Plumbing traps and the trap seal field guide for plumbers

The trap and the trap seal

A plumbing trap is the U of pipe under a fixture, and the water it holds is the trap seal. That seal is a plug of water sitting in the bend, and it is the only thing between the room and the open sewer on the other side of the drain. Every fixture that connects to the drainage system gets a trap, and the seal is the whole point of the fitting. Take the water out and a trap is just a bent pipe doing nothing.

The drain under a sink does not end at the sewer in a friendly way. It ends in a pipe full of sewer gas, kept moving by the vent system but always there. The trap lets waste water pass through while keeping a standing plug of water in the bend at all times, so air cannot travel back up the pipe and into the room. Water flows over the weir and down the drain. The seal stays behind.

The trap pairs with the vent, and neither one works alone. The trap holds the seal. The vent keeps a draining fixture from pulling that seal out, and how that works is covered in the DWV and venting guide. Get the trap right and skip the vent, and the seal disappears the first time the fixture drains hard. This guide is about the trap and the seal. The venting that protects it, and the rough-in that sets it, have their own guides.

What sewer gas is and why the seal stops it

Sewer gas is the mix that builds in any drainage system: hydrogen sulfide, methane, carbon dioxide, ammonia, and whatever else the waste stream throws off as it breaks down. The rotten-egg smell is hydrogen sulfide, and it is the one that does real harm. At low concentration it stinks and gives people headaches and nausea. At high concentration it deadens the sense of smell, so the warning goes away while the danger climbs, and at that point it is toxic.

Methane is the other problem, because it is flammable and lighter than air, so it collects high in a closed space. A drainage system venting methane into a room is an explosion risk nobody is watching for. Add the nuisance side, drain flies and roaches and even rodents traveling up an unsealed drain, and the trap is earning its keep on more than odor.

The seal is the barrier, and it is a low-tech one: a couple inches of water in a bend. That is all that stands between the building and the gas. People treat a missing trap or a dry trap as a smell complaint. It is a containment failure, and the smell is just the part you can detect.

How deep does a trap seal need to be?

A trap seal is commonly between 2 inches and 4 inches deep, measured from the top of the dip in the trap to the bottom of the weir, the lip the water spills over. Less than 2 inches and a single hard drain or a small pressure swing clears it, because there is not enough water to hold against a siphon. More than 4 inches and the trap runs sluggish and fouls, because the deep leg does not scour clean and solids settle in it. The 2-to-4-inch window is the common code range, but confirm it against the adopted code and the fixture.

Deeper is not safer, which surprises people who think more water means a stronger barrier. A deep-seal trap holds against siphonage better, true, and deep-seal traps exist for exactly that on unvented or hard-to-vent fixtures. But the standard fixture trap is built to the 2-to-4-inch range so it self-scours, and a homemade deep seal on a normal fixture trades one problem for another.

The seal depth is fixed by the trap's geometry, not something you set in the field. You pick the trap, the trap sets the seal. What you control is whether the vent protects it and whether the fixture gets used often enough to keep it full.

The P-trap, the standard fixture trap

The P-trap is the trap on nearly every fixture installed today, and the shape is in the name: a U turned on its side, so the outlet runs horizontal into the trap arm instead of straight down. Water comes down the tailpiece, fills the U, spills over the weir, and runs out the arm to the drain and the vent. The bend holds the seal. The horizontal arm gives the vent a place to connect, which is what makes the P-trap ventable and the S-trap not.

It self-scours, which is the quiet reason it won. Each time the fixture drains, the slug of water moving through the U scrubs the trap and carries the solids out instead of letting them settle. A P-trap sized to the fixture keeps itself clean, which is why the trade dropped the large-volume traps that came before it.

On a sink or lav, the visible P-trap under the cabinet is usually a slip-joint assembly you can take apart by hand to clear a clog or fish out a ring. That serviceability is a feature, not a compromise. A tubular P-trap with slip nuts is the most repairable fitting in the house, and it is the standard for a reason.

Why is an S-trap not allowed?

An S-trap is prohibited in new work because it siphons its own seal. The shape tells the story: instead of running horizontal to a vent like a P-trap, the outlet turns back down right after the weir, forming a full S. When the fixture drains, the column of water falling down that second vertical leg pulls a vacuum behind it, and that vacuum reaches straight back through the trap and drags the seal out with the waste. No vent gets a chance to break it.

You will find S-traps under old sinks all the time, and they appear to work, which is the trap people fall into. It works most of the time because the seal does not always fully siphon, and it refills a little on the tail of the drain. But it self-siphons enough to lose the seal often, and an S-trap that has lost its seal is an open pipe to the sewer that looks completely normal. The smell comes and goes, which is the tell.

Find one on a remodel and replace it with a vented P-trap and a proper trap arm. It is not grandfathered safety. It is a defect that has been getting away with it.

Drum, bell, bottle, and house traps you will still meet

Several older trap types are still out there in old buildings, and most are prohibited in new work for the same reasons. Know them so you can identify one on a service call and explain why it is going away.

The drum trap is a large cylindrical canister, usually under a tub, with the inlet and outlet near the bottom and a screw-off lid on top. It holds a big volume of water, which made it hard to siphon, but the same volume does not self-scour, so it packs with hair and sludge and the lid corrodes shut right when you need to clear it. It is obsolete, replaced by the P-trap, and prohibited in new work in most codes, with narrow exceptions like a solids interceptor or a chemical-waste system.

The bell trap, and the similar bottle trap, seal with a bell or cup over a center standpipe instead of a water bend. The bell trap sits in a floor and loses its seal the moment the bell is knocked loose or the part rusts away, which is most of them, so it is prohibited. The bottle trap, common on imported lavatory fittings, seals in a bottle-shaped body and is restricted or disallowed under US codes because it does not scour and the seal is shallow and easy to lose.

The running trap, also called a house trap or building trap, is a U set in the building drain itself, near where it leaves the building, to seal the whole house off from the sewer at one point. Older cities required them. They clog, they have no good cleanout on the street side, and a house trap with no relief vent ahead of it chokes the drainage. Most modern codes no longer require or allow them in new work, and many jurisdictions are pulling the old ones. If you find a house trap backing up, that single fitting is often the whole problem.

The grease interceptor, a different job

A grease interceptor, what the trade still calls a grease trap, is not a trap seal device at all. It does a different job: it catches fats, oils, and grease, the FOG, from a commercial kitchen before they hit the sewer, where they cool, congeal, and block the main. The name overlaps with the fixture trap, but the purpose does not. A grease interceptor is an interceptor, not a seal against sewer gas.

It works by residence time. Kitchen waste enters a baffled tank, slows down, and the grease floats to the top while the cooled water draws off the bottom and leaves. The grease layer gets pumped out on a schedule. A small under-sink unit is a hydromechanical grease interceptor. A large in-ground tank outside the building is a gravity grease interceptor, sized for the kitchen's flow.

The fixtures upstream of the interceptor still need their own traps for sewer-gas protection, because the interceptor does not seal the line the way a fixture trap does. Grease sizing, the cleaning interval, and the FOG program are their own subject, off the project documents and the local sewer authority's rules. The point here is to keep the two ideas separate: the trap seals against gas, the interceptor catches grease.

Why does my floor drain smell?

A floor drain smells because its trap has gone dry and lost the seal. A floor drain has a trap like any fixture, a P-trap buried under the slab, but the fixture above it is not used on a schedule. Nobody runs water into a mechanical-room or basement floor drain for weeks, and the seal evaporates a little every day until the bend is empty and the sewer gas comes straight up. The smell is the proof the seal is gone.

The first fix is the cheapest: pour a bucket of water down it. That refills the seal and the smell stops, which also confirms the diagnosis. If it comes back in a few weeks, the trap is evaporating faster than the drain gets used, and the answer is to keep it primed automatically rather than ask someone to remember the bucket.

That is the whole reason trap primers exist, and they get their own section next. The other thing to check on a floor drain that smells right after you poured a lot of water is whether the trap was actually built with a seal, because a floor drain plumbed with no trap, or with the trap bypassed, smells no matter how much water you pour. Pour the bucket first. If that fixes it for a while, it is evaporation. If it never fixes it, the trap or the vent is the problem.

Trap primers and trap-seal devices

A trap primer is a small valve that feeds a little water to a floor-drain trap on a schedule, so the seal never evaporates dry. It is the standard answer for any trap that does not get used often enough to refill itself: floor drains, floor sinks, mop-basin drains, and emergency drains in mechanical rooms. Where a trap is subject to evaporation, the code generally requires it be kept primed, and a primer is how.

There are a few types, listed to different standards. A water-supplied trap seal primer valve taps the potable supply and dribbles water to the trap when it senses a pressure drop from a nearby fixture being used; it is listed to ASSE 1018. A wastewater-supplied primer takes clean discharge from a nearby fixture, like a lavatory tailpiece, and routes a share of it to the floor-drain trap; that device is listed to ASSE 1044. There is also a barrier-type trap-seal protection device, a flexible flapper that sits in the drain and opens to let water pass, then seals shut against gas when there is no flow; it conforms to ASSE 1072 and needs no water supply at all.

Pick by the situation. A primer needs a water source and a line run to the trap, which is easy at rough-in and a pain to add later. A barrier device drops into an existing drain and is the common retrofit for a floor drain that keeps drying out in a finished building. Whichever you use, it has to be accessible, because a primer line or a barrier flapper is a maintenance item, not a forget-it install. Verify which method the adopted code accepts for the fixture.

How a trap loses its seal: siphonage

Siphonage is the most common way a trap loses its seal, and it comes in two forms. Both pull the seal toward the drain by dropping the air pressure on the drain side below the pressure in the room. The trap is just water in a bend. Drop the pressure on one side and the water gets pushed and pulled out.

Self-siphonage is the fixture emptying its own trap. When a fixture drains and the trap arm runs full, the slug of water moving away from the trap pulls a vacuum behind it, and with no vent to break that vacuum, the seal follows the waste down the pipe. This is exactly what the S-trap does by design and what an over-long or over-pitched trap arm does by accident. The cure is a vent within the trap arm limit, covered in the DWV and venting guide.

Induced siphonage is another fixture stealing your seal. A fixture upstream, or a stack running full beside your trap arm, drops the pressure at the junction and siphons a trap that was sitting still, doing nothing wrong. The lav on the same unvented branch loses its seal when the tub drains, even though nobody touched the lav. Both forms are pressure problems, and both are why the vent exists. A correctly vented trap holds because the air can move without going through the seal.

Back pressure and blowout

Back pressure pushes the seal the other way, up into the room instead of down the drain. It happens when the pressure on the drain side of the trap goes positive, above the pressure in the room, and the higher pressure shoves the seal back up and out of the bend. On a bad one you see water bubble up out of the fixture and you smell the gas that comes with it.

The cause is a column of waste falling down a stack and compressing the air below it, usually at the base of the stack or at a horizontal offset where the falling water seals off the pipe like a piston. The fixtures low on that stack take the hit, because that is where the pressure piles up. A floor drain or a first-floor fixture that gargles and spits when the building above it drains is a back-pressure symptom, not a clog.

The fix is venting again, specifically the relief and yoke venting that gives that compressed air a path out before it reaches a trap. A deep-seal trap resists back pressure better, which is one reason they show up on fixtures at the base of tall stacks. But the real answer is a vent system sized and arranged so the pressure never builds against the seal in the first place, which the DWV and venting guide covers.

Evaporation and the dry trap

Evaporation empties a trap that does not get used. Standing water in a warm, dry, moving-air space goes away, slowly, and a trap that nobody refills loses maybe an inch of seal a week, faster in a heated building with dry winter air. Give it long enough and the bend is empty and the gas comes up. This is the floor-drain problem, and it is also the guest-bathroom and the seasonal-cabin problem, any fixture that sits unused for weeks.

It is the easiest loss mode to diagnose and the easiest to miss, because nothing is wrong with the plumbing. The trap is fine, the vent is fine, the seal just dried out. A house closed up for a season comes back smelling of sewer from every dry trap at once, and the owner thinks the system failed when it only needed water.

The fixes scale with how often the fixture gets used. A bucket of water handles a fixture you can refill by hand. A trap primer handles a floor drain in service. A barrier-type seal device handles a drain in a finished space where running a primer line is not practical. And a little mineral oil floated on top of the water in a rarely-used trap slows the evaporation, an old trick for a fixture that will sit idle, because the oil floats and the water under it lasts longer.

Capillary action, momentum, and wind

A few less common loss modes round out the list, and they catch people because the plumbing looks right. Capillary action is the classic one. A piece of string, a rag edge, or a length of hair draped over the weir wicks water out of the trap a drop at a time, like a siphon made of fabric, until the seal is low enough to fail. You will not see it unless you look, and the fix is to clear whatever is hanging over the weir.

Momentum is the seal getting thrown out by a sudden heavy discharge, a bucket of water dumped fast into a small trap, where the water's own momentum carries the seal past the weir before it can settle. It mostly matters on oversized discharges into undersized traps.

Wind effect, or waving out, is pressure oscillation at the trap from gusts across a vent terminal or surging in the stack. The seal sloshes and a little spills over the weir with each wave until it is low. It shows up on tall buildings and exposed vent terminals more than on houses. None of these three is as common as siphonage or evaporation, but a trap that keeps losing its seal with a good vent and regular use is worth checking for a rag over the weir before you blame the system.

How does a vent protect the trap seal?

A vent protects the trap seal by giving air a path to the drain that does not run through the trap. The trap and the vent are a pair, and the trap cannot hold without it. When a fixture drains, air has to move to replace the water leaving the pipe. With a vent, that air comes from the vent opening. Without one, it comes from the only other path available, back through the trap, and it takes the seal with it.

The vent equalizes the pressure on both sides of the seal. It breaks the vacuum that causes siphonage and bleeds off the positive pressure that causes back pressure, so the seal sits at near-atmospheric pressure on both faces and stays put. That is the entire relationship. A trap with a good seal and no vent is a trap that will lose the seal. A trap with a vent within the limits holds.

How vents are sized, which method fits which fixture, and the trap-arm distance that ties a trap to its vent are all in the DWV and venting guide. The point to carry here is that you cannot solve a trap-seal problem by working on the trap alone if the real issue is the vent. Check the vent first on any trap that will not hold.

The trap arm and the trap-to-vent distance

The trap arm is the run of pipe from the trap weir, the downstream lip of the trap, to the vent fitting. It is the single dimension that decides whether a vented trap actually stays protected, and it is the one most often built wrong. The arm has a maximum length set by its pipe size, because the longer it runs, the more it flows full, and a full arm siphons the trap before the vent can break the vacuum.

There are two limits working at once. The length limit caps how far the vent can be from the trap, longer for bigger pipe. The fall limit caps how much the arm can drop on its way to the vent, generally not more than one pipe diameter, because once the weir sits below the vent opening the trap self-siphons no matter what the length is. The full tables, the length-by-size limits, and the slope rules live in the DWV and venting guide, and they are worth knowing cold.

The number to carry: the trap arm is trap weir to vent, not the whole fixture drain, and an inspector measures it with a tape, not an eye. A 2-inch arm run 10 ft to reach a vent passes a glance and fails the measure. Get the vent inside the arm limit at rough-in, because moving it after the wall closes is a demolition.

Trap material and size by fixture

A fixture trap is sized to the fixture, never smaller than the fixture's drain outlet and never so large that the discharge stops filling it. Oversizing a trap is its own mistake: too big and the flow no longer fills the trap enough to scour it, so it fouls. The trap size matches the fixture, off the code's fixture-trap table.

The common minimum trap sizes run by fixture type. A lavatory takes a 1-1/4 inch trap, a kitchen sink and a tub and most residential fixtures take 1-1/2 inch, a shower commonly takes a 2 inch trap, and a floor drain is usually 2 inch or larger. A water closet has its trap built in and does not get a separate one, which the next section covers. Verify the sizes against the adopted code's fixture table, because the minimums are set there and a few fixtures differ.

Material is PVC or ABS plastic for most drainage traps today, with a slip-joint tubular trap, chrome-plated brass or plastic, under exposed lavs and sinks where it shows. The old cast and lead traps are gone from new work. Plastic tubular traps are cheap and serviceable; chrome brass is chosen where it is visible and wants to look finished. What matters more than the material is that the trap is the right size, holds a maintained seal, and connects to a proper trap arm and vent.

The cleanable trap and the cleanout

A trap has to be cleanable, because it is the low point where things settle and the place a clog forms first. The trade handles this two ways: a trap you can take apart, or a trap with a cleanout plug built in. The slip-joint tubular P-trap under a sink is the take-apart kind. Loosen the slip nuts by hand, drop the trap into a bucket, and the clog or the lost ring is right there. That serviceability is why the visible trap under a lav is a slip-joint and not a glued joint.

Where the trap is not accessible to take apart, a glued or cast trap below a floor, it gets a cleanout. The cleanout is a threaded plug at the bottom of the trap or just downstream, sized to the pipe, that opens to let a cable or a snake reach the trap and the line past it. A trap with no way to clean it is a service call waiting to become a demolition.

The rule on every trap is the same: there has to be a way in. Glue a trap solid under a slab with no cleanout and the first clog means breaking concrete. Set the slip-joint where it can be reached, or set the cleanout where a cable can actually enter it, facing the right direction. The access is not optional, and the inspector checks for it.

Prohibited trap practices

A handful of trap practices are flat prohibited, and they are the ones an inspector fails on sight. The S-trap leads the list, any trap that drops vertically into the drain with no vent between, because it siphons its own seal. The bell trap, the crown-vented trap, the drum trap in new work, and any trap that depends on moving parts or an interior partition to hold the seal are all prohibited, because each loses the seal in a predictable way.

Double-trapping is the one that catches good plumbers. You cannot put two traps in series on one fixture or one drain run. Two traps trap the air between them, and that trapped air has nowhere to go when the fixture drains, so the section between the traps airlocks and the fixture drains slowly or not at all, and one of the seals gets pulled. One fixture, one trap. A fixture with an integral trap, like a water closet, does not get a second trap added downstream.

A trap with no vent is prohibited in effect, because an unvented trap siphons. So is a trap set too far below its fixture: the vertical drop from the fixture outlet to the trap weir is limited, commonly to 24 inches, because a long drop lets the discharge build speed and self-siphon. And a trap has to be the right type for the fixture and the waste. Verify the prohibitions against the adopted code, because the list and the narrow exceptions, like a drum trap used as an interceptor, are written there.

The water closet's integral trap

A water closet does not get a separate trap, because the trap is built into the china. Look at the side of a toilet bowl and the S-curve cast into the body is the trap, with the seal sitting in the bottom of the bowl, the standing water you see in it. That standing water is the trap seal, the same barrier against sewer gas that a P-trap holds under a sink, just integral to the fixture instead of a separate fitting.

This is why you never add a trap below a toilet. The closet flange connects the bowl's integral trap straight to the drain, and a second trap in that line would double-trap the fixture and airlock it. The bowl traps itself. The drain just carries it away. The same goes for other fixtures made with an integral trap; the code exempts a fixture with an integral trap from the separate-trap requirement.

The toilet's seal is refilled every flush, so it does not evaporate the way a floor drain does, but it can still be lost. A blocked or undersized vent siphons a bowl, which shows as a low water line after a flush, less seal than there should be. A bowl that keeps reading low is a venting problem at the fixture, the same physics as any other siphoned trap, covered in the DWV and venting guide.

Why does a fixture smell like sewer?

A fixture that smells like sewer has lost its trap seal, and the job is figuring out which way it went. Pour water in first. If the smell stops and stays gone, the trap was dry from evaporation, and a fixture used that rarely needs a primer or a barrier device. If the smell stops and comes back in days or weeks, the trap is being emptied actively, by siphonage or back pressure, and the vent is the suspect.

Walk it in order. Confirm there is a trap at all, because a fixture plumbed with no trap, or with an S-trap, smells no matter what. Check the trap arm length and the vent, because an unvented or over-long arm siphons the seal every time the fixture drains. Look for back pressure if the fixture sits low on a tall stack and gargles when the building drains. Look for a rag or hair over the weir wicking the seal out by capillary action. And on a toilet, a low water line in the bowl after a flush points straight at a siphoning vent.

The mistake is chasing the smell with bleach and gels. Those mask it for a day. The smell is sewer gas coming through a lost seal, and until the seal is restored and protected, it comes back. Find why the seal is gone, fix that, and the smell ends.

Floor-drain and trap-primer programs on commercial and critical buildings

On a commercial or critical building, dry floor-drain traps are a managed problem, not a surprise. A big building has dozens of floor drains, floor sinks, and mechanical-room drains that almost never see water, and every one of them is a trap that will dry out and let sewer gas into the space if nobody keeps it primed. The smell complaint from a mechanical room or a back-of-house corridor is almost always a dry trap, and at scale it is a maintenance program, not a one-off bucket.

The design answer is primers at rough-in. Every infrequently-used drain gets a trap primer fed from a nearby fixture or the potable supply, or a barrier-type seal device, so the seal is maintained without anyone tending it. The facilities answer, for the drains that slip through or whose primers fail, is a route: a list of every floor drain, on a schedule, with someone pouring water and checking the primer. The ones that keep going dry get a primer added or a barrier device dropped in.

On a data center the stakes change but the rule does not. A dry trap in a room next to live equipment is a sewer-gas path into a space people work in, and hydrogen sulfide and the corrosion that comes with it are a problem around electronics. The mechanical makeup water for humidification and cooling is its own piping with its own backflow protection, off the project documents, and separate from the domestic floor drains. Keep the floor-drain trap-primer program documented and on a schedule, because on a critical building a forgotten dry trap is a callback that should never have happened. The fixture rough-in that sets these drains is covered in the fixture rough-in and setting guide.

What to document

Without a written trap schedule, nothing shows each fixture got the right type and size with a seal the vent protects, and the dry-trap callback later starts from guesswork instead of a record. The record is what proves each fixture has a trap of the right type and size, with a maintained seal and a vent that protects it, and it is what the next plumber reads when a fixture keeps smelling.

Capture the trap type and size at each fixture, whether the seal depth meets the code range, the trap arm length and fall against its limit, the vent method, and the primer or seal device on any floor drain or infrequently-used trap. Record which code and edition you worked to, because the seal depth, the trap-arm limits, and the prohibited types only mean something tied to the code that set them. For a floor-drain program, record the drain locations, the primer type, and the maintenance schedule, so the dry-trap callback has a route behind it.

Common mistakes

• Installing an S-trap, or any trap that drops vertically into the drain with no vent, so it siphons its own seal.
• Double-trapping a fixture, two traps in series on one run, which airlocks the section between them.
• Leaving a trap unvented, or running the trap arm past its length or fall limit, so the fixture siphons the seal.
• Setting a floor drain or other infrequently-used trap with no primer or seal device, so it evaporates dry and passes gas.
• Building a trap seal outside the 2-to-4-inch range, too shallow to hold or too deep to scour.
• Leaving a rag, string, or hair draped over the weir, wicking the seal out by capillary action.
• Oversizing or undersizing the trap for the fixture, so it will not scour or it restricts the drain.
• Adding a second trap below a water closet or other integral-trap fixture, which double-traps it.
• Gluing a trap solid with no cleanout or take-apart joint, so the first clog means breaking it open.

Field checklist

Standards and references

The framework lives in the two model plumbing codes, and the adopted edition with local amendments controls. The IPC, published by the ICC, and the UPC, published by IAPMO, both cover traps and trap seals in their traps chapters, along with the prohibited trap types, the seal-depth range, and the requirement that every fixture be trapped and every trap be vented. The common rules, a trap seal of 2 inches to 4 inches, one trap per fixture, no S-traps or bell or drum traps in new work, and a limited vertical drop from the fixture to the trap, all sit there. The section and table numbers shift between editions, so confirm them against the edition the jurisdiction enforces before citing one on a permit set.

Trap primers and seal devices run on their own product standards, referenced where the code accepts them. A water-supplied trap seal primer valve is listed to ASSE 1018, a wastewater-supplied primer device to ASSE 1044, and a barrier-type floor-drain trap seal protection device to ASSE 1072. The venting that protects the seal, the trap-arm limits, and the vent methods are in the DWV and venting guide, and the fixture rough-in that sets the trap and its drain is in the fixture rough-in and setting guide. Grease interceptors and the FOG program run off their own code provisions and the local sewer authority's rules.

The standard that controls any given call is the one the AHJ has adopted and enforces. The model code is the starting point. The adopted edition, the local amendments, and the inspector's interpretation govern the work, and the seal depth, the trap-arm distance, and the trap sizes in this guide are common figures to verify against that code, not numbers to install from memory.

Units and terms

Trap work carries its own vocabulary, and the same part shows up under different names across a plan set, a code book, and a supply-house counter.

The trap seal is measured in inches of depth. Trap and pipe sizes are nominal diameters in inches. The terms below are the ones a plumber and an inspector use to talk about the same parts of a trap without crossing wires.

Trap

The U-shaped fitting under a fixture that holds a water seal against sewer gas

Trap seal

The plug of water held in the trap bend, commonly 2 in to 4 in deep, that blocks sewer gas

Weir

The downstream lip of the trap where water spills over toward the drain

Trap arm

The drain pipe from the trap weir to the vent fitting, limited in length and fall, covered in the DWV and venting guide

P-trap

The standard fixture trap, a sideways U with a horizontal arm to a vent, that self-scours

S-trap

A prohibited trap that drops vertically after the weir with no vent and siphons its own seal

Trap primer

A device that feeds water to a trap on a schedule so the seal does not evaporate, listed to ASSE 1018 or 1044

Trap seal device

A barrier-type flapper that seals a drain against gas with no water supply, listed to ASSE 1072

Integral trap

A trap built into the fixture, as in a water closet, exempting it from a separate trap

Grease interceptor

A tank that catches fats, oils, and grease from a kitchen, a different job from a fixture trap

Deep-seal trap

A trap with a seal deeper than the standard range, used to resist siphonage on hard-to-vent fixtures

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