Why is my drain gurgling?

Oatey, a plumbing-parts manufacturer, gives a direct answer: "Gurgling (a hollow bubbling sound) while draining is usually a sign of a partial blockage in the system." It explains that vents in the drain, waste and vent system "promote airflow throughout the piping system," and that "as water flows through the pipe, a partial blockage will cut off that airflow." When a blockage closes the pipe completely, Oatey says the air escapes "via the drain instead," and that is the gurgle.

A drain and its vent work as a pair. Water leaving a fixture needs air behind it, the way a can with one hole glugs and one with two holes pours smoothly. The Phoenix plumbing code describes the vent system's job this way: it must "permit the admission or emission of air so that the seal of any fixture trap shall not be subjected to a pressure differential of more than 1 inch of water column." Put simply, the vent keeps the water in each trap from being pulled or pushed out.

So a gurgle points to one of two things:

• A partial clog in the drain, which slows the water and squeezes the air out through the nearest trap.
• A blocked or poorly working vent, which starves the system of air so the water has to pull air through a trap.

### What is a trap, and why does it make the noise?

Every fixture has a trap, the curved pipe that holds water. The 2024 code that Phoenix follows says "each fixture trap shall have a liquid seal of not less than 2 inches (51 mm) and not more than 4 inches (102 mm)." That water seal blocks sewer odors from the building. When air is forced through it, you hear it bubble.

Look at where and when the gurgle happens.

One fixture, only when it drains. A sink that gurgles as it empties, and drains slowly, has a partial clog in its own line, most often in the trap or just past it. Try the steps on our slow-draining sink page.

One fixture, when a different fixture runs. If the bathroom sink gurgles when the toilet flushes, or the kitchen sink gurgles when the dishwasher or washing machine drains, the fixtures share a line or a vent. See sink gurgles when I flush, kitchen sink gurgles when the dishwasher drains and sink gurgles when the washer drains.

A toilet that gurgles. That may be a clog in the toilet's own line or farther down. Our toilet gurgling page covers it.

Several fixtures at once. Oatey lists water coming back up in other fixtures as a sign of a blockage and says a full blockage farther down the line can cause backups from other fixtures. Gurgling at more than one fixture, especially with slow drains, points to the branch or main line. Our main line vs branch clog page shows how to tell them apart.

Gurgling with a sewer smell. Oatey says that poorly installed, blocked or weather-damaged vents can create negative pressure that "can pull water out of P-traps or force gases back through fixtures," and lists "gurgling drains, slow drainage, and intermittent sewer odors" as common signs. If you smell sewage as well, do not wait.

### Why does a blocked vent matter?

The code's own introduction to its vent chapter says a drainage system depends on keeping an air path open so waste and odor cannot blow back into fixtures and trap seals are not siphoned. Oatey describes the same effect, saying negative pressure can "pull water out of P-traps." A trap that has lost its water seal no longer blocks odors, which is why a vent problem can show up as a gurgle and a smell together.

These are safe first checks, and none of them takes tools.

1. 1Run one fixture at a time. Note which one gurgles and whether it is slow. Then run a second one. The pattern tells you whether the problem is local.
2. 2Check for a slow drain. Fill the sink and let it drain. Oatey suggests filling a tub or sink and letting it drain to see whether it flows freely, and not leaving it unattended, as a fixture can overflow.
3. 3Clear the local clog. For a sink or tub, clean the stopper and strainer, then try a plunger. Our safe alternatives page has the steps. Avoid chemical cleaners in a drain that is gurgling, because a partial blockage farther down means the liquid may not reach it.
4. 4Look at the roof vent, from the ground. Plumbing vent pipes come through the roof, and code requires each open vent to terminate outdoors. Do not climb on the roof. If you can see a vent from the ground that looks damaged or covered, tell a plumber. Our roof vents page explains what they do.

### Can a dry trap make a drain gurgle?

A trap that has dried out, in a guest bathroom or a floor drain that rarely runs, lets sewer air into the room. Oatey lists checking P-traps for water as a step when you have a sewer smell. That is a smell problem more than a gurgle, but if a rarely used drain smells, pour water into it and see if the smell goes away.

Since a partial clog is the usual cause, keeping clogs from forming is the best prevention. Oatey's list for keeping lines clear includes using food strainers or hair catchers, flushing the drains monthly by filling the tub or sink and letting it drain, not flushing anything but toilet paper, and having the sewer line scoped every two years if you have had problems. Phoenix Water Services says cooking grease should be allowed to cool and then be thrown in the garbage, not poured down a drain, and it names grease and roots as the two main causes of sewer backups. Our why the same drain keeps clogging page has more on repeat clogs.

Call if any of the following is true:

• More than one fixture gurgles.
• Drains are slow in several places, or water backs up in a tub, shower or floor drain.
• You smell sewage along with the gurgle.
• The gurgle returns within days of clearing a clog.
• You suspect a vent problem. Vents run through walls and roofs and are not a homeowner job.

Phoenix Water Services says grease and roots are the two main causes of sewer backups, and that the owner is responsible for the line from the house to the city main. A gurgle at several fixtures can be an early sign of either. Oatey recommends that anyone who suspects a blockage address it "as soon as it arises." A camera inspection shows what is in the line.

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