A toilet backing up after you run the washing machine might feel random or frustrating, but it’s actually your plumbing system sending you a clear warning. When two fixtures in your home start interfering with each other, it usually means your drainage system isn’t working the way it should—and the problem often runs deeper than the bathroom or laundry room.
Let’s break down why this happens, what it means for your plumbing system, and how to fix it properly.
How Are the Toilet and Washing Machine Connected?
In most homes, the toilet and washing machine both drain into the same main sewer line. This main line carries wastewater from all fixtures—sinks, tubs, toilets, and appliances—out of your home and into the municipal sewer system.
When you run your washing machine, it releases a large volume of water very quickly. If the drain line is clear and properly vented, that water flows out without any issues. But if there’s a restriction or airflow problem, the water has nowhere to go.
Instead of moving forward, it pushes back—and the toilet is often the easiest place for that pressure to escape.
This is why homeowners often notice:
- Gurgling sounds from the toilet
- Bubbling in the bowl
- Water rising or backing up after a laundry cycle
Why Does the Toilet Back Up Instead of the Washing Machine?
Washing machines drain using a powerful pump, while toilets rely mostly on gravity. When there’s a blockage or pressure imbalance in the system, the washer forces water into the drain faster than it can exit.
The toilet, being a lower and less resistant fixture, becomes the path of least resistance. As a result, wastewater is pushed back through the toilet trap and into the bowl.
This is rarely a toilet problem—it’s almost always a main drain or venting issue.
Common Causes of Toilet Backups After Laundry
Partial Blockage in the Main Drain Line
This is the most common cause. Over time, materials like:
- Grease
- Soap and detergent residue
- Hair and lint
- Food particles
- Mineral buildup
can collect inside your main drain line. The pipe may not be fully blocked, but the reduced capacity can’t handle the sudden surge of water from the washing machine.
At first, the backup may only happen occasionally. As the blockage worsens, it becomes more frequent and more severe.
Blocked or Improper Plumbing Venting
Your plumbing system relies on vent pipes to regulate air pressure and allow wastewater to flow freely. These vents exit through the roof and can become blocked by:
Leaves or trash
Ice or snow (common in Toronto winters)
Bird nests or small animals
When airflow is restricted, pressure builds up in the drain lines, leading to slow drainage, gurgling, and backups—often at the toilet.
In some homes, the venting system may also be undersized or poorly designed, especially when older plumbing meets modern appliances
Poor Drain Slope or Plumbing Design
Drain pipes must be installed at the correct slope to move waste efficiently. Pipes that are too flat allow debris to settle, while pipes that are too steep can cause water to outrun solids.
If the toilet and washing machine share a poorly sloped or poorly designed drain path, the system may not be able to handle the washer’s discharge—leading to repeated toilet backups.
Sewer Line Damage or Deterioration
Older homes and properties with large trees are especially vulnerable to sewer line problems. Common issues include:
- Tree root intrusion
- Pipe corrosion
- Cracks or collapsed sections
- Sagging pipes due to soil movement
When the sewer line is compromised, it restricts flow. The high volume of water from a washing machine can then force sewage back into the home—often through the toilet or basement drains.
The only reliable way to confirm this is with a camera sewer inspection.
Why You Shouldn’t Ignore This Problem
A toilet backing up after laundry is not a harmless inconvenience. It’s an early warning sign of a larger drainage issue that will only get worse over time.
Ignoring it can lead to:
- Full sewer backups
- Water and sewage damage
- Foul odors
- Health risks from contaminated water
- Costly emergency repairs
Addressing the issue early can save you significant time, stress, and money.
Why DIY Fixes Rarely Solve the Issue
Plungers, chemical drain cleaners, and store-bought drain snakes may provide short-term relief, but they do not fix deep blockages, venting problems, or sewer line damage.
Chemical cleaners can even make things worse by:
- Damaging older pipes
- Weakening joints
- Creating leaks
For recurring backups, professional diagnosis is the only long-term solution.
What a Professional Inspection Can Do
At Infinity Luxury Plumbing, we use advanced drain camera inspections to look directly inside your pipes. This allows us to:
- Locate blockages precisely
- Identify venting issues
- Detect sewer line damage
- Recommend the correct solution—without guessing
Based on what we find, we may recommend drain cleaning, hydro jetting, vent repairs, or sewer line service to permanently resolve the problem.
Schedule Drain Cleaning or Repair in Toronto & the GTA
If your toilet backs up after running the washing machine, your plumbing system needs attention—and fast.
Infinity Luxury Plumbing provides expert drain cleaning and sewer inspections across Toronto, North York, and the GTA. Our licensed plumbers deliver honest assessments, long-term solutions, and reliable workmanship.
Call now: 647-996-7574