Gurgling Drains and Water Backing Up When the Washing Machine Drains

Why drains gurgle

When water flows down a pipe it pushes air ahead of it and pulls air behind it. If the pipe is partly blocked, or the vent that lets air in is blocked, that air has to come from somewhere — so it bubbles up through the nearest trap. That is the gurgle you hear in the sink or the toilet.

Each gurgle also pulls water out of the trap, which is why gurgling drains often come with a sewer smell. See bathroom smells like sewer.

The washing machine test

A washing machine pumps out a lot of water very fast. If the shared waste pipe is partly blocked, that water has nowhere to go and comes back up the nearest opening — the kitchen sink, a floor drain or the shower.

Water appearing in the kitchen sink when the machine empties means the blockage is in the pipe they share, beyond the sink's trap. Cleaning the sink trap will not fix it.

Your flat or the building?

Only your fittings affected, only when you use water: the blockage is in your flat's pipes.

Gurgling or water coming up when neighbours use water, in the lower flats especially, or problems in several flats: the building's shared column or drain. That is for the condominium — see who pays for a blocked building drain.

What you can do

Clean the traps of the affected sinks and the washing machine's drain hose connection — see kitchen sink blocked.

Check that the washing machine hose is not pushed too far into the standpipe or trap; it needs an air gap.

Avoid running the washing machine and dishwasher at the same time until it is fixed.

What a plumber does

We clear the shared pipe with a drain snake and, where grease and scale have narrowed it, with high-pressure jetting so it stays clear.

If gurgling continues with a clear pipe, the vent is the likely culprit. That usually means access from the roof or a shared area, arranged with the building.

Why it gets worse if you wait

A partial blockage catches more and more waste. What is a slow drain this month is often a sink full of dirty water next month — typically when the washing machine is halfway through a cycle.

How washing machine drains are connected

The drain hose either hooks into a standpipe (a vertical pipe with a trap) or connects to a spigot on the sink trap. The hose must not be sealed airtight into the standpipe and should not be pushed too deep, or the machine can siphon water back out and the drain can gurgle.

Many Lisbon kitchens connect the machine to the sink waste — convenient, but it means sink and machine share a narrow pipe.

Upper floors versus lower floors

On upper floors, gurgling is more often a venting issue: fast-flowing water in the column pulls air through your traps. On lower floors, water coming back up is more often a blockage further down the column or the building drain.

Either way, if more than one flat notices it, tell the building administrator.

Temporary measures

Run the washing machine at times when others use less water. Avoid using the machine and dishwasher at the same time. Keep plugs in rarely used drains to reduce smells until it is fixed.

These buy time; they do not fix the cause. Clearing starts from 75 €.

Frequently asked questions