The unsung hero of your sanitary drain system is no doubt the P-Trap. An inexpensive device found on every drain except the toilet in your home or, business that serves as a barrier between your nose, the foul gases, and, diseases that lurk in the sewer system of your town, or, city.
The P-trap is a “U” or “J” shaped bend that holds water in that shaped section providing a liquid seal between you and those sewer gases. When water goes down the drain the excess water is water over the dam so to speak allowing the drain water to enter the sewer system without the seal ever breaking.
In the past less effective methods were used to provide the seal. Those methods included bell traps, s-traps, drum traps, and a few others. These all had limitations in the sense that they either failed to provide good reliable drainage, were difficult to clean, or, were subject to the trap siphoning due to not being vented properly because of the design. In 2003 an outbreak of SARS in China was traced to ineffective traps.
A common mistake by DIY’ers is to not install the drain pipes under the sink
properly, or, to remodel a kitchen installing a deeper sink and garbage disposer without paying attention to the height of the drain in the wall. This results in the trap being installed in a manner that alters the depth of the water seal on the trap. This results in poor drainage, odors, and possible early failure of the disposer. The depth of the water in the trap is a set depth and must not be altered..
Today’s p-trap properly installed and vented is very reliable at draining and provides excellent sealing. There are rules at to how it is installed and vented. Generally the wall arm coming out of the trap is horizontal with a pitch of 1/4″ per foot minimum downward into the wall. Once inside the wall it can either turn down into a vertical drain or continue on a pitched horizontal run. However in any case it must have a vent before the drain turns downward. There are also limitations of the total developed length of the horizontal run before having a vent. Consult your local code so your installation will meet the code in your area. Failure to properly vent the p-trap can cause siphonage of the water out of the trap allowing sewer gases to enter your home.
When your sink becomes clogged it is a good idea if you can remove the trap and inspect it and the pipes from the sink drain to the wall to see if the blockage is in any of those pipes. Use a bucket under the trap to catch any water remaining in the sink and drain pipes to keep the mess under control. If the blockage is not in those pipes but into the wall you would probably be best served by calling a plumber to clean the drain line.
What’s a ptrap again? I really didn’t get it, can you explain it again please?
The “un sung hero” until it leaks of course and then it can be the hero once again when the plumber writes the bill !
Just kidding but if you think about it the P trap probably is one of the most repaired things
Thanks for the great article!
Your welcome!
Of course most of them go for years and years without any problem at all.
I firmly believe most of the ones that have problems are the result of a bad installation.
In the decades past I have been plumbing, the brass p-trap has had the most problems. I have seen more brass ptraps with rotten holes in them than any other material. Stick with a good solid plastic trap and it will last forever, you can’t argue that!
Sometimes I wish all clogged drains were at the trap. Would make my job quite easy! The hardest ones to maneuver through are the old type brass ones that have an abrupt short turn. Hard on the cables as well.
Those old tight brass & galvanized ones can be a bear!
Throw in a cleanout plug at the bottom that won’t come out and the fun really starts!
I’m glad I read this article as well! We didn’t think about any of the plumbing when remodeling our kitchen, and now I’m worried that we committed a serious error. We bought a sink that is about 1.5″ or 2″ deeper than our previous sink. Now the disposal sits lower than it used to. What exactly are the critical things I need to measure from the outlet of the disposal to the pipe in the wall? Isn’t everything okay as long as the outlet of the disposal is at the same height, or higher than that of the pipe in the wall? I’d better go and double check this stuff.
If the outlet of the disposal is *lower* than the pipe in the wall, then it seems that the disposal will always have water trying to get pushed in from its outlet. If this is the case, do they make disposals that can deal with this situation, or will I need to get a plumber to come in and repipe that section of my kitchen? Am I totally hosed?
Thanks!!!
Hi Dave,
The key is that no matter what the depth of the trap seal that is shown above in the picture can’t be altered.
Turning the trap bend backwards, or using flex trap, or other fittings to make up a deeper trap bend are bad news.
When the water remains flooded in the pipes the drainage can be slower and the drain more susceptible to clogs.
If the disposer is flooded then things can get a little foul and the service life of the disposer is reduced.
A picture to see what you have would be great if you can get one. info411plumb@gmail.com
Sometimes it’s just a matter of piping things differently other times things have to be changed inside the wall.
Even changing things inside the wall is not the end of the world.
If the drain pipe in the wall is vertical it can be fairly easy to accomplish.
If there is a horizontal arm then it’s more difficult.
This error is actually very common with new cabinets and counter tops from the large home centers.
It would be nice if they brought this detail to peoples attention before the installation.
It would make the life of the DIYer or plumber much easier.
Redwood
Ok, I’ll try to get a picture in a day or two. I’m actually not attacking this project for another week or so, and nothing right now is really connected. When I have time, I’ll put on the disposal and we’ll see how things look. I’m getting a little concerned here.
Dave,
What make and model disposer do you have?
Some are taller than others from the sink bottom to the disposer outlet.
Redwood
Hi again, sorry for the delay. I’ve got two pictures, each of a different bathroom sink. one seems to have stopped leaking, but I want to fix it again anyway. Please let me know if you see anything other than the obvious misalignment!
http://www.mediafire.com/?yznjowygzzm
http://www.mediafire.com/?gyiy02wjkym
Hi Dave,
The sink #1 picture Darn thats so close to making it into alignment. If the vanity was 1/2″ to the left it would have been perfect. There is something staining on the pop-up tailpiece just above the p-trap on the left is that a leak from above?
Other than that all I see is a alignment problem.
Sink #2 what I would do there is put the bend on the pop up tailpiece all square and pretty. Then play with the length of the p-trap outlet so it perfectly rests into the joint of the J-bend. Once that is set hook it up with the nuts.
Also look and see if any of the beveled connections have gotten cracked with all the action they have seen. usually a cracked bevel joint will leak worse when you tighten it more.
I installed the disposal and dishwasher today — believe it or not, no leaks! Thank goodness the disposal still sits a couple of inches over the drain, even though I bought way too deep of a sink. Phew.
Hopefully, I’ll have time to redo the bathroom sinks tomorrow.
Home builder installed 4 inch p-trap with vent and septic tank. The p-trap has a history of backing up although septic system has been pumped and line leaving the trap is clear. Waste collects in the p trap. Can’t find any web info detailing the correct installation/pitch for the p trap.
Hi Klebrun,
I’m going to gaze into my crystal ball and guess that you live in New York state.
Its one of the few places on this world where those nightmares are required.
What you have is a “House Trap” and is no longer required under many codes except yours.
The drain line and trap should maintain a pitch of 1/4″ per foot.
Which basically says the outlet should be about 1/4″ lower than the inlet.
That said I will now tell you that many people have house traps and do not have problems with them.
I recently did a job for a customer that had lived in the home for 50 years without a problem until recently.
I properly cleaned the house trap and he probably is good for another 50 years.
House traps when they have been installed for some time accumulate debris at the bottom of the trap and a sludge forms that eventually completely blocks or catches something that blocks the trap.
In this case the drain cleaner should push a spinning shake with a 3″ diameter blade on it through the trap to scrape the sludge off the trap walls and then vacuum the debris out of the trap with a wet/dry vac.
With you talking about the builder I’m guessing that your installation is newer and sludge buildup is not the problem.
A few things that may help is switching toilet paper brands to one that falls apart once it gets wet.
Some brands such as Cottonelle and Charmin are very thick and stay intact all the way to the septic tank without disintegrating. while other brands are more septic friendly and will probably disintegrate as they pass through the trapway of the toilet or a short distance down the drain pipe.
Another nemesis of house traps is the old grease down the drain…
Even if you are very good, small amounts of grease will emulsify with hot water and detergent while doing dishes and go down the drain. When the waste water containing the emulsified grease hits the cold water in the trap the grease immediately solidifies and floats across the top of the water sticking in place. Eventually the grease build up will clog the house trap. Of course grease in large quantities will clog the trap immediately.
Hopefully these tips will help you live with a house trap.
Of course if there is a mechanical problem with the house trap such as pitch problems, or, misalignments they should be corrected.
Here in areas where we are allowed to remove house traps we often find it to be a simple operation of just putting in a straight piece of pipe and perhaps a clean out where the house trap was connecting with shielded couplings like a Fernco Proflex, Mission coupling, or, No-Hub coupling.
Hopefully this information helps you out,
Redwood
Hello,
I was happy to come across this thread. I have a problem similiar to Dave M’s regarding the placement of the garbage disposal outlet after installation of a new sink.
I had a new countertop and sick basin installed. Now the drain hole in the sink is both lower and closer to the back wall. This new placement results in the outlet from my garbage disposal is only inches above the inlet for my new p-trap. Maybe 2 inches at most. This is the physical hole in the garbage disposal and not the end of the disposal drain pipe. This leaves little room after the bend in the garbage disposal outlet pipe. I may not have enought room to place a compression nut on the garbage disposal outlet where it meets the p trap inlet. If I could drop the p trap 2 additional inches, it would make life much easier to fit things together. But after reading this thread, it sounds like this is not a good idea.
I am also installing a new p trap because the old p trap outlet (90 degree connection) was directly underneath my new sink drain hole. The center of the garbage disposal needs to be in location. The drain pipe is horizontal to the floor as it goes into the wall. I have about 4 inches of drain pipe between the wall and the outlet connection of my new p trap. There is not a vent on this drain and the house is 10 years old.
Can you please provide some suggestions?
Thanks
Hi Al,
You do not have to install the disposer with the drain facing straight back towards the drain in the wall.
You may use the swing of the trap to make the connection, and you may also come out of the wall with an elbow so the drain is offset to one side.
That said, if the drain in the wall is too high I would recommend opening the wall and installing the drain lower rather than rigging the trap and risk the water level of the trap flooding the disposer where it will cause drain odors, poor drainage and a shortened service life of the disposer.
Thanks for posting,
Redwood
Redwood,
Thank you for the quick response. Great tip to use an elbow, where the pipe comes out of the wall, to offset to one side. I have done some additional measuring and it appears that the center of the outlet hole in my disposer is only 1/4 inch higher, at most, than the center of the outlet at the wall.
I’m going to follow your recommendation to open the wall and install the drain lower to solve my problem.
Thanks again,
Al
Hi – my question is about a bathroom. I replaced the single handle faucet with a double – no problem. When I replaced the pop-up drain mechanism, I discovered that it leaves about 1/4″ gap so that I cannot connect the p-trap to the pipe leading into the wall. The p-trap cannot go further up the pop-up drain b/c it is stopped by the white ball and stick that extends into the pop-up itself.
Do they make an extension for between the p-trap and the wall pipe so that they can connect? Or would that compromise the performance of the p-trap?
Your help would be greatly appreciated! Thanks.
Hi Michelle,
If I understand you correctly the height of the pipe is correct but it is too short?
If this is the case a new p-trap may have a longer outlet tube that has not been cut to fit the old piping.
It is also possible to use and extension tube to make it longer but if the new trap would make it work I would go that route before creating an additional slip joint in the mix.
Redwood
Hi Redwood – thanks for the prompt reply!
Let me try to re-explain the situation: the faucet I bought is a standard Delta home fixture – came with the pop-up drain included. I removed the old pop-up drain in the sink and slid the new one into place. When I took the old p-trap and slid it as high as it will go onto the pop-up drain, it cannot reach the exit pipe leading into the wall; it is short by about 1/4 – 1/2″. The nut on the pvc will not reach to tighten onto the p-trap.
So what are my options? Do I simply have a new pop-up drain that is too short for my sink? Or is there a piece of pvc sold that bridges the gap between pipes that can be of use in this situation? And finally, if there is such a piece, will it compromise the functionality of the p-trap as it will lengthen (in effect) the p-trap?
Hope this is a clearer explanation. Thanks again for your response!
Hi Michelle,
I’ve got it now! You need a shorter pop up assembly so the p-trap can go up higher and connect.
Unfortunately the Delta faucet came from a big box store and they used one of these (Click Here).
Your best bet is to pick up a new chrome plated brass Pop up Assembly making sure that it has a shorter installed height and use that instead. Those are quite a bit taller I believe.
Redwood
Hi Redwood,
Thanks a bunch; your response is disappointing but sadly not unexpected. Clearly I will move on to Plan B. Happy Valentine’s Day!
We are having a similar problem, but with our bathroom sink. We purchased a new sink and put the new faucet and when we dropped it into place, the p trap doesn’t line up to new tail piece. The new tail piece is not only too long, but it also is about just outside of the p trap. Help! It appears that they put in 6″ pipe and p trap and when we visited the home center they said that was much too big and should only be 2″ pipe and they don’t have anything that would accommodate this for a bathroom sink with piping this big.
Any advice you give is much appreciated.
Thank you in advance.
Devin & Kelly