Weir-d Science, how a P-trap works

This picture post will attempt to show how a P-trap functions, or more importantly what makes them function.

Notice how the weir can see the vent opening?

P-traps have for the most part replaced most other kinds of traps—notably the S-trap.  The toilet is one notable exception—it is designed to be siphoning—the more siphoning the better.  You want to hear that glug glug glug.


Many inspectors are under the false impression that merely lengthening the trap arm will eliminate the S-trap, but what this actually does is increase the potential for siphoning of the trap as the slug of water attempts to go down the drain.  It is a similar principle to how a toilet siphons.

Still an S-trap

The configuration still makes an “S,” just an odd shaped S is all.

What makes the P-trap function is the vent, and the rules that guide where that vent has to be. 

Essentially the vent has to be at least two pipe diameters away from the Weir.  The Weir is where water either starts to go down the drain after the trap or stops going down the drain.  So in a typical 1-1/2″ kitchen sink drain that would be a minimum of 3″ away.

The P-trap rules

We also have to be careful to not get the vent too far away from the weir as the trap arm could flood and block the vent–essentially returning the assembly to more of an S-trap. 

The rules of how far away it can be has to do with the diameter of the pipe (trap arm).  The bigger the diameter the longer the weir can “see” the vent.

Charles Buell, real estate inspections in Seattle

This is just weir-d!

One of the things I like to do in my Inspection Reports, when writing about concerns involving water leaks, is to give a rough “visual idea” as to what the issues are by doing a color “overlay” of the issue.  Whether it is suspected water behind the tiles of a tub/shower enclosure, water under the vinyl floor around a toilet, or an improperly installed sink trap, I think these pictures help get the point across—-and worth the few seconds it takes me to do it.

The particular defect that I am going to discuss today, of an improperly installed sink trap, is sometimes difficult to convey to the buyer just why the installation is wrong—-after all—-water is going down the drain.

This first picture shows a normal trap and the overlay shows the amount of water that is in a properly installed trap.  Anyone that tries to explain what a “weir” is appreciates how useful pictures can be.  The small amount of water in the trap is easily pushed over the dam (weir) and down the drain.

The normal amount of water in a p-trap

The normal amount of water in a p-trap

This next picture shows an improperly installed trap with an overlay that shows the amount of water trapped in the pipes. (Note how the water level actually keeps the disposer on the left flooded.

TOO much water in the p-trap

TOO much water in the p-trap

From the picture you can imagine the extra force necessary to push all that water out of the trap. The result is poor drainage with the likelihood of clogging over time.  Sure it will still work—-just not as well as the first picture. 

Seems like it is not so weir-d after all.


By Charles Buell, Real Estate Inspections in Seattle

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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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