Physics of Buildings

Physics of Buildings · Drains

Why your drain gurgles or smells like sewer

Under every sink a few inches of water hold the sewer back, and a vent keeps the air from pulling it out. Here is that system cut open and running. Flush, drain, block the vent, or pick your symptom.

A basin, its P-trap, the vent and the drain it shares with a toilet, cut open and simulated (not a recording; the trap is drawn enlarged). Flush or drain, then drag the red-dotted handle beside the roof vent down to block it. The meter and the trace show the air pressure at the trap in inches of water: amber is the ±1 in. band vents are designed to hold, the red mark the suction that breaks this seal.

Every drain in a house has a trap: a bend that stays full of water, normally 2 to 4 inches deep, which keeps sewer air on its own side of the pipe [1]. That seal is held in place by nothing but its own weight, so it has a weakness. Change the air pressure in the drain by a couple of inches of water and the seal is pulled out or pushed back. The vent is the pipe that stops that from happening: it lets air in and out of the drain so the pressure at every trap stays within about one inch of water of the room’s [2][3]. Gurgling, a trap that will not stay full, and sewer smell are the ways that arrangement fails.

Figure 1 The same simulated basin, trap, vent and drain, pinned while you read. Each step switches it to one situation; drag the vent handle or change anything to take over, and use Return to the story to hand back. Simulated with representative numbers; the trap is drawn enlarged.#

1. A healthy trap and vent

At the fixture: nothing. The toilet flushes and the basin stays quiet. In the pipe: the water falling through the shared drain drags air along with it, and that air has to be replaced. With the vent open, it comes down the vent from the roof. Pulling 5 litres a second through 20 feet of 1½-inch pipe costs about a quarter of an inch of water, so the pressure at the trap dips by that much and recovers.

Why the trap survives: a trap is a U-tube manometer. A suction of a inches of water lifts the outlet side and lowers the inlet side until the two levels differ by a [4]. A quarter of an inch spills a little water over the weir and leaves the seal at about 1.9 of its 2 inches. That is the vent doing its job: designers size it to hold the pressure within ±1 inch [3].

Question 1 Choose an answer before you scroll on.#

2. No vent: the basin siphons its own trap

At the fixture: the basin drains, then gives a loud gulp at the end. In the pipe: the meter swings to the seal’s limit and stays pinned there until the water has gone. The toilet’s bowl, on the same branch, feels the same pull: its water drops too.

Why: with no vent near the trap, the water falling through the drain below it acts as the long leg of a siphon. It needs air to follow it, and the only air available is on the far side of the trap. Suction builds until it equals the seal depth, the inlet side reaches the dip, and room air bubbles through: the gurgle. This is self-siphonage, the fixture emptying its own trap, and its severity depends on the drain’s length, slope and diameter, the trap and the rate of discharge [1].

Also: codes deal with it by limiting how far, and how far down, a trap arm may run before it meets its vent; the Uniform Plumbing Code, for example, does not let the vent opening sit below the trap’s weir [5]. Requirements differ between the codes in use (IPC, UPC and local amendments), so what is allowed where you live is a question for the local code and a licensed plumber.

3. Blocked vent: the sink gurgles when the toilet flushes

At the fixture: flush the toilet and the basin (or a tub, or the toilet’s own bowl) gurgles. In the pipe: the same flush as step 1, but the vent is 88 percent blocked, and the suction runs straight past the band to the seal’s limit.

Why: the flush needs the same air as before. Through a nearly closed vent it can get only part of it, so the pressure in the branch falls until the next path opens, and the next path is the basin’s trap. Water spills over the weir, the inlet side drops to the dip, and room air is pulled through in gulps. In this idealised model the seal is left half full after one flush. Losing seal to another fixture’s discharge is what the NBS researchers called siphonage, to tell it apart from self-siphonage [1].

Also: a vent can be choked by leaves, a nest, ice or debris at the roof, or be too small or too long for the flow it serves. Drag the vent handle in the figure: the trap holds until the vent is roughly three-quarters blocked, then fails quickly. A drain that is slow without any gurgling points first to a partial blockage in the trap or the drain line, which no vent can fix [1].

4. S-trap: the shape that siphons itself

At the fixture: the basin drains fine, sometimes with a slurp, but there is a faint sewer smell near it. In the pipe: the outlet of the trap turns straight down, and the vertical leg runs full of water as the basin empties.

Why: an S-trap carries its own siphon leg. Our drawing gives it 18 inches of full-bore drop under a 2-inch seal, and nothing to let air in at the top of that leg. Every time the basin drains, the seal is pulled to the dip and air breaks through. The idealised model leaves half the water behind; real S-traps often lose more, because the water keeps moving after the flow ends and bubbles drag water out with them [1]. A half-empty seal then holds only half the back-pressure it did, so the next push of air down the drain lets drain air through. The Uniform Plumbing Code prohibits S-traps outright [5].

5. Dry trap: the smell from a drain nobody uses

At the fixture: a sewer smell from a guest-bathroom shower, a basement floor drain or a utility sink that has not run for weeks. In the pipe: no event at all. The meter reads zero; the trap is empty.

Why: water evaporates from both surfaces of the trap. With no water there is no seal, and drain air drifts into the room with no pressure difference needed. A dry trap is the most common cause of sewer odour indoors [6], and a university safety procedure traces localised sulfur smells to dry sink traps and unused floor drains, prevented by running water into them at least once a week [7]. How fast a trap dries depends on heat, dry air and air moving over it: measurements from the 1880s found anywhere from about one inch a week to more than two [8]. The figure assumes a slower quarter-inch a week, which empties a 2-inch trap in 8 weeks.

Also: codes require trap seal primers, devices that top up the water, on floor drains that see little use [5]. A smell that comes back right after the trap is refilled points elsewhere: a cracked or loose drain or vent pipe, a failed wax ring under a toilet, or a trap whose seal is being pulled out (steps 2 to 4).

6. Air admittance valve: a vent that only breathes in

At the fixture: nothing, as in step 1. In the pipe: when the flush pulls, the valve’s seal lifts and room air flows in; the suction stops at a little more than half an inch. When the flow stops, the valve drops shut.

Why: an air admittance valve (AAV) is a one-way vent. It opens under a small suction (one maker lists −70 Pa, about a quarter of an inch of water) and is tight at zero or positive pressure, so sewer air cannot come out of it [9]. That is also its limit: it cannot let air out. Pressure pushing up the drain must still escape through an open vent, which is why the International Plumbing Code still requires at least one vent pipe to the outdoors on every building drain, and sets limits on where AAVs may be used [10].

Also: AAVs must stay accessible for replacement and be installed the way the code and the maker require [10]. Whether one is acceptable in your house is a local code question.

7. Wind over the roof: gurgles on a gusty day

At the fixture: a gurgle or a faint whiff on windy days, with nothing draining. In the pipe: the meter rocks below zero with each gust.

Why: air moving fast across the open top of the vent is at lower pressure than still air (Bernoulli) [4], and the vent connects that pressure straight to the trap. At 55 mph, with an illustrative pressure coefficient of −0.5, gusts pull about three-quarters of an inch of water: inside the design band, but each gust spills a little and rocks the seal. Real traps lose more than this quasi-static model shows, because the water sloshes in time with the gusts.

Also: a vent terminal close to a roof edge, a wall or a ridge sits where the wind speeds up most. Changing a vent terminal is roof work and a code matter: a job for a licensed plumber.

Common causes, by symptom

The tour simulates one mechanism for each symptom. Real problems overlap, so here is the longer list. It is a guide to describing what you hear and smell, not a diagnosis; a licensed plumber confirms the cause before pipes are opened.

  • A sink, tub or toilet gurgles when another fixture drains. Air is being pulled through its trap because the drain cannot get enough from the vent: a blocked or undersized vent, or a fixture with no vent at all [1][2].
  • A basin gurgles as it finishes draining. Self-siphonage: a trap arm that is too long, too steep or not vented, or an S-trap [1][5].
  • Sewer smell in the bathroom. Most often a dry trap on a fixture or floor drain that is rarely used [6][7]. If refilling does not cure it: a seal being siphoned (an S-trap, a missing or blocked vent), or drain air escaping from a cracked pipe, a loose joint or a toilet’s base seal.
  • Slow drain. Without gurgling, usually a partial blockage in the trap or drain line, which limits how fast the fixture can empty [1]. With gurgling, also suspect the vent.
  • Gurgling or smell on windy days. Wind across the roof vent changing the pressure in the drain [4].

Sewer gas is more than a smell. Its rotten-egg note is hydrogen sulfide, noticeable at about half a part per billion; at high concentrations people stop smelling it within minutes, and very high levels can cause sudden collapse [11]. Sewer gas can also carry methane, which is odourless and explosive between 5 and 15 percent in air [12]. A strong or worsening smell, or any smell with headache or dizziness, means ventilate and leave the space, and in a sewer or a pit never go in to look. Vent and stack repairs, roof access, and anything involving the main drain are jobs for a licensed plumber; codes differ from place to place, and nothing here says what your local code allows.

What a trap and a vent do

A trap is a U-shaped bend in the drain right below the fixture. Water fills it up to the weir, the lip where the outlet turns toward the drain; the dip is the top of the bend’s lower surface. The height from weir to dip is the trap seal, normally 2 to 4 inches [1], and the Uniform Plumbing Code requires a seal in that range on every fixture [5]. Air cannot pass the trap without pushing its way through that water.

A vent is a pipe that connects the drain, just downstream of the trap, to the outdoors (usually through the roof). It does two jobs. When water rushing down the drain needs air to follow it, the vent supplies that air instead of the trap. When water pushes air ahead of it, the vent lets the air out. The International Plumbing Code states the goal in pressure terms: vents keep the difference across any trap seal within one inch of water column [2], the same ±1 inch criterion the National Bureau of Standards used in the research behind modern vent tables [3].

Why gurgling means air is going through the seal

The trap is a manometer: the difference between the water levels in its two sides is the pressure difference between the drain and the room, in inches of water [4]. Pull on a full trap from the drain side and the outlet side cannot rise past the weir, so water spills over and the inlet side falls. When the suction equals the seal depth, the inlet side reaches the dip and the next bit of suction pulls air through: that is the gurgle [1]. Push from the drain side instead and both sides move, one up, one down, so a full trap holds twice its depth against back-pressure. The essay on siphonage hands you a single trap to pull and push, so you can find the breaking points yourself.

A gurgle is therefore a symptom of two things at once. The pressure in the drain has gone past the seal, and the trap is now partly empty, so the next push of drain air (back-pressure) breaks it more easily, and real traps often lose more than the model’s half. That is why a gurgle is worth tracing even when nothing smells yet.

Air admittance valves

An air admittance valve is a vent that sits indoors, usually under a sink, instead of running to the roof. It opens when the drain pulls a small suction and closes at zero or positive pressure [9][10]. It protects a trap from siphonage, which is most of what a vent does, but it cannot relieve pressure, so it does not replace the building’s open vent to the outdoors [10]. Where an AAV is allowed, which fixtures it may serve, and how high above the drain it must sit are set by the code in force and the maker’s instructions.

Go deeper

The essay Why a trap loses its seal explains the physics behind every step above: why a full trap resists back-pressure twice as well as suction, why traps are 2 to 4 inches deep and not 12, why an S-trap fails, and how a vent’s size sets the pressure. The trap seal calculator tells you whether a given suction, back-pressure, wind or dry spell breaks a given seal. For another building system explained the same way, see how a well pressure tank works.

Physics of Buildings explains the systems inside homes and commercial buildings through their physics. Every model is checked by an independent verifier against the sources cited; see about and the sources.

Sources

  1. John L. French and Herbert N. Eaton (1951). Self-Siphonage of Fixture Traps, Building Materials and Structures Report BMS 126. National Bureau of Standards, Washington, DC. government research report Foreword: the water seal is normally 2 to 4 inches deep; lost seal lets drainage-system pressure fluctuations bubble sewer air back through the trap. §1 fig. 1: definitions of trap weir, depth of seal, remaining seal, seal loss; siphonage by other fixtures vs self-siphonage. §2: inertia of the water at the end of the discharge, air bubbles dragging water, the inlet leg pulled down to the dip so air passes; refill by trail discharge. §5.2: discharge set by orifice, trap, drain and vent fitting.
  2. Gary Gauthier, Building Safety Journal (2025). Code Corner: 2024 International Plumbing Code Chapter 9: Vents. International Code Council. code body article IPC 901.2: vent piping permits the admission or emission of air so the pressure difference across a trap seal stays within 1 in. of water column; positive pressure pushes a seal back, negative pressure pulls it over the weir; §1002.4 has provisions for traps subject to evaporation.
  3. Robert S. Wyly and Herbert N. Eaton (1961). Capacities of Stacks in Sanitary Drainage Systems for Buildings, NBS Monograph 31. National Bureau of Standards, Washington, DC. government research report §4.5: vents are sized to hold pneumatic pressures within +1 to −1 in. of water column of atmospheric, so pressure effects on trap seals are negligible; vent sizes for a 1 in. drop computed with Darcy–Weisbach plus loss coefficients. §7: vent stacks sized so pressures do not destroy trap seals. Measured air-to-water flow ratios in a stack.
  4. Samuel J. Ling, Jeff Sanny and William Moebs (2016). University Physics Volume 1, §14.2 Measuring Pressure and §14.6 Bernoulli’s Equation. OpenStax, Rice University. textbook §14.2: a manometer column of height h balances a pressure difference hρg; gauge vs absolute pressure. §14.6 (https://openstax.org/books/university-physics-volume-1/pages/14-6-bernoullis-equation): p + ½ρv² + ρgh = constant along a streamline; pressure drops where a fluid moves fast.
  5. IAPMO (2018). Uniform Plumbing Code (2018), Chapter 10: Traps and Interceptors (Nevada adoption, printed copy). IAPMO; copy hosted by the City of Lynnwood, WA. plumbing code 1002.1 every trap protected against siphonage and back-pressure by a vent; 1002.2 trap arm length limits (table); 1002.4 vent opening not below the trap weir; 1004.1 S traps, bell traps and crown-vented traps prohibited; 1005.1 liquid seal 2 to 4 in.; 1007.1 trap seal primers for infrequently used floor drains. Described, not reproduced.
  6. Union Sanitary District (2024). Prevent and Report Odors. Union Sanitary District, Union City, CA. public utility A dry trap is the most common cause of sewer odors indoors; every shower, tub, floor drain and toilet has a trap; when the trap is dry, odors may enter the building.
  7. Colorado State University Pueblo, Environmental Health and Safety (2019). Preventing Odors from Dry Sink Traps, Standard Operating Procedure EHS-017. CSU Pueblo Facilities Management. university procedure Localised sulfur-like odours frequently trace to a dry sink trap or unused floor drain; without water in a trap, sewer gas is drawn into the room; prevent by running or pouring water into drains at least once a week.
  8. J. Pickering Putnam (1911). Plumbing and Household Sanitation (chapter: The Siphonage and Evaporation of Traps). Doubleday, Page & Co.; reproduced at chestofbooks.com. historical text Reports evaporation experiments signed June 1884 (Putnam and Rice): about 1/7 in. a day in traps vented 3 in. past the crown, over 1/3 in. a day with the vent at the crown; fastest in winter and with heated flues, slowest in damp summer weather; a 1¼ in. S-trap seal lost in four to eleven days.
  9. Studor (2018). Studor Mini-Vent Air Admittance Valve: Product Information/Specification Sheet. Studor (IPS Corporation); copy at plumbingsupply.com. manufacturer data sheet Opens and admits air under the negative pressure of a discharging fixture; closes by gravity when flow stops; opening pressure −70 Pa; tight at 0 Pa or higher; air flow capacity 7.5 L/s (EN 12380).
  10. International Code Council, PMG Membership Council (2023). CodeNotes: Installation of Air Admittance Valves (based on the 2021 IPC and IRC). International Code Council. code body guidance An AAV lets air into the drainage system under negative pressure to prevent trap siphonage; closes and seals at zero or positive pressure; does not relieve positive pressure, so at least one vent must extend outdoors; ASSE 1051 (branch) and 1050 (stack) valves; must be accessible; manufacturer's instructions apply.
  11. Agency for Toxic Substances and Disease Registry (2024). Hydrogen Sulfide ToxFAQs. U.S. Department of Health and Human Services (CDC/ATSDR). public health agency Rotten-egg smell noticeable at about 0.0005 ppm; often called sewer gas; smell lost after 2–15 min near 100 ppm, so odour is not a reliable warning; very high levels (> 500 ppm) can cause sudden unconsciousness or death; flammable.
  12. ILO and WHO (2018). International Chemical Safety Card 0291: Methane. International Labour Organization. international safety card Colourless, odourless gas; explosive limits 5–15 % by volume in air; can cause suffocation by lowering oxygen in confined areas.