Why Does My Basement Drain Keep Backing Up?
A basement drain that backs up repeatedly is almost always a sign of a blockage or restriction somewhere in your home’s sewer system — not a random plumbing quirk. The most common causes are a buildup of grease, debris, or tree roots in the main sewer line, a collapsed or broken pipe, or a failing sewer system that needs cleaning or repair. A one-time backup during heavy rain might be a fluke. A drain that keeps backing up is telling you something is wrong — and it won’t fix itself.
There’s nothing quite like the sinking feeling of walking into your basement and finding water backing up out of your floor drain. Again.
If it’s happened more than once, you already know that mopping it up isn’t a solution. The water comes back. Maybe it’s only after a load of laundry. Maybe it happens when you flush the toilet upstairs. Maybe it seems to come out of nowhere. Whatever the pattern, one thing is consistent: your basement drain is telling you something is wrong with your sewer system.
BEST Plumbing & Drainage has been diagnosing and fixing basement drain problems in London and Southwestern Ontario for over 50 years. Here’s what we see most often — and what you need to do about it. For urgent situations, call us anytime at 519-438-6565.
First — Understand How Your Basement Drain Works
Your basement floor drain is connected to your home’s main sewer line — the pipe that carries all wastewater away from your house to the municipal sewer system. It sits at the lowest point of your drainage system for a reason: it’s a safety valve. When something goes wrong further down the line and water has nowhere to go, it backs up through the lowest available opening — which is almost always that basement floor drain.
So when your basement drain backs up, the problem usually isn’t the drain itself. It’s something further downstream. That’s important to understand, because it means clearing the drain opening won’t fix it. You need to find and address the actual blockage. Our drains and sewers service page covers the full range of solutions BEST offers.
The Most Common Reasons a Basement Drain Keeps Backing Up
1. A Partial Blockage in the Main Sewer Line
This is the most common cause we see. Grease, soap scum, hair, and debris accumulate on the walls of your sewer pipe over years and gradually narrow the passageway. Eventually, the line gets restricted enough that heavy water use — a shower running, a load of laundry, flushing multiple times in a row — overwhelms its capacity and water backs up. Professional drain cleaning in London is the right solution for most partial blockages.
A partial blockage tends to get worse over time. What starts as occasional backups during heavy use becomes more frequent, and eventually more severe. Getting the line professionally cleaned before it becomes a full blockage is always cheaper than dealing with it after the fact.
2. Tree Roots in the Sewer Line
This is more common than most homeowners expect — especially in London’s older neighbourhoods where large trees have had decades to grow. Tree roots are naturally drawn to the moisture and nutrients in sewer pipes. They find tiny cracks or joint gaps, work their way in, and over time grow into a mass that catches debris and causes repeated backups.
Root intrusion won’t go away on its own. A professional drain cleaning can clear the roots temporarily, but if the pipe is significantly damaged, repair or replacement may be the right long-term call.
3. A Collapsed or Damaged Pipe
Older London homes — particularly those with clay or Orangeburg sewer pipes — are susceptible to pipe collapse, cracking, and joint separation over time. A damaged pipe creates a low spot where debris collects, or an outright blockage where flow stops entirely. This can’t be cleaned away. It requires repair or replacement.
The only way to know for certain whether you have pipe damage is a video camera inspection — a camera run through the line that shows exactly what’s happening inside your pipes. If we haven’t done one in your home, it’s the first thing we recommend when a backup keeps recurring.
4. A Blocked or Malfunctioning Backwater Valve
If your home has a backwater valve — a device that prevents municipal sewer backups from entering your home — and it’s partially blocked or stuck, it can restrict flow and cause your basement drain to back up. Backwater valves need to be inspected and cleaned periodically. Learn more about backwater valve installation in London, or give us a call if you’re not sure whether yours is working.
5. The Municipal Sewer System Is Overwhelmed
If your backups happen specifically during or right after heavy rainfall, the problem may not be inside your home at all. When the municipal sewer system gets overwhelmed by stormwater, it can back up into private connections — including yours. This is a different problem that requires a different solution, and it’s worth paying attention to whether your backups have a clear weather pattern.
What to Do When Your Basement Drain Backs Up
- Don’t ignore it — even a single backup can indicate a developing sewer problem. Repeated backups mean the problem is already developing.
- Note the pattern — does it happen after laundry? After a shower? During rain? When multiple fixtures are used at once? This information helps diagnose the cause.
- Skip the chemical drain cleaners — they don’t clear sewer line blockages and can damage older pipes.
- Stop running water — if the drain is actively backing up, minimize water use in the house until the line is assessed.
- Call a professional — a camera inspection and professional drain cleaning is the only reliable way to diagnose and fix a recurring backup.
How BEST Diagnoses and Fixes a Recurring Basement Drain Backup
When you call BEST about a recurring basement drain backup, here’s how we approach it:
- We start with a video camera inspection to see exactly what’s happening inside your sewer line — blockage, root intrusion, pipe damage, or collapse
- Based on what we find, we recommend the right solution — high-pressure hydro jetting to clear a blockage, root cutting and cleaning, or pipe repair or relining if there’s structural damage
- We explain clearly what we’ve found, what your options are, and what each one costs before any work begins
- For homes at risk of municipal sewer backups, we discuss backwater valve installation as a preventive measure
We’re available 24 hours a day for emergency drain situations. Call 519-438-6565 or request a service call online.
Frequently Asked Questions
Are cracks in a basement foundation normal?
Yes, small ones are. Thin vertical hairline cracks are very common in poured concrete walls and usually come from shrinkage as the concrete cured. They’re only a concern if they leak, widen, or change over time.
How wide does a foundation crack have to be before I should worry?
A good rule of thumb is 1/4 inch (about 6 mm). Cracks wider than that, or cracks that keep getting wider, should be looked at by a professional. Width matters less than direction, though. Even a narrow horizontal crack deserves attention.
Are horizontal foundation cracks worse than vertical cracks?
Usually, yes. Vertical cracks tend to come from shrinkage or settling. Horizontal cracks usually mean soil or water pressure is pushing the wall inward, which can lead to bowing and structural damage if it isn’t addressed.
Can a foundation crack cause a wet basement?
Absolutely. Cracks are one of the most common ways water gets into a basement, especially during heavy rain and the spring thaw. Even a small crack can let in enough water to cause mould, stained walls, and damaged flooring.
Do foundation cracks get worse in winter?
They can. Freeze-thaw cycles cause the soil around your foundation to expand and contract, which puts extra stress on your walls. Many homeowners in Southwestern Ontario first notice new cracks or leaks in late winter and early spring.
Should a foundation crack be sealed from the inside or the outside?
It depends on the crack and the water source. Crack injection from the inside works well for many leaking cracks in poured concrete. Exterior waterproofing stops water before it reaches the wall and is often the better long-term fix when there’s ongoing water pressure or drainage failure.
Can I just use caulk or hydraulic cement on a foundation crack?
For a dry, cosmetic hairline crack, a DIY patch may be fine. For a crack that leaks, the patch often fails because water is being pushed in from outside. It can also hide changes in the crack that you’d want to catch early.
How long does it take to repair a foundation crack?
A crack injection is often completed in a few hours. Exterior waterproofing takes longer because the soil has to be excavated along the wall. Your contractor should give you a clear timeline after assessing the crack.
Will home insurance cover foundation crack repair?
It depends on your policy and the cause. Many policies don’t cover cracks from gradual settling or wear, but some cover sudden water damage. Check with your insurance provider before repairs begin.
Does BEST repair leaking foundation cracks in London, Ontario?
Yes. BEST Plumbing and Drainage Contractors repairs leaking basement cracks and provides wet basement waterproofing across London and Southwestern Ontario. Call 519-438-6565 or request a quote online to book an assessment.
