Why does my toilet gurgle when the washing machine drains?

A washing machine empties its drum in a minute or two, far faster than a sink. That slug of water moves down the pipe like a piston, pushing air ahead of it and leaving a vacuum behind. Normally the vent pipe through the roof lets air in and out, so the water in each fixture's trap stays put. When a clog narrows the drain, or the vent cannot breathe, the system takes air from the nearest trap instead, usually the toilet's. That is the glug, and sometimes the bowl's water level drops afterwards.

Is it a clog or a vent? Narrow it down

Note for a day which fixtures gurgle, what drains slowly, and whether water appears where it should not.

Why does a partly blocked drain make the toilet gurgle?

A clog below the point where the washer and toilet drains meet is the most common cause, because it squeezes water and air into a smaller opening. The backed-up water compresses the air, which escapes through the toilet's trap as bubbles; as it drains away, the vacuum can siphon water out of the bowl. Hair, soap, washer lint, kitchen grease and so-called flushable wipes collect at the same low points.

A clog in one branch affects only the fixtures on it. A clog in the main building drain, or in the sewer lateral to the street, affects everything, and water shows up first at the lowest opening, usually the basement floor drain or a basement shower. If you see that, it is a main line problem, not a toilet problem.

How do I know if my plumbing vent is blocked?

A blocked vent usually means gurgling in more than one fixture while the drains still run at close to normal speed. The bowl's water level may drop after another fixture drains, you may notice a faint sewer smell, and it often starts after leaves come down in fall or during a long cold snap, when a roof vent can collect snow or frost.

Clearing a vent means working on the roof and running a snake or water down the stack. Leave it to a plumber, and never go up on a wet or icy roof.

Could an air admittance valve be the problem?

Yes, if your laundry, a basement bathroom or a kitchen island was added in a renovation. An air admittance valve is a one-way plastic vent cap, under a sink or behind a panel near the laundry, that opens to let air into the drain and closes to keep sewer gas out. As its seal ages it can stick shut, causing gurgling and slow draining, or fail to close, letting sewer smell into the room.

The Ontario Building Code allows these valves only in limited cases, such as renovation fixtures where tying into a vent is not practical. Each valve must stay accessible and out of places where it could freeze, and every house still needs at least one vent to open air. A valve sealed behind drywall or in a cold garage is a common culprit. A plumber can replace it, or tell you the fixture needs a real vent.

When does gurgling mean a main sewer line problem?

It points to the main line when every fixture is slow or gurgles, when the lowest drain shows water during laundry or showers, or when the problem returns every few months, often roots in an older lateral. That pipe is split between you and your municipality, so find out whose side the blockage is on.

• Toronto: the City owns the pipe from the street main to the property line; the homeowner owns the rest. Call 311 first: Toronto Water checks the City sewer and can camera the service line. If the City side is clear, you hire a licensed plumber.
• Newmarket: the homeowner is responsible for the lateral leaving the house; the Town looks after the part on Town property. Call the Town at 905-895-5193 (extension 3 after hours). Before anyone clears the lateral, the Town requires a free Sewer Safety Inspection through Ontario One Call (1-800-400-2255) to check no gas line runs through the sewer pipe.
• Aurora, Markham, Richmond Hill and Vaughan have their own rules; see the sibling article on main sewer line clogs.

In Toronto and York Region homes, what makes this more likely?

Older houses are where drains and laterals give trouble. About 30% of Newmarket's homes were built before 1981, and in Toronto 29% date from 1960 or earlier. Their laundry rooms are often in a basement finished later, with drains and vents added in stages. Moving or adding plumbing needs a permit and inspection in Newmarket and Toronto, so if your laundry was moved without one, have the venting checked.

What should I tell the plumber?

• When it gurgles: only when the washer empties, with showers, with any flush, or after snow or a cold snap.
• Which fixtures gurgle or drain slowly, and on which floor. Say whether water has ever come up in the floor drain, a basement shower or a tub.
• Whether the toilet bowl's water level drops after the gurgle, and whether you smell sewer gas anywhere.
• The house's age, whether the basement or laundry was renovated, and whether you have seen a vent cap under a sink or near the washer.
• Where the main cleanout is, if you know, and whether the lateral has been cleared or replaced before.
• A short video of the toilet while the washer drains helps more than a description.

What does it cost to fix on Doyo?

Most gurgling toilets are fixed by clearing the drain. On Doyo, Drain Cleaning is $253, the full price with Doyo's fee included, and Toilet Repair is $218.50 when the toilet itself is the problem. A blocked vent, a failed air admittance valve or a broken pipe is Pipe Repair, by written quote after the plumber has seen it. Plumbers on Doyo hold an Ontario Certificate of Qualification (306A), which Doyo checks before they are offered plumbing work.