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What to check, and in what order

A drain or toilet gurgles when something else is used

Gurgling is air being pulled through a trap's water seal, because the pipe cannot get air the way it is meant to. It is worth acting on rather than living with: the same pressure that makes the noise can empty the seal entirely, and the seal is what keeps drain air out of the building.
  • 6Candidate causes
  • Most commonMost likely
  • No — candidates onlyA diagnosis

Where this starts

Gurgling is a pressure symptom, not a water symptom. Every fixture has a trap holding a small depth of water, and that seal is the only thing between the room and the drain. When water discharging somewhere in the system moves faster than air can follow it, it creates a partial vacuum behind itself — and the pipe takes air from wherever it can get it, which is through the nearest trap. The noise is air dragged through that water.

The reason it matters is what happens next. The same pressure difference that pulls air through a seal can pull the seal out altogether, and a trap with no water in it is an open pipe. That is the point at which drain odours, and in some circumstances gases, reach the room.

So the question is always where the air is supposed to come from and what is stopping it. A system has a designed air path — a vent stack, individual vents, or an air admittance valve — and the causes below are the ways that path fails or is overwhelmed.

The one distinction worth making early: gurgling that happens ONLY when a specific other fixture is used points at venting or at a shared branch; gurgling accompanied by slow draining anywhere points at a partial blockage, which is a different problem with a different urgency.

What it could be, and what tells them apart

These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.

  1. A partial blockage further down the system

    Most common

    Slow draining as well as gurgling, and usually worsening over weeks. More than one fixture affected points further downstream than one.

    How to check: Time how long a full basin takes to empty, and whether more than one fixture is slow. Two or more slow fixtures locate the restriction below where they join.

  2. A blocked or restricted vent

    Common

    Normal drainage speed but gurgling whenever another fixture discharges. Often seasonal — a vent terminal blocked by leaves, a nest, or frost.

    How to check: Whether draining is actually slow. Gurgling with normal speed is a venting symptom rather than a blockage one.

  3. A failed air admittance valve

    Common

    Gurgling that began after an installation or alteration, in a property where the vent is a valve inside rather than a pipe to the roof. Sometimes a smell near the valve.

    How to check: Find the valve — commonly in a loft, a cupboard or behind a panel — and listen at it while another fixture drains. These are wearing parts with a finite life.

  4. A fixture connected without adequate venting

    Common

    Present since the installation rather than newly developed, and tied to one specific fixture added or moved.

    How to check: Whether it ever did not do this. A symptom that dates from an alteration is a design issue rather than a fault.

  5. A long or steep branch siphoning its own trap

    Less common

    The fixture gurgles when IT drains rather than when another one does, sometimes with the seal audibly pulled at the end of the discharge.

    How to check: Note whether the noise comes at the end of that fixture's own discharge. Self-siphonage is a branch-length and gradient problem.

  6. A shared or overloaded stack

    Less common

    In a flat or a multi-storey building, gurgling tied to use elsewhere in the building rather than in the dwelling.

    How to check: Whether the timing matches your own use at all. If not, it is a building system rather than a dwelling one.

What to check, in order

The order matters — each step is here because of what it rules out for the ones after it.

0 of 5 checked
  1. 1

    It is the single split that separates a blockage from a venting problem, and everything after it branches on the answer.

  2. 2

    The pairing locates the problem: two fixtures that affect each other share a branch, and a fixture that gurgles at everything is closest to the failing air path.

  3. 3

    A blocked terminal is common, seasonal, and visible from the ground with binoculars. It is the cheapest cause to eliminate.

  4. 4

    Valves fail, they are a wearing part, and replacing one is inexpensive — but only if it is the cause rather than an assumption.

  5. 5

    It tells you whether the seal is merely being disturbed or is actually being lost, which changes this from a noise to a health matter.

The calculations behind this

Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.

Drain Pipe Slope Calculator →Storm Drain Pipe Slope Calculator →Domestic Water Pipe Sizing by Fixture Unit Calculator →

When to stop and get somebody in

  • Any persistent smell of drains inside the building, which means a seal is being lost rather than disturbed.
  • Waste water backing up into a bath, shower or gully, which indicates a blockage downstream of several fixtures rather than a venting issue.
  • Gurgling that started after building work, where a fixture may have been connected without the venting the layout needs — a design question rather than a clearance.
  • Anything in a shared building where the stack serves other dwellings, since the cause and the remedy may not be within your property.
  • Repeated blockages in the same place, which usually indicate a defect in the pipe — a displaced joint, a root, a sag — rather than what is being put down it.

This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.