Gurgling drains and what the noise is telling you

What a gurgle actually is

Every fixture has a trap: the U-bend under a sink, the cast-in bend inside a toilet, the drum or P-trap under a tub. The water sitting in that bend blocks sewer gas. When a large slug of water rushes down the stack and the plumbing cannot pull enough air from the roof vent, it sucks air through the nearest trap instead. You hear that as a gurgle, and you may smell it too, because a trap that is siphoned dry no longer seals.

The usual causes we find

• A partial clog in the branch line, so water backs up and drags air behind it
• A roof vent capped by a bird nest, leaves or, in February, a frost plug
• A mechanical air admittance valve under an island sink that has stuck shut
• A main line slowed by roots or a sag, so the whole system drains under strain
• An S-trap left from an old renovation, which siphons itself on every use

The check you can do first

1. Run the bathroom sink and watch the tub and toilet beside it. If they bubble, the shared branch is the problem area.
2. Fill a bucket and dump it quickly into the toilet. A slow swirl and a gurgle from the basement floor drain point at the main line.
3. In winter, look at the vent pipe on the roof from the yard. Visible frost build-up at the opening is a strong clue.

What we do on the visit

Off comes the trap (or the cleanout plug) so we can feed a cable through the branch, then flow test with full basins. If the branch is clear, the vent goes on the list: a garden hose down the stack from the roof shows us whether it is open, and a camera tells us where the blockage sits. Air admittance valves get swapped for a new one rated for the fixture load. A buried S-trap is cut out and rebuilt as a vented P-trap so it stops siphoning.

When it cannot wait

A gurgle that comes with water rising in the basement floor drain or the lowest toilet means the main is close to blocking. Stop running water and laundry, and call.