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.
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.
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.
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Charles Buell, Seattle Home Inspector
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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
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.
Great explanation, great photos. I’m going to start linking to this blog post in my inspection reports.
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 🙂
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.
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.
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?
Guadalupe, you will have to do the Metric conversion, but this diagram pretty much explains your options:
Drain installation”
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.
Brian
Master plumber
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).”
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.
I LOVE that word!
Many thanks for your help. I had a drain problem at my kitchen sink, and your information saved me a plumber call.
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!
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.
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.
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
sounds like it might not be vented properly
I had my bathroom dual sinks replaced and did the installation of the p trap myself. Now I have a smell coming from sinks every time I use the faucets. Could there be some leveling issue with the P traps where water is allowing fumes to come back only during use?
Post some pictures please
Thankful to see your experts sight has so many positive comments. I have a problem. A handy man hooked up a sink for me, and we are now getting sewer smells coming from the sink drain. Here is a photo of the p-trap etc. One thing that is hard to see in the photo, is that the black pip coming from the wall is not level, it tilts downward out of the drywall until it connects with the white pipe. Hope you can help me figure out this problem. Peace.
Well certainly the black pipe should slope toward the wall from the P-trap. Odors would not be expected from this setup unless there is no sanitary tee fitting inside the wall resulting ins siphoning of the trap. In other words, the drain may not be properly vented.
“..it tilts downward out of the wall…”. Shouldn’t the black pipe tilt upward —from wall to the p-trap?
It does, it is an optical illusion
Thank you. There is a vent pipe inside the wall that goes up to the roof. So I can only assume that the black pipe sloping downward from the wall, as opposed to downward from the trap, is cause the problem. I know it it is hard to see, but the black pipe is pitched about 3/4″ from the wall to the fitting of the white pipe. I put a small level on the black pipe and to get a level reading it was 3/4″ lower than at the wall. Any further comments you can provide would be much appreciated. Again, thank you for taking the time help me with this problem. Grateful.
I think you likely have something else going on. While the trap arm is going to be flooded because of the reverse slope, this would result in gases being less likely, not more likely. Perhaps the vent is blocked?
Thank so much.
Hi, can someone tell me if this is proper installation of a kitchen sink p trap?
I think it will work—it would be nice if there was a little more fall between the arm and the outlet of the disposal—but I don’t see anyway you are going to make it better given where the wall outlet is.
I actually ended up plumbing it into the clean out that is below the inlet that you see pictured. That way I won’t have water standing in my disposal.
So the high Tee is the original drain? And the pipe that goes off horizontal above that is the vent because there is none inside the wall?
I have a wall outlet in my bathroom sink installed at 13′ high, but the vanity my wife bought has a cross bar exactly around that height. I want to solve without cutting up vanity too much. Seen all kinds of discussions as to whether S trap is ok, etc. and seems a bad idea if anything because it is out of code where I live. I was thinking of raising the p trap so that its bottom is inside the cabinet and connecting down to the wall outlet with two 90 degree elbows. Is that going to work, or is it the same as an S trap, with the same possible smell and venting problems?
If I understand you correctly I suspect it would still amount to an S-trap. can you move the drain in the wall up a bit?
My son and I remodeled his kitchen and have now run into a plumbing problem under his sink. The drain is unusually high which makes the installed P-trap high, too. The new basin sink is fairly deep and, to make matters worse, he has a dishwasher discharge connection on his tailpiece. (I don’t have a picture, but could ask my son to take one if needed.) You’ll probably understand our problem when I say that the trap, when installed correctly, is above the level of the dishwasher connection on the tailpiece. Obviously this doesn’t work… I can only make the connection if I reverse the trap. What other options could you recommend? Thanks in advance!
Henry, I’m having the same problem. What did you end up doing?
Heather and Henry, I’m having the same problem. What did you end up doing?
Hi Charles,
Really appreciate all the info you have to share. My originally installed P trap barely cleared the shelf of my vanity by 1/8″. I am trying to raise it up a bit and came up with this configuration. I’ve read that the distance from weir (where water flows out to the drain) to the top of the dip ) should be 4″ max. Mine measures 5-1/2″ if I measure from the dip of the upside down p trap to the dip of the “normally” installed p trap. Is this the only thing wrong with my set up?
John
Help, I have the following configuration and the smell coming from the drain is bad. I have checked and the trap is not drying out. If I take the p trap drain off I do notice that there is a slight buildup in the cap. Did I set something up wrong. The pipe going right is for the double sink and the pipe in the back upper right is the vent pipe coming from the basement.
I don’t like that it is all hard piped and the trap arm is technically too short—but I see nothing that would account for odors. What is the Tee abvoe the drain in the vent and where does the vent go to the right?
The vent from the right comes up from the basement where there is a bathroom. It goes into the wall and vents to the roof.
Does the lateral PVC between the sinks have the correct slope so that solids don’t get stranded along it and sit to cause an odor?
Good question
Slope is correct. Triple checked it and even increased the slope slightly when I first noticed the odor.
The smell is gone. I did 3 things. I lengthened the pipe from the first tee to trap, I replaced the trap with the cleanout to one without, and lengthened the trap arm a couple inches. It has been a few weeks and the smell has not come back. Thank you for your suggestions.
Will this work for our shower?
If you are going to connect that to where that clean out is, no, that would not be proper.
Can p-traps be stacked? If there is one on the second floor and another inline in the basement do they cancel out each other? Is this a problem?
Each trap should serve the fixture on each floor level
Thank you for all the Q&A in this post, I’ve learned a lot.
I have a similar issue in a new kitchen remodel. The plumber installed the garbage disposal and ran the line from the disposal outlet in such a way that it has to ‘push’ wastewater uphil to get to the drain line. Is this acceptable? What are the alternative drain configurations I can suggest?
That would certainly be wrong. no way this was done by a real plumber.