A toilet backing up after running the washing machine might seem like your plumbing is throwing a tantrum, but it’s actually trying to tell you something. When two fixtures in your home start to interfere with each other, it’s a sign that your drainage system isn’t handling waste the way it should. And the culprit usually lies deeper than the bathroom or laundry room.
Let’s break down why this happens, what it means for your plumbing, and how to fix it for good.
How Are the Toilet and Washing Machine Connected?
In most homes, the bathroom and laundry room drains are tied into the same main sewer line. This main line carries wastewater from every fixture in your home out to the municipal sewer or septic system. Each drain — whether it is from your sink, tub, washer, or toilet — feeds into this central channel.
When you run the washing machine, it discharges a high volume of water at once. This sudden surge flows into the shared drain line. If the path is clear, the water continues to the sewer without any problems. But if there is even a partial blockage or an issue with venting, the water has limited space to move. It starts to push back, and the toilet is often the easiest spot for that water to reappear.
The result? Gurgling, bubbling, or a full backup into your toilet bowl.
Why Does the Toilet Back Up Instead of the Washer?
Washing machines are designed to pump water out forcefully. Toilets, on the other hand, rely on gravity. When a blockage or pressure imbalance occurs in the plumbing system, the washer’s forceful output causes a backup, and the toilet becomes the path of least resistance. Water can be pushed back through the trap and into the bowl.
This kind of backup is often a symptom of a larger issue that involves your main drain line, your venting system, or both. To understand what is really going on, it helps to break down the most common causes.
What Causes This Kind of Backup?
Several different issues can lead to a toilet reacting badly after a laundry cycle. These are the most common problems we find in homes:
Partial Blockages in the Main Drain Line
One of the most common causes of a toilet backup after running the washing machine is a partial clog in the main drain. These clogs can form over time from grease, detergent residue, hair, food particles, or other debris that slowly builds up inside the pipes. The line may not be fully blocked, but the restricted flow limits how much water can move through it at once.
When the washer drains, the surge of water hits the blockage and has nowhere to go. Instead of continuing toward the sewer, it pushes back into nearby fixtures. Since the toilet is usually closest to the main line, it is often the first to show signs of trouble.
The backup may not happen every time at first, but as the blockage grows, the problem becomes more frequent and more severe.
Blocked or Inadequate Venting
Your plumbing system has vent pipes that help regulate air pressure and allow wastewater to flow smoothly. These vents connect to the drain lines and extend up through the roof. They allow air to enter the system, which prevents vacuums and allows waste and water to move through the pipes without interruption.
If a vent becomes blocked by debris, nesting animals, or even snow, it can prevent air from entering the system. That lack of airflow causes slow drainage, gurgling sounds, and sometimes full backups.
In some cases, the venting system may have been poorly designed or undersized for the needs of the home. If the vent cannot keep up with the volume of water from modern appliances, the result is the same. The system cannot balance the pressure, and water backs up into lower fixtures like the toilet.
Poor Drainage Design or Improper Slope
Not all plumbing systems are designed with ideal flow in mind. If your home’s drains were installed with improper slope or configuration, water may not flow out as efficiently as it should. Pipes that are too flat or too steep can both cause problems. Water might move too slowly and allow solids to settle, or it might move so quickly that solids are left behind.
In homes where the toilet and washing machine are connected by a poorly sloped or poorly planned layout, the risk of backup is much higher. The system simply cannot handle the fast-moving discharge from the washer, and the result is a toilet that reacts every time you do laundry.
Sewer Line Damage or Collapse
Older homes or properties with mature trees may be at risk for sewer line issues that are not obvious at first glance. Shifting soil, root intrusion, corrosion, or age-related decay can cause the sewer line to crack, sag, or collapse. These issues block or restrict wastewater flow, creating a backup risk.
Washing machines introduce a large amount of water in a short period. If the main line is compromised, the pressure from that water can cause sewage to back up through the toilet or other low drains in the home.
Since the sewer line is underground, the only way to know for sure is to inspect it using a specialized camera.
Why You Should Not Ignore the Signs
Toilet backups caused by laundry cycles are not random plumbing quirks. They are red flags that indicate something is wrong in your drainage system. While the symptoms might seem manageable at first, the underlying issue will likely get worse over time.
If left unaddressed, you risk a full sewer backup, which can affect multiple drains at once and cause costly water damage, foul odors, and exposure to unsanitary waste.
Ignoring this problem can also put stress on your plumbing fixtures and appliances, leading to more frequent repairs or replacements down the road.
Why DIY Fixes Rarely Work
Many homeowners try to resolve these problems with plungers, chemical cleaners, or even store-bought drain snakes. While those tools might help with small clogs near the drain opening, they do not address deep blockages, vent issues, or structural problems in the main sewer line.
Chemical drain cleaners may seem like a quick solution, but they can damage older pipes, cause leaks, and only offer temporary relief. In many cases, they dissolve just enough of the clog to allow water to pass briefly, but the buildup returns.
To solve this problem for good, you need a clear diagnosis and the right tools for the job.
What to Expect From a Professional Inspection
At Priority Plumbing & Drains, we offer drain camera inspections that allow us to look inside your plumbing system without digging or guessing. This technology shows us exactly where the problem is, what is causing it, and how to fix it properly.
Once we know the source of the issue, we can take the appropriate next step. That might include removing a clog, updating your venting system, or repairing a damaged sewer line. We tailor the solution to your home and your plumbing layout, so you get lasting results.
Schedule Drain Cleaning or Drain Repair in Toronto, ON, Today!
If your toilet backs up after running the washing machine, your plumbing system is trying to get your attention. This is not something that will go away on its own. In fact, it usually gets worse over time.
At Priority Plumbing & Drains, we offer expert-level drain cleaning in Toronto, ON, and the surrounding areas. Our technicians are trained to spot problems quickly and fix them the right way. We never guess. We rely on clear inspections, transparent recommendations, and proven solutions.
Contact us today to schedule a drain repair or cleaning service! Let us help restore your plumbing system and keep everything flowing the way it should.