Everything You Need to Know About Plumbing Traps

If you decide to take on a plumbing project, you’re bound to hear about plumbing traps. Plumbers talk about them all the time. Grease traps, p-traps, s-traps, drum traps, and more. But what the heck are they and what on Earth do they actually do anyway?

If you want to tackle a DIY project that involves plumbing, you definitely need to know the answers to these questions before you start. But we also want to make sure you understand what products to use, how to install them, and what other supplies you’ll need to successfully complete your mission.

We’ll make sure you know what plumbing traps actually do and what kinds are on the market today. In addition, we also want to make sure you know what traps you may want to steer clear of to save yourself a headache. Don’t let plumbing intimidate you. With the right information, you can be on your way to DIY success. And that’s exactly where we come in.

Let’s look at some tips and tricks that will make your life easier when you decide you’re ready to handle the plumbing projects around your house. As a bonus, here are a few of the best plumbing traps you can use.

Basics of Plumbing Traps

Sewage disposal systems produce some pretty nasty odors while others can even come to a point where they are considered dangerous. In order to protect us from these risks, a barrier is placed between the plumbing fixture and sewage waste system.

Sewage disposal systems start in the production phase of the shower, bathtub, or toilet. Then it enters the disposal stage, sending the sediment waste through a series of inline steps. This helps ensure waste doesn’t overflow.

Related Read: All Plumbing Terminology

Common Gases that Are Produced in a Sewage System

  • Methane.
  • Hydrogen Sulfide.
  • Nitrogen.
  • Carbon Monoxide.

Having said that, any plumbing fixture directly connected to the sanitary drainage system must be equipped with a water seal trap. That means every single plumbing fixture used to evacuate waste from a building should have its own plumbing trap.

Plumbing Trap Definition

By definition, a plumbing trap is a device that keeps a small amount of liquid every time the fixtures is used. The amount of retained liquid is called a trap seal. This trap seal prevents sewage system odors, gases, and vermin (mice, insects, etc.) from entering the living or workspace.

Trap seal is the maximum vertical depth of liquid that a trap will retain measured from the crown weir and the top of the dip of the trap. The most common of all plumbing traps is the p-trap. This is used with kitchen sinks, lavatories, and laundry sinks. Check out the diagram in Figure #1.

tubular p-trap graphic

Figure #1 Tubular P-Trap

Plumbing Codes Restrictions

Most Plumbing Codes place restrictions on how a p-trap is used and manufactured. Here are some of those limitations, restrictions, and clarifications.

  • No trap that must depend on moving parts to retain its seal can be used.
  • No trap outlet can be larger than the fixture drain to which it is connected. The sizes of the trap outlet matter. Don’t be flexible with this. For instance, you can’t put a 1 1/2 inch p-trap on a fixture with an 11/4 inch drain.
  • Fixture traps must be self-cleaning by nature. This means that with each use, it must be able to flush away unhindered by lint or hair.
  • You must install a trap level in relation to its trap seal. If the trap isn’t level, it can be susceptible to back-siphonage. Check out the diagram in Figure #2.

two diagrams of P-Trap Level and Not Level

Figure #2 – P-Trap Level and Not Level

  • No plumbing fixture can be double trapped. This is best illustrated using a water closet as an example. Put simply, waste cannot discharge through the integral trap of a water closet and then go through a second trap. Check out the diagram in Figure #3.Double Trapped Water Closet Diagram

Figure #3 Double Trapped Water Closet

  • Most codes allow double bowl kitchen sinks, triple compartment sinks, and laundry sinks to be furnished with one p-trap connected by a continuous waste. The compartments must be next to one another. Each compartment cannot be spaced more than 30” inches apart center to center of the drains. Check out the diagram in Figure #4.

Three Compartment Sink Spacing

Figure #4 Two & Three Compartment Sink Spacing

  • There is usually a maximum allowable vertical drop from the fixture outlet to the water seal. Why is that so? Because the further the liquid waste drops, the greater the velocity of the water.

The greater the velocity of the water rushing through the plumb fixture drain, the more likely the drain will siphon the trap seal. The vertical drop for sinks, lavatories, showers, and bathtubs is between 18” and 24” but not more than 24”. The shorter the vertical distance, the more efficient the trap functions. Check out the diagram in Figure #5.

Horizontal Distance of Tailpiece Drop of a lavatory

Figure #5 Horizontal Distance of Tailpiece Drop

When a water closet has flushed, the velocity of wastewater removes the trap seal completely. The trap seal would remain compromised if the trap seal wasn’t restored by the refill tube on a tank-type toilet. Concealed traps used for bathtubs, showers, etc. cannot have integral cleanouts. Check out the diagram in Figure #6 to observe the v parts of a p-trap.

important part of a trap seal depth

Figure #6 Important Part of a P-Trap

Explaining Deep Seal P-Traps

Deep seal traps have vertical depths of 4″ or more. They are used in a limited number of applications. There are some advantages to using deep seal traps. These benefits are specific to their respective application.

Advantage #1

A deep trap has a deeper seal with more liquid than others. Therefore, the seal doesn’t evaporate as quickly.

Application. Use a deep trap when you install a floor drain in a remote location. This can be a large warehouse. In these cases, the application needs very little water to reseal the trap.

Advantage #2

The deep seal trap has a much greater capacity for resealing. Because the trap is usually quite a bit larger than a traditional trap, it can handle a much greater flow of water. As a result, the trap is less likely to lose its seal because of the extra amount of water.

Application. Use a deep seal trap when a fixture or piece of equipment calls for an indirect waste connection. This can be a commercial ice maker, salad bar, or in some cases a triple pot sink.

Advantage #3

Thanks to its depth, a deep seal trap is less likely to lose its seal due to the backpressure or trap siphonage.

Application. There are instances where a fixture or trap cannot be properly vented. In cases such as these, a deep primer seal trap would be ideal. All of the aforementioned advantages apply here as well.

Deep Seal Traps Don’t Work for All Jobs

Here are a couple of reasons why deep seal traps are not the norm for all applications.

  • The traps are deeper, therefore, larger and tougher to fit into confined spaces.
  • Because of their increased depth and capacity, they are more resistant to flow from a standard fixture. Therefore, they impede drainage and make the fixture drain sluggishly.

Related Read: Plumbing Tools The Homeowner Would Use

What Are the Types of Plumbing Traps that Most Plumbing Codes Prohibit?

commonly prohibited traps

Figure #7 Commonly Prohibited Traps

The majority of plumbing codes agreed on avoiding certain types of plumbing traps. These are:

  • Bell Traps.
  • Running Traps.
  • Crown Vented Traps.
  • ¾ S Traps.
  • Full S Traps.
  • Drum Traps.

I think drum traps need a little bit more explanation. Codes don’t necessarily prohibit them. However, professionals simply choose not to use them in new construction anymore. Drum traps played their part when installing bathtubs and lavatories. A trap adapter is used in residential and commercial drain, waste, and vents systems.

Figure #8 Drum Trap

Figure #8 Drum Trap

A drum trap is a circular metal barrel canister with an inlet near the bottom of the trap. The waste outlet is at the top with a removable cover.

Ideally, professionals install the drum trap in a place with easy access. This way, you can remove the cover to clean. Unfortunately, many drum traps we’ve encountered are behind a wall. In other cases, people have it hard to remove the cover which fused together by time. Therefore, when drum traps stop, they are extremely difficult to clear by rodding. That’s due to their inlet and outlet configuration.

Drum traps existed so that people could find jewelry or valuables if lost down the drain. The trap would catch and guard the ring or earring by sinking to the bottom. The likelihood that a valuable would flow into the waste opening at the top of the trap is highly unlikely. A sand trap also helps to separate sand and oxidized organics found in water supplies.

What Is a Building Trap?

Back in the day, before people used or understood system venting, vermin like rats or insects could move freely from building to building, house to house. Not to mention the sewer gas odor could be unbearable because of backpressure and trap siphonage. Health officials knew that this could pose a serious health risk, especially in heavily populated, wet, and dry areas.

Therefore, to combat the issues above a house, a building trap is required in each building. The building trap provided a secondary line of defense against the vermin and sewer gas. Most times, the building trap was a large diameter S-trap. Today, most codes do not require a building trap. In fact, in most cases, they are against code.

typical Building Trap diagram

Figure #9 Typical Building Trap

Reasons Why Plumbing Trap Seals Break down

Trap Siphonage

Trap siphonage can look like a low negative pressure within the fixture drain. However, you can grasp this concept more easily by describing it through a simple visual image. Picture a large amount of wastewater hitting a waste stack at one time. As the wastewater goes by the other fixtures connected to the stack, it can pull the water from their respective trap seals.

Another scenario that could cause trap siphonage is a fixture using an S-trap. If the fixture is full and the wastewater is released, the water will rush through the trap, with the waste pipe carrying some of the trap water. What happens is that not enough liquid is left to form an adequate trap seal. A similar scenario can occur if people install a fixture on a long run of piping with no ventilation. If the water vacates a vent fixture into the trap, it could build up enough velocity to drain the trap.

Back Pressure

Above we’ve described instances when a trap is compromised by being pulled out of the trap. However, there are instances when water can blow of the trap into the fixture. Subsequently, the water enters the building. This can happen when a large amount of waste flows into the drainage system. The water will compress the air in front of it. If the fixture at the point of compression has no proper ventilation, it will blow out the trap.

bathroom shower head running hot water from white tiles

One of the responsibilities of owning a home is maintenance. Often, you may not realize there is a maintenance problem until you see the damage. A leaking shower is not only a nuisance but can raise your water bill and result in costly repairs.

The shower leaks that you can see are the ones that you quickly notice and are easier to repair. The leaks you cannot see are the ones that are most problematic. Shower leaks behind walls can do internal damage long before you detect them.

Related Read: Shower Leaks Behind Wall: How to Fix Them in 7 Steps

Wind Effect

Have you ever been in your bathroom on a windy day and see the water level in the toilet bob up and down? The same action can compromise the traps in your home. The pressure or suction caused by the strong winds can cause the water to rise and fall into the trap. If the fluctuations are big enough, a small amount of trap seal may spill into the waste system. This incident is going to compromise the trap. As you can imagine, the seal is more susceptible to both backpressure and trap siphonage.

Evaporation

This is a very common occurrence especially in instances where a fixture or drain has a sporadic activity. The water in the trap then evaporates when not in use, at least once a week. Laundry room floor drains, remote floor drains, fixtures and all traps in a summer home are all more vulnerable. Under these circumstances, evaporation has a high chance of happening.

Capillary Action

This doesn’t happen often. However, if there is a constant issue with a fixture losing its trap seal, this is something you can rule out. I think an example will serve this concept better than a lengthy explanation.

diagram of Capillary Attraction

Figure #10 Capillary Attraction

Maybe a cleaning person runs out of clean water while mopping a large public toilet room. They may not want to fill up the bucket again. The alternative is that they flush one of the water closets a few times to make sure the water is clean. They rise the mop off in the toilet. One of the strands of the mop dislodges and gets stuck inside the trap seal. The piece stretches to the fixture branch and into the waste system. The water from the trap will pick up the string and into the waste piping, draining the trap seal.

Minimum Drain Sizes For Plumbing Fixtures

So many people have asked about trap/drain sizing for fixtures we decided to give you at least a loose guide for each type of plumbing fixture. Please note, although these sizes are pretty universal they are still subject to local plumbing codes.

Toilet Drain Size – The universally accepted toilet drain size is 3″. Please note, closest flanges have a 4″ inlet and a 3″ outlet.

Shower Drain Size – The universally accepted shower drain size is 2″

Bathtub Drain Size – The universally accepted shower drain size is 1 1/2″. Please note this includes combination tub and shower fixtures.

Kitchen Sink Drain Size (Domestic) – The universally accepted kitchen sink drain size is 1 1/2″. Please note this size assumes there is no dishwasher or garbage disposal installed.

Bathroom Sink Drain Size (Lavatory Sink Drian Size) – The universally accepted bathroom sink drain size is 1 1/4″

Bathroom Sink Drain Size In Sets (If you have two lavatory sinks side by size) – The universally accepted bathroom sink drain size is 1 1/2″

Washing Machine Drain Size – The universally accepted washing machine drain size is 1 1/2″

Dishwasher Drain Hose Size – The universally accepted dishwasher drain size is 1 1/2″

Bidet Drain Size – The universally accepted bidet drain size is 1 1/4″.

Drinking Fountain Drain Size/Water Cooler Drain Size – The universally accepted drinking fountain/water cooler drain size is 1 1/4″.

Washfountain Drain Size (This depends on the discharge of the fixture) – The universally accepted wash fountain drain size is between 1 1/2″ and 2″

Urinal Drain Size – The universally accepted urinal drain size with an exposed trap is 1 1/2″ if the trap is integral to the fixture the trap drain size is 2″

Laundry Tub Drain Size – The universally accepted laundry tub drain size is 1 1/2″

Bar sink Drain Size – The universally accepted bar sink drain size is 1 1/2″

Related Read: How To Properly Vent Your Pipes: Plumbing Vent Diagram

Final Word on Plumbing Traps

Now that the plumbing traps are no longer a secret of the trade to you, you will be able to understand your system better and visualize how it truly works. You can now choose the right type of device for your home and make sure you leave no loopholes for a breakdown.

If you have any tips or tricks regarding plumbing traps, share them in the comment section below. You’ll provide additional insight to our readers and maybe even save someone from a plumbing disaster.

Last update on 2021-09-18 at 01:41 / Affiliate links / Images from Amazon Product Advertising API

83 comments

  1. What is wrong with a double P trap, and what is wrong with one of slightly larger diameter?

    My mother in law’s house is quite jerry-rigged. Her tub (1 1/2″) drains 3+ feet straight down to a 2″ galvanized 8″ deep seal P-trap right at the sanitary tee hub on the main stack. It rotted out and I’m looking to replace it with a PVC solution. Looking up at the basement ceiling, it looks like the bathroom sink and the kitchen sink also drain into this line, and I know both those sinks have p-traps directly under them. But now, with the basement P-trap leaking, sewer smell is present at the kitchen sink and in the bathtub. Seems that a deep seal lower P-trap and regular upper P-traps were doing the job. Pretty sure both sinks use 1 1/2″ pipe too.

    1. Hello I am Naveen. Our customer has done a core cutting with 270 mm distance from the wall where as the S trap distance of the closet is just 210 mm. Is there any way it can be brought to 270 mm. Is there a product to cover the distance of 60 mm

      1. Wow I’m gonna have to think about it for a minute. A closet bend would get help you if you were 12″ to 36″ of but you’re just 21/2″. That is such a tight offset. If it were me I’d probably remove the toilet and reset if possible. Is this a basement foundation core? Wall? What floor? If you can snap a few pictures so I can see the layout.

        Sean K

  2. I am planning to close a toilet and instead modify the plumbing to connect to a washing machine. Is it advisable to make the modification ?

    1. There is no inherent reason why you can’t cap off the waste from a toilet and use the waste piping for a laundry. I would say you should try and dump the laundry waste as close to the end of the waste run as possible. You don’t want a dead leg at the end of the waste line, it’s not the end of the world but you want to try and minimize. Just remove the toilet and closet collar if you can, stuff the pipe with newspaper or burlap and pour concrete over the top. 2″ to 4″ should suffice.

      admin@theplumbinginfo

  3. hi,in my building combord pipeline,sink pipeline and waste water pipelines in toilet are joined together by means of a deep trap plumbing system.is it advisable?.in general people are questioning how can you join both combord line and waste water line.odour will come through waste water line?.please clarify

    1. Hello Jana,

      I have never heard of the term “combord” before so I can’t answer your question. Please clarify and I’d be glad to help.

      Sean K

  4. I’m adding a prefab shower in a tight space upstairs–the drain has to fall over a 2×8 joist ,which i notched a bit to get the drain body to sit where it needs to be..this means I have to offset the tailpiece a bit to connect to the trap (in other words it isn’t straight out of the drain body like all pictures show) –we’re only talking about a 2-3″ offset to reach the trap.. is this going to be a problem?? Thanks…

  5. Wanting to hook up a single line to a sink and washing machine.
    Was planning to have a P-Trap for both , the washing mashing is first in line and more then likely the most active of the two!

    My question is ” Will one P-Trap create suction on the other causing it to drain out? “

    1. @Dan good question. Most codes read that each plumbing fixture shall be individually trapped so two traps is the right way to go. I do not believe if the fixtures are vented that you run the risk of siphoning a p-trap.

      Sean K

      1. Awesome thank you for the peace of mind.
        I have ran two P-Traps and so far so good as the washing machine line is also acting as a vent too…

    1. You can tie into it however the utility sink must be independently trapped.

      Sean Kavanaugh

  6. Hi,
    is there a diameter difference in piping/water traps between european sink standard and USA? I have read some materials that e.g. bathroom vanities from European market cannot be (or have hard time) to be applied to US norms.

    Have you heard anything about it?

    Thanks!

    1. There are several difference actually. #1 in Europe water piping is sized in millimeters by the outside diameter or O.D. in the U.S. water piping is sized in inches using the nominal dimension which is the inside diameter. When using European faucet in the U.S. there are usually adaptors included so that it can be made to work.

      Sean Kavanaugh

      1. I was wondering if a p-trap has to be set straight up and down or if it can be tipped onits side a little bit. So I dont have to cut hole in floor deck.

        1. The p-trap must be installed horizontally with the trap straight. If you do not you will almost certainly lose your trap seal.

  7. Hi,

    I installed a deeper kitchen sink and now my trap is about 2 inches lower that the outlet. What can I do to reconnect it. Thanks.

    1. Cut kitchen tailpiece that drops directly down from the sink 2″ and reinstall. That will raise the kitchen waste piping up 2″ so you can connect it to the waste opening.

      Sean Kavanaugh

  8. If I dropped a ring down the sink, how long will it stay in the P-trap? It happened a month or two ago. I have been using the sink, and figured it was gone. Someone told me it may still be there, but someone else said, “if you don’t get it right away, once the water flushes through, it will be gone.”. which statement is true?

    1. The truth is if the ring is heavy enough it will probably still be in the p-trap. It really all depends on the weight of the ring and the speed of the water going down the drain. If the water is slowed down by a pop-up drain there is a good possibility it’s still there.

      Sean Kavanaugh

  9. Hello, I have been told that for a tub you need a specific tub Trap, is this true? And if so what are they called?
    Thanks so much.

    1. You need a waste and overflow drain with a p-trap. The p-trap is a common p-trap nothing special. The connection to the material you are using for the Waste and Overflow will determine the type of connection but other than that you’re good. If you’re in a hi-rise building hit me up again.

      Sean Kavanaugh

  10. I have a 1st story bathtub with a 1-1/2″ PVC P trap connected within 18″ to a 3″ cast iron waste stack and vent. The waste stack runs to the basement and changes from 3″ to 4″ cast iron 3 feet above the basement concrete slab. There is a P trap in the 4″ cast iron under the basement slab. Is this situation considered a double P trap or does the 3″ vent stack between the two traps negate this problem? I am going to renovate the bathroom, but prefer to not rework any of the waste lines since all of the fixtures are going back into their original locations. Thanks!

    1. You really have to check with your municipality. We haven’t put house traps in, in the Chicagoland area for over 50 years. My house in a Chicago suburb doesn’t have one and it was built in 1955. From a functionality standpoint the vent would take care flow or back pressure issues so I wouldn’t remove it, it’s not necessary. From a code perspective if it’s illegal in your town the house would be grandfathered I would imagine.

      Sean Kavanaugh

  11. My tub drains into an elbow then into the waste line. I smell sewer gas whenever the shower is used. Several plumbers I have called in and they are adamant that the elbow joint is a trap, I know they are wrong, a downward bending Elbow can not be a trap. Who is correct ?

    1. So the tub drains directly into a 90 and that connects to a branch waste line? I’d love to see a picture if you have one. It’s hard for me to believe that two different plumbers got it wrong. Are you sure that someone didn’t make a trap from fittings? Let me know I’d like to see it. skavanaugh@todaysgrowth.com

      1. My outlet pipe to my s bend in my kitchen is too short….
        How would you recommend fixing this – apart from getting another s bend? It’s literary only short 2 millimeters.

        1. S-Traps are illegal in most areas in the US. Because of your use of a metric measurement I assume you’re outside the US. The only way to fiz this is to purchase a new S-trap.

  12. I am installing a shower in a first floor location that is cantilevered out from the house. The floor joists are 2 x 10s and the joist cavity is insulated. Problem is the P-trap almost touches the bottom of the joist cavity and is likely to freeze being very close to the exterior of the house. If I install a trap inside the house proper (a running trap) freezing will not be an issue. The run from shower drain to the trap would be about 4 feet. I do not know why codes do not allow running traps for this kind of application. What kind of problems would a running trap, in this application, be likely to create? Thanks.

    1. Running traps are illegal in most municipalities. If this were my house I would install the p-trap where it is supposed to be and run heat tracing tape around the p-trap. This way in the winter it’s always kept warm.
      http://www.homedepot.com/p/Frost-King-6-ft-Electric-Water-Pipe-Heat-Cable-HC6A/202262328?cm_mmc=Shopping%7cTHD%7cG%7c0%7cG-BASE-PLA-D26P-AirCirculation%7c&gclid=CKyW-5X43s4CFQuNaQod8TsBEw&gclsrc=aw.ds

      It’s not what we use on the commercial/industrial side but this more than adequate.

      Sean K

    1. I don’t quite understand the question. Are you talking about the horizontal piping from the trap to it’s connection to the waste branch or the vertical drop from the waste outlet of the sink to the trap? The maximum length of a drop from a sink to a trap is 18″ and the maximum length of horizontal 2″ waste piping to it’s connection to a waste branch is 5ft.

      Sean Kavanaugh

  13. Can anybody give me an answer, the p trap in my basement utility rm has a p trap with a 3 slotted vertical spacer in it. What is the purpose? It is not draining the condensate from my a/c coil.

    1. A slotted tail piece or indirect funnel interceptor on a p trap was used for something like a drip condensate drain or similar device. It is normally open to be able to visualize that the drain is plugged or stopped up.

      1. and to spill out on the floor if a backup occurs. There is a natural air gap when an indirect waste is installed.

  14. I am removing a bidet can just cap the hot and cold copper water lines and the drain(which joins the toilet drain line leading to the basement.

  15. I am installing a new tub with 2″ drain. Is there anything wrong with running a horizontal line with(1/4″ drop per foot ) about 2 feet before adding the P trap ?
    Because of space their isn’t much room to add it right below the drain.
    Thanks

    1. Most state codes state that no traps shall be more than 18″ away from the fixture being served. Please check your local codes for the most accurate information. 😉

      Sean K

      1. Hello Sean,

        Thanks. I’m in a highrise. I am running a linear drain with a horizontal outlet (instead of vertical) as this allows me to not have to break a hole in the concrete floor and access/install plumbing in the apartment below mine.
        Coming out of my linear drain, I will run my 2″ PVC about 12-16″ horizontally (with 1/4″/foot slope) into the chaste, and there I will put the trap (and connect vent pipe as well in the chaste). Can you please send me info/code requirements for this horizontal run before installing p-trap? I know most shower drains are trapped directly below the drain, however, my application does not allow for this configuration, therefore, I want to find the appropriate codes to support my configuration. Does this make sense, or am I leaving out any relevant details?

        1. http://danny79m.tripod.com/sitebuildercontent/sitebuilderfiles/allcode.pdf

          There really isn’t a maximum distance between plumbing fixture and trap. In Chicago’s code on page 99 it’s states “trap shall be installed as close to the fixture as possible” The issue becomes the distance between the outlet of the fixture and weir of the trap. That can’t exceed 24″. That is universal although you can read it in the Chicago Code.

  16. If I want to permanently remove a tub, would I have to cap off the drain under the p trap….it is under the concrete slab.

    1. Remove the tub, cut the waste off flush to the floor, stuff burlap or newspaper into the waste opening and fill with concrete for a few inches. You may have to grind down the waste piping a bit to get it flush with the floor but it’s not too difficult.

      Sean K

      1. Remove the tub,pull out the tailpipe and screw a test plug in the pipe below the floor. Because if the pipe ever stop up or back up it wouldn’t come back out your pipe,and cause you a problem.

  17. Hi! We have a 4″ concrete slab with a drain pipe with no p trap. Are there any kinds of bathtubs that have a p trap built in (kinda like a toilet)? Or do we need to put the tub on a platform highbenough for a p trap under it? Cutting the concrete is not an option. Thanks so much!

    1. This is a great question because we have always done the latter. We put the bathtub on a platform so the p-trap fits underneath. I did do a little research and to my knowledge there are still no manufacturers that make a bathtub with an integral trap. If you do run across one please leave a link in this thread.

      Sean K

  18. rental property that has had several main waste line backups over last 2 to 3 years. In just about half the cases, maybe 8 to 10 now over 3 years, we have found evidence that things being flushed that clogged the lines (ie., tampons, feminine pads, baby wipes, grease) but recently we have been stumped that it seems like every few days we are noticing a slow flow as if normal waste or toilet paper is backing up the line. We have easily remedied the backup by snaking at the house trap (just inside the foundation wall) which is buried in the basement floor, but the cap is accessible. We have had this property for going on eighteen years now and are beginning to wonder, ether the trap has deteriorated or become full of sediment or otherwise compromised. We are still trying to find the right equipment to maybe camera or scope out the trap, as this seems to be the area that once snaked, quickly relieves the back up. Any experience or suggestions welcome. thanks!

    1. You most certainly need to camera the sewer line. There is probably a dip or a separation in the piping that is causing things to hang up at the same spot. We use a Rigid Sea Snake. I’ve used plenty of other units but this is still the Cadillac of sewer cameras.

  19. Thank you for the helpful info! We are in the middle of having a washer installed…the plumber installed most of the plumbing and left it over the weekend, to be finished later this week. Over the weekend, a distinct sewage smell and flow of air was coming out of the open standpipe and we noticed that no p-trap was installed at the bottom.

    We asked our plumber and he said that “that type of piping doesn’t need a p-trap” and he’ll fix it so that it doesn’t smell anymore. Is this a legitimate practice, i.e. is there any setup where a p-trap wouldn’t be needed?

    Thanks again!

    1. A washing machine standpipe should be trapped and be within 5ft of a vent. If not the line should be independently vented. That is a false statement.

      Thanks for asking.

      Sean

  20. Not sure if you can help me as I’m doing a kitchen reno atm. When the S trap is in place the water would back up into the dual sinks. I removed the S trap and plumbed directly into the waste pipe and never have a problem with water backing up. Is there another option or is there a reason this is happening?

    Cheers

    1. S-traps are almost completely obsolete. You should remove the s-trap and add a p-trap. If you have already done so you’re in good shape if not you most certainly need one. As to why it backed up I’m not sure. It could be that the trap was obstructed.

  21. I’m installing a pedestal sink in a bathroom. The drain is 1 1/4″, therefore I’m assuming the trap needs to be 1 1/4″ as well. I’d like to increase the pipe size as soon as possible after the trap. Is there a minimum or maximum distance required before increasing pipe size?

  22. Hi! would I be able to install a sink from water line that used to be a shower? Do you need to change the trap? or would the same trap work for the new sink?

    1. You can absolutely use the water supply that used to service your shower. You cannot, however, use the existing trap. The maximum distance between the sink drain and the p-trap is 24″.

  23. does there need to be a trap in the 4 inch waste line from my bath to the main sewer if the appliances in the bath are trapped. the trap that is there now has waste from the commode floating.

    1. I’m not sure I understand the question completely however I will give it a try. Most plumbing fixtures are trapped and most municipalities do not require the house to be trapped (There are some, Lancaster PA is one that still requires a house trap) So it’s most likely you do not need a trap on the 4″ line. The thing that is throwing me off is that you reference appliances in the bath and that there is raw waste in the water closet. I’m not making the connection to the two fixtures. They should both be independently trapped.

  24. Is there a problem with having a 4-foot vertical drop from a water closet before to starts to run horizontally. I cannot find where this is prohibited by code, but it seems like it could siphon the water out of the bottom of the toilet. Also seems like the liquids could out-run the solids and create buildup in the vertical section of pipe.

    1. Hey John, thanks for reaching out. I’m not super familiar with California’s plumbing code although I did go through it and didn’t find a section mentioning disallowed traps. It may be I don’t know where to look. I’ve reached out to a friend in Cali to get a definitive answer. Bottle traps are not allowed in many municipalities across the US. I will have an answer for you shortly.

  25. My Indian toilet has backwater from the septic tank as the toilet outlet pipe in below the septic tank level. What should i do? Should i install a Western toilet with some kind of backflow stopper? Will it help? And how should i seal my Indian toilet? Please help.

    1. I don’t think your Indian toilet is the issue. May I ask where the heck did you purchase an Indian toilet in the US? There aren’t many distributors here in the states and certainly no fixture manufacturer based in India that have gained a foothold. To my knowledge, there are no toilets that have an integral check valve. You could install a water closet with an ejector pump that sits above the floor like this

      And then pipe it back into the waste line. This way your water closet is divorced from the waste line. It’s basically a forced main.

  26. I am responsible for maintenance in an apartment building that is nearly 20 years old. Whenever a resident moves out, one of the many things I update is to switch the original chrome plated p-traps serving the kitchen (1.5″) and bathroom (1.25″) sinks to plastic.

    I have noticed that at least some of these plastic traps come loose over time and leak into the sink cabinet, causing serious damage to the cabinets and rollout drawers. This is an expensive problem, especially if it is widespread.

    Can anyone explain why plastic traps come loose over time? My hypothesis is that the expansion and contraction effect of running hot and cold water causes the threads to loosen ever so slightly upon contraction, but shouldn’t this occur with threaded metal pipes also? My experience with chrome plated metal pipes is that when they leak, it is due to physical damage or rusting out from the inside, hence the reason I have been switching to plastic.

    The domestic hot water in my building is communal and capped around 120-125 F by the mixing valve so supply water is not particularly hot, although residents who regularly boil pasta, eggs, etc. can introduce much hotter water to their drains. I see this loose trap phenomena most often at the kitchen drains–which fits my theory–but it has also occurred in the bathrooms as well.

    Should I be performing a periodic trap inspection? If so, how often is recommended?

    I have been using basic plastic drains from HDSupply exclusively. Should I try a different brand of plastic drain?

    Thank you for reading this; I appreciate any insight you can offer.

    1. This is a great comment and observation. I don’t want to sound wishy-washy but both work well depending on the type you use. I agree with you that most chrome plated p-trap I’ve replaced are replaced because of deterioration. The joint is fine but the metal has corroded and worn away. However, when you invest in something like the Brasscraft or basically any p-trap or tubular products that are 17 gauge will give you the durability and rigidity you’re looking for. The same is true for plastic fittings. Most plastic tubular p-traps you buy from the home centers are not heavy-duty which makes them way more susceptible to heat. Pay the extra money for commercial grade tubular p-traps. Having said all of the above the biggest issues I’ve experienced with leaking p-traps is with homeowners store things in the cabinets containing the tubular fittings. As people pull out and put back cleaning supplies and other things they hit the trap compromising the joint. Tell people to be careful if they are going to store things in the cabinets.

  27. Wow, this blog is so fantastic. There’s so much great info here. I work in the grease trap industry, but this is such a great resource to send people for those that want more in-depth knowledge about drainage and plumbing.

    1. Thank you so much. Tell me more about your business. I’ve certainly put in and worked on enough grease traps in my day. Rodding super grease traps are super fun. I think I’ve mentioned that before.

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