Charleston Home Inspector discusses plumbing traps, arms and vents

Submitted by Ray Thornburg on Mon, 11/05/2012 - 14:33


Blue Palmetto Home Inspection serves Charleston




Blue Palmetto Home Inspection Discusses all those things we find under the kitchen sink like p-traps, arms, tailpieces and vents.


A P trap is that u shaped pipe under the kitchen sink. Its purpose is to hold a little water in the trap in order to keep sewer gases from entering the building. Sewer gasses can be dangerous so we don’t want them in the home. In order to keep the sewer gasses out and sewage flowing smoothly code officials developed rules that must be followed for proper drainage and safety. In addition to traps; vents are needed to keep pipes from gurgling and to help liquids to flow through the pipe.

Yes it's true that a lot of people think plumbing is easy as I've heard it said time and again " hot on left - cold on right -s*** runs down hill ". I say we should give credit to the man who keeps things running smoothly and thank him. A lot of special knowledge and a lot of specialized tools are needed.

So below is list of the most common things to look for when taking a peek under the kitchen sink. First we’ll talk about Traps. References IRC 3201.2-6

·         Each fixture requires a trap except toilets which have a integral trap and fixtures which are close together can use the same trap.

·         Fixtures sharing a trap max 30 inches apart horizontally.

·         Trap seal minimum 2” to maximum of 4”. (This is how much water is held in the trap see pic)

·         Set traps level and protect from freezing.

·         No S traps, bell traps, drum traps, or traps with moving parts or interior partitions.

·         Sized per 2006 irc table 3201.7

·         Trap size equal or greater than fixture outlet size.

·         No double traps.


     Study the picture to the left carefully ( courtesy of internachi). Click the image to enlarge in a new window. Notice the tail piece, trap arm and the trap seal depth.







The fixture tailpiece is the pipe that comes straight down from the drain. Here are some rules for them. References IRC 3201.6,2706.2

·         Fixture tailpiece maximum 24” vertical distance with the exception of a washing machine standpipe which is 18-30” per IRC 2706.2.

·         Maximum 30” horizontal distance between vertical tailpiece and p trap vertical tailpiece.

·         Directional fitting required for the discharge of the disposer or dishwasher. (i.e. wyes, combos, or tees with baffles)

p trap parts

    On the left is a picture of the tailpiece,trap arm,p trap, aav (air admittance valve)  correctly installed. Click photo to enlarge. Notice also two fixtures can share the same trap if closer than 30" as is pictured here. This is the usual way of doing this. The trap arm is after the p trap as a reader pointed out.






The trap arm is the horizontal pipe that extends from the p trap to the vent stack or waste stack. References IRC 3201.6-7, 3105.1-3,3201.1

·         Trap must be same size as trap arm.

·         Trap arm length must be a minimum 2x trap arm diameter.

·         Trap arm length per 2006 irc table 3105.1

·         Maximum Slope of pipes less than 3” diameter is ¼” per foot.

·         Total fall of trap arm is one pipe diameter. ( i.e. if the minimum pipe size is 1 ¼” then the max trap arm length is 5’. 5/4 divided by ¼ = 5’)

·         Only 1 trap permitted per trap arm except 2 traps allowed to join through a double-wye fitting to a common vent.

·         Tubing trap require trap adapter

·         Max 1 slip joint allowed on outlet side of trap.

·         Clean out required for change in direction of more than 90 degrree in a 3” or less diameter pipe.

·         Slip joints required to be accessible.

Vents prevent pressure differences in the pipe so that water doesn’t get sucked out of the trap.

plumbing trap requirements picture

    To the left is another picture of the typical requirements of trap design. Click picture to enlarge.







You can see plumbing vents sticking up through the top of the roof of most homes. There are a lot of rules about venting so I’m only going to touch on the most general ones here. References IRC 3101.2.2.1, 3101.3, 3104.3, 3104.2, 3105.2-3

·         All fixture traps require venting

·         Vent system is not to be used for any other purpose.

·         Slope vents to drain to soil or waste piping.

·         No vent opening below trap weir except toilets.

·         No crown vents – must be min. 2 pipe diameters from trap.

·         Horizontal dry vents min 6” above flood level rim of fixture. Not ok under FLR.

·         Minimum 1 vent to outside.



There is a type of vent called an air admittance valve. AAV is used where it is impractical to put in a regular vent like an island sink fixture. Some of the same rules apply to AAV but here are a few particular to AAVs. References IRC 3114.2-8

·         OK at individual, branch, circuit and stack vents.

·         Individual and branch type AAV to vent only fixtures on the same floor level and that connect to a horizontal branch drain.

·         Individual AAV min 4” above fixture drain.

·         Stack type AAV min ^” above FLR of highest fixture.

·         AAVs terminating in attic min 6” above insulation.

·         Space containing AAV must be accessible and ventilated.

·         AAVs not ok for sumps or tanks.


     Here is a typical method of installing an AAV type vent. Picture courtesy of internachi.







washing machine standpipe

    The picture on the left (courtesy of internachi) show the correct way to trap for a washing machine standpipe.






improper trap example

  At left is an example of how not to vent a fixture. Water rushing down the tailpiece into the drain will create a pressure difference which may siphon the trap dry allowing sewer gases into the home. The fall of the trap arm cannot exceed one pipe diameter.







trap arm incorrectAt left is a field example of just how not to vent a trap from the above illustration. The trap arm cannot exceed one pipe diameter. In this example it actually makes the trap way too deep so it is also subject to clogging.




Combination Waste and Vent

The one exception to having the trap arm directly connected to a vent/drain is when the drain diameter is much larger than the trap piping size . For instance a sink draining into a 3 or 4 inch drain line which extends up vertically past the trap arm by at least 4 inches. The idea is that water will never completly fill the drain pipe so the trap will not siphon dry. P3111.1 The rule is that a combination waste and vent shall not serve fixtures other than floor drains, sinks and lavatories. The sink cannot have a garbage disposer attached to it.


combination waste and vent example

  Here is another example of a combo waste and vent with the rules per 2015 IRC.  Note* That this is not allowed on a shower or tub. Also note that the larger diameter pipe starts at the trap arm.










Bends, Elbows, Tees, and Wye's

Before we move on to trap installation defect pictures I thought I'd spend a moment talking about fittings in drain lines. Basically there are two parts of a plumbing system, vents and drains. Essentially drain line fittings need to be directional whereas vent couplings can have a sharper turn. A 1/4 bend (1/4 of a circle or 90 degrees) is used in the drain line whereas the 90 degree elbow in the picture below (in red) is used for the vent portion of the plumbing.  Using a sharp elbow in the drain line will cause a clog point so you should restrict their use to the venting system (like the one in yellow below).

bends, elbows, sanitary teeThe sanitary tee at left (in blue) is only used where a drain goes from horizontal to vertical. It cannot be used in any other position. A regular tee (not pictured is way too sharp to use for anything but a vent.

The wye (in green) is used in a variety of circumstances (except fixture connections).  A long turn wye (not pictured) should not be used in trap arms.

There are many different types of fittings but the bottom line is directional fittings are required to be used in the drainage portion of the plumbing system.  So if you see a 90 degree elbow used in part of a drain (after the trap) it is wrong.



Now lets take a look at some more crazy do it yourself plumbing pics.


rusty sink

  Amateur work here rusting out the sink, improper trap. Time to hire a plumber. If you look closely you can see some kind of glue or caulk someone used to try to stop the leak. Also visible is a flexible drain transition pipe. Most inspectors agree this type of pipe is not approved because the inside walls are not smooth and will hold gunk. This kind of pipe is thinner and prone to leakage and is a sign of amateur work. Frequently we find other issues whenever this stuff is used.


s trap

    S traps are not allowed.... there is no trap arm.....not vented...... another do it yourself project.





corrugated trap

   Here is another one of those flexible pieces used improperly for a p trap. Leak waiting to happen.







trap installed wrong

     Here's an improperly installed trap. This one can actually allow sewer gases into the house. Trap should be on the left side.






improper island vent    In this photo they are trying to use traditional island venting but the trap is way way too deep. It'll have to be reconfigured.





double trap Can you see the double trap in this picture. Only one trap is needed under the tailpiece of the sink. The other one needs to be removed. The dishwasher drain hose needs to be tied up under the sink too in order to prevent back flow of sink water into the dishwasher.




horizontal vent slope example     At left is an example of a vent stack (horizontal portion) which is improperly sloped. It should tilt towards the drain. Vent stacks are open at the top so can fill with water (like this one did) and prevent proper venting of the plumbing system.


double trapBesides the obvious supply leak what you cannot see in this picture is the trap under the bathroom sink directly above the trap pictured. That makes this double trapped (and more than 24" below the fixture) so is not allowed. Also the AAV vent is in the wrong location. It should have been 4" above the trap arm of the fixture it is serving. Essentially almost every plumbing rule was broken here...and it's leaking!





Toilet Venting

One subject I haven't touched on is toilet venting....yes toilets need venting too. On a toilet the trap is built into it. If you look carefully at the side of the toilet you can see it. So the vent would come off the toilet drain directly below the toilet flange in much the same way as a sink vent comes off the trap arm. Vents for a toilet are harder to confirm because they are mostly out of sight (behind walls, under the slab, etc.). If you hear gurgling in a nearby drain or small bubbles rising up through the toilet bowl then suspect a vent issue. Frequent clogging can suggest a vent issue, an improperly sloped pipe under the home (or other issues).








If you found this information useful then leave a comment. We'd love to hear from you!

Submitted by Ray Thornburg on Mon, 11/05/2012 - 14:33


Anonymous | Sun, 01/02/2022 - 07:57

Before my question - a big thank you so much for this very clear and well set out exposition - I always enjoy the challenge of learning something new and found your posts really helped me understand. I believe I have a tub drain which is not up to code and would be so grateful if you could verify that and my proposed solution! I am replacing a built in tub with a new free standing tub. The existing drain system has a shower drain of 2" running into a drain of 3". At one end that 3" drain drops vertically down and connects to the main stack vent. At the other it serves a toilet, which also has a vent stack. That all appears to be per code. The tub has a p-trap and a 1.5" trap arm which has a wye connection to the 2" shower drain. BUT the trap arm drops about 5" over 4ft (it is not straight - there is one 90 and 2x45 turns). I think this is a violation?! I believe the best solution would be to cap off the current 1.5" connection and splice in a new wye connection into the 2" with straight 1.5" trap arm with a less than 2" drop - and achieve that by lowering the p trap with a longer distance between tub and p trap (but less than 24"). Is that correct?! (The alternative could be to cut into the 3" drain rather than the 2" shower drain?)

Anonymous | Tue, 03/16/2021 - 13:22

Hi just a question, can't seem to find an answer anywhere. How far away can you place the p trap from the tub drain? My floor joist is in the way for directly under so I was thinking of putting it a little down the line with one bend to bring the drain through the joist. This was a DIY from the prev homeowner I believe.. Trying to work with this but I need to replace cuz the old galvanized pipe is plugging frequently. Picture from under the bathroom tub: They put the trap sideways. I was gonna do the same thing but put the trap in right side up but after running the pipe through the joist. Is this problematic?

Ray Thornburg | Thu, 03/18/2021 - 11:29

Thank you for your question.....The P trap should be directly under the tub drain or the tub overflow pipe. Avoid a "running trap" which is what I think you're suggesting as there might be no way to clean it and they would tend to clog easily..... However...having said this R3201.6 of the 2012 IRC indicates 30" max distance from the fixture drain to the trap inlet. Traps cannot be set sideways as they would not hold water. If you think outside the box you'll see you can do this without breaking any rules (though it might be more work) it will be worth it do do it right. Hope this helps....Ray

Anonymous | Sun, 01/24/2021 - 04:11

Contractor said 8" or 9" sink due to drain height. I asked which is it? he got upset and said 8" then! okay?? I will have to figure it out myself. I only want to get the deepest sink, the 8" depth sinks are not so nice. Assuming drop in sink, how to I calculate the sink depth I can use? There is only a drain hole in the wall. Should I buy a P trap and put it in the hole to help measure? thanks

Ray Thornburg | Mon, 02/08/2021 - 06:51

Thanks for your inquiry..Without seeing what you're dealing with it's hard to say but I would think helps to see what is possible when you have the differerent pieces in hand....anything is possible...the question is how much work are you willing to go through to make it happen....good luck and God bless...Ray

Anonymous | Sat, 01/09/2021 - 06:54

Trying to put a vanity on the other side of our bathroom. Challenging. I am trying to figure out the proper abs fittings to go between the p trap below the sink and a horizontal drain pipe. Just happened to be a 2” t off of the main stack 3 feet away. I can reroute the vent if I use this abs a drain. From the p trap I cannot use a 90 but it seems odd to use a wye

Ray Thornburg | Mon, 01/11/2021 - 20:27

Thank you for your question. I'm not exactly sure if I picture your situation...normally a 90 comes off the p trap and a sanitary T is used at the vent stack connects to the trap arm. See the pictures on this page. Hope this helps...Ray

Anonymous | Fri, 11/29/2019 - 08:04

Not sure how to add a photo

Ray Thornburg | Mon, 12/02/2019 - 06:36

I'm not sure from your end.... I guess I need up modernize my website but it's a lot of work so I'm putting it off as long as possible....

Anonymous | Fri, 11/29/2019 - 08:02

We have a floor pipe and the s-trap does not extent out far enouph to reach the strainer pipes so we rigged up a bendy pipe for now just to make it useable for Thanksgiving day. Wondering if we could install a directional fitting then trap arm and just do a p-trap instead? And do we still need the Air admittance valve?

Ray Thornburg | Mon, 12/02/2019 - 06:32

Thank you for your question...I guess anything will work for emergency but you eventually want to hook it up correctly. Normally the sequence is drain-tailpiece-trap-trap arm-vent following the prescribed distances for each. So the trap should go right below the strainer because if you move it "in line" it will clog and be difficult to clean. Yes you always need a vent after a trap. If it's a sink an AAV should work. I hope I read your question right....Good luck..

Anonymous | Mon, 10/28/2019 - 05:09

Wonderful post! Keep it up! Another services provide (link removed by moderator).

Anonymous | Tue, 10/09/2018 - 11:30

I'm trying to design a sunken tub below the surface of a slanted floor which has a shower and linear drain on a separate system and also used as the overflow drain for the tub. When overflowing the water would move about 10" along the back wall of the tiled room. With the 2" drain requirement I have no room to place the p-trap under the tub so would like to run a 3" pipe about 72 inches at 1.5" slope to and opposite wall and place my trap in the wall. Sort of like a reverse trap arm. I envision a sweep under the tub or a short 90 if allowed housing the foot lock stopper. Wish there were low profile (flat) drains like those in Europe. Would this pass code? Thanks enjoyed your informative well illustrated site.

Ray Thornburg | Wed, 10/17/2018 - 07:53

Trap should be directly below the fixture if possible...on sinks the offset from the fixture drain to the trap is limited to 30".  So it sholdn't be much different for a tub. On tubs sometimes the trap is offset a bit for a joist etc.. So if I understand your question correctly then would not be allowed.  You might want to contact a plumbing contractor about this to be sure.

Anonymous | Sat, 03/24/2018 - 21:42

more too it than i thought

Anonymous | Mon, 05/29/2017 - 10:26

Nice piece....Nick, Cali Home Inspector

Anonymous | Mon, 03/20/2017 - 07:02

What causes the water standing in the p-trap to start flowing toward the drain?

Ray Thornburg | Wed, 03/22/2017 - 10:11

You may have to be a little more specific with you problem but I'll take a shot at it anyway. The P trap is meant to be full of water all the time to keep sewer gases out of the home. Some appliances like washing machines and dishwashers drain extremely fast. This can overload a drain which may be partially clogged or not vented well or not within the specifications above. The extra agitation is what is causing the soap bubbles to come up. Changing soap brands may help. I have found that many washing machine stand pipes are not vented correctly....especially in older homes. It's hard to check if all your piping is behind wall coverings. If your home has a crawlspace and the washing machine is on the first floor it may be possible to spot the drain line from under the home. That may give you a clue as to what's going on. They also make a kind of adapter that may help.  It's a rubber plug the washing machine drain hose fits just provides a tight fit around the hose and has some small vent holes to balance the air pressure. Hope these ideas help.

Anonymous | Tue, 08/05/2014 - 11:00

In your diagram where you show that the trap-arm is exceeding one pipe diameter....refering to the one below where the trap-arm has bends rather than straight like the one above:..Are you saying that the below diagram is correct? Because I have learned that a step-up is not allowed because it breaks the vent and renders it ineffective.

Ray Thornburg | Fri, 08/08/2014 - 06:13

If you're talking about the drawing with the big red x in the middle then you are right as both illustrations show incorrect methods. Trap arm cannot exceed one pipe diameter. Thanks for visiting my site.....

Anonymous | Mon, 07/21/2014 - 21:53

I have a trap for my washing machine that I don't think is working properly. Occasionally I can smell sewer gas though I don't hear any obvious siphoning. The stand pipe comes down about 24" to the trap. There is a 90 and attached to that is another 90. So there is basically no trap arm. This goes under the house another 18" or so and meets up with a T for the vent. This is all in 2" pipe. Does there need to be a trap arm?

Anonymous | Sat, 03/01/2014 - 11:52

The pictures, the narrative, the simple, bullet-pointed code elements -- all very helpful. I'm a novice DIYer who's competent at finding lots of 'information' on the web, but realizes that 'knowledge' comes from experience. I'm grateful that you've shared yours.

Anonymous | Mon, 11/18/2013 - 14:18

All in all this is great. I would note that in one illustration, the way the phrase "trap arm 30" max" appears on the pipe connecting two sinks (before the p trap)is misleading. But you do clearly state several times that the trap arm is the piping after the p trap. In any event, very helpful presentation.

Ray Thornburg | Tue, 11/19/2013 - 10:55

You're quite right.... Thanks for the correction. I have noted the correction in the photo.