe from the garbage disposal causes the y-connector to be lower than the drawing on the right in which my a is the same distance as your a. My question is… How larger can b be Thanks!

    • Charles Buell says:

      Sorry, I actually don’t know the answer—but I would think not very much. The deeper it is the more difficult to flush the trap.

      • Joe Coletta says:

        Mr. Buell,
        After receiving your response, I spent much more time researching the question than I should have. I should have spent my time wisely and taken my kids to the movies.
        I did however find two prevalent requirements in local ordinances and state codes: (1) A distance that allows the trap to easily flush and not permit the release of waste odors (My ignorant interpretation of complex legal jargon.) , and (2) 2-4 inches. Although most plumbers offering online advice recommend less than 3 inches.

  8. thom dege says:

    My P trap on my washing machine discharge pipe appears to allow terrible sewer smell
    into my house after a load is washed. I checked the pipe size (2″) and the p trap height difference is only 2″. could the washer machine be providing to much force and not allow any water to stay in the P trap ? Please help

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