Plumbing · Lesson 3 of 13Beginner11 min read

Drains, Traps and Vents

Drains look simple: water goes down. But a drain system has a hidden job. It has to carry waste away while blocking sewer gas from coming back up, and it does that with nothing more than a bend of pipe, a little water, and a pipe through the roof.

Gravity does the work

Nothing pushes water through a drain. Each horizontal pipe slopes gently downhill so gravity carries water and solids along. Too flat, and solids settle out; the Army manual and HUD’s standard for manufactured homes both use a quarter inch of fall for every foot of pipe. HUD allows an eighth of an inch per foot only where a quarter inch isn’t practical, and then only with a full-size cleanout at the upper end. The model code where you live sets the exact slopes for each pipe size.

A quarter inch per foot doesn’t sound like much, but it adds up. A 20-foot run falls 5 inches.

fall = slope × run 20 ft × ¼ in per ft = 5 in (about a 2 percent grade)

Slope exaggerated so you can see it. A quarter inch per foot is about a 2 percent grade.

The trap and its water seal

Sewers are full of gases that smell bad and can be harmful. If a sink drain ran straight down to the sewer, those gases would flow right up into the room. The fix is the trap: a U-shaped bend under every fixture that stays full of water after the fixture drains. HUD’s inspection guide describes it as designed to hold a water seal that blocks sewer gas from coming up through the drain.

The seal depth is the height of water between the low point of the top of the bend (the dip) and the lip where water spills out (the weir). HUD’s manufactured home standard calls for P-traps with a seal of 2 to 4 inches. It also bans S-traps, drum traps and bell traps, and HUD’s inspection guide explains why S-traps are no longer allowed by plumbing codes: they tend to siphon themselves dry.

A trap is a pressure gauge in disguise. A seal 2 inches deep can hold back a pressure difference of 2 inches of water. NIST puts an inch of water at about 249 pascals, so a 2-inch seal resists only about 0.07 psi (0.5 kPa), hundreds of times less than the pressure in your supply pipes. Tiny pressure changes in the drain can push or pull that water out. That’s the job of the vent.

The seal depth is the height of water between the dip and the weir. HUD’s standard for manufactured homes calls for P-traps with a seal of 2 to 4 inches; your local code may differ.

a seal of depth d holds back a pressure difference of d inches of water
2 in × 249 Pa per inch ≈ 498 Pa ≈ 0.07 psi

Drain the sink and watch the seal. Then block the vent and try again. The pressures shown are illustrations; real values depend on the pipe sizes, lengths and flow.

Why drains need vents

When a slug of water rushes down a drain, it acts like a piston. Ahead of it, air is pushed; behind it, air is pulled. Without a way for air to get in, that pull can suck the water right out of a trap, which is called siphoning. HUD’s inspection guide puts the vent’s job in one line: vents equalize the air pressure in the drainage system to prevent siphoning. HUD’s manufactured home standard requires every fixture trap to be protected against siphonage and back pressure.

Vents also let sewer gas out above the roof, where it can drift away, and they keep the drains flowing. The Union Sanitary District in California notes that when a vent is blocked, air can’t move and drains won’t work correctly. A vent through the roof has to stay open. HUD’s inspection guide says vents should be high enough above the roof that snow can’t close them, and HUD’s manufactured home standard sets a minimum of 2 inches above the roof.

The trap can’t be too far from its vent, either, or the trap arm itself can run full and siphon. HUD’s standard for manufactured homes sets these limits; your local code has its own table.

Maximum distance from a trap to its vent, by drain size (HUD standard for manufactured homes)
Fixture drain size	Maximum distance to the vent
1¼ inches	5 feet
1½ inches	6 feet
2 inches	8 feet
3 inches	12 feet

How a trap loses its seal

Siphoning. A full pipe flowing past, or the fixture’s own rush of water, pulls the seal out when the vent can’t supply air.
Back pressure. A surge of air pushed ahead of water in the stack can blow the seal up into the fixture.
Evaporation. In a drain nobody uses, such as a basement floor drain or a guest shower, the water slowly evaporates. The Union Sanitary District suggests running a little water into each trap every month.

HUD’s inspection guide gives an inspector’s test: run several fixtures and watch the trap water. If it moves more than an inch, the venting is probably inadequate or blocked.

You canYou can refill a dry trap

A sewer smell from a drain you rarely use is often just a dry trap. Run water into it for a few seconds. If the smell keeps coming back, or comes from a drain that’s used every day, the vent or the drain needs a plumber’s look (Lesson 11).

Stacks, the building drain and the sewer

Branch drains from each fixture run to a vertical pipe called a stack. A stack that carries toilet waste is a soil stack. At the bottom, the stack turns into the building drain, which the Army manual defines as the pipe that receives the waste from within the building. Just outside the foundation it becomes the building sewer (the Army manual calls it the house sewer), which runs to the public sewer main or a septic tank. Above the highest fixture, the stack keeps going up through the roof as a vent.

Drains turn corners with gentle fittings, such as wyes and sanitary tees, so waste doesn’t slam into a wall of pipe and stop. And every system needs cleanouts: capped openings where a plumber can run a cable. HUD’s inspection guide says to put them near where the main vertical drains join the building drain.

Simplified. Each fixture has a trap, and each trap has a vent within a set distance. Drains are dark gray; vents carry only air.

Drain, waste and vent parts
Part	What it does	Signs of trouble
Trap	Holds a water seal that blocks sewer gas.	Sewer smell, gurgling, a slow drain from a clog in the bend.
Trap arm	The sloped pipe from the trap to the vent.	Too long or too steep and the trap can siphon.
Vent	Lets air in and out so traps keep their seal; releases gas above the roof.	Gurgling, slow drains, smells; blocked by debris, nests or snow.
Branch drain	Carries one or more fixtures’ waste to the stack.	Slow drains on one fixture or one room.
Stack	The vertical drain and vent pipe that collects the branches.	Noise, leaks at joints, backups on lower floors.
Cleanout	A capped opening for clearing clogs.	Missing or buried cleanouts make clogs costly.
Building drain	The lowest drain inside the building.	Backups in the lowest fixtures or a floor drain.
Building sewer	Carries everything to the public sewer or septic tank.	Several drains slow at once; tree roots; sewage in the yard.

Septic systems

EPA’s SepticSmart program explains the conventional system step by step. All the water from the house runs out one main drain pipe into a septic tank: a buried, watertight container, usually concrete, fiberglass or polyethylene. Solids sink to the bottom as sludge; oils and grease float to the top as scum. Compartments and a T-shaped outlet keep the sludge and scum in the tank, and the partly treated liquid, called effluent, flows out to the drainfield. That’s a shallow, covered excavation in unsaturated soil. The soil accepts, treats and disperses the effluent as it soaks through, removing harmful bacteria, viruses and nutrients. Send too much water to a drainfield and it can flood, with sewage coming up to the surface.

EPA’s care rules: have a typical household system inspected at least every three years by a septic service professional, and expect the tank to be pumped every three to five years. Flush only human waste and toilet paper. Avoid chemical drain openers. Don’t park or drive on the drainfield, keep trees at a distance, and send roof drains and sump pumps away from it.

A conventional septic system. Source: U.S. EPA SepticSmart.

StopWhen drains back up in more than one place

If water comes up in a tub when the toilet flushes, or the lowest drains back up, the blockage is probably in the building drain or sewer, or the septic system is full or failing. Stop running water and call a plumber, or a septic professional if you have a septic system. Being an Adult 101 says the same (Unclog a Sink or Toilet).

Check yourself

Tap an answer to check it, or open “Show the answer.”

How far does a drain fall over a 12-foot run at a quarter inch per foot?
Show the answer
3 inches. 12 × ¼ = 3 inches.
What does a trap’s water seal do?
Show the answer
Blocks sewer gas from coming up the drain. The plug of water in the bend blocks sewer gas.
Why do drains need vents?
Show the answer
To let air in so flowing water doesn’t siphon the traps dry. Vents equalize the air pressure in the drains so traps keep their seals.
A basement floor drain that’s never used starts to smell like sewer. What’s the likely cause?
Show the answer
Its trap has dried out. Water in an unused trap evaporates. Pour water in to restore the seal.
According to EPA, how often is a typical household septic tank pumped?
Show the answer
Every three to five years. Every three to five years, with an inspection at least every three years.

Go deeper

Union Sanitary District (California): Maintaining Traps and Vents A sanitary district’s short guide to traps and vents.
U.S. EPA SepticSmart: How to Care for Your Septic System EPA’s SepticSmart care guide.
24 CFR 3280.606, Manufactured Home Standards: Traps and Cleanouts (via Cornell LII) An official code text on traps and cleanouts (manufactured homes).
Being an Adult 101: Unclog a Sink or Toilet When the drain is just clogged.

Sources

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A free course from Teddy Rook Book. It explains how plumbing works in general and follows U.S. sources; your local plumbing code, the authority having jurisdiction and manufacturers’ instructions are the final word.