Infiltration & Inflow Flooding in Toronto: A Silent Siege

Toronto's most dangerous basement floods don't always arrive with a dramatic wall of water. Sometimes, they arrive quietly. Rain falls. Snow melts. Groundwater rises. Water finds cracks, defective pipe joints, aging drainage systems and overloaded sewers. Eventually, it can find its way into your basement. This is where infiltration and inflow, commonly called I&I, become important.

The Water You Can't See

Infiltration and inflow are two different ways water can get into a sewer system. Infiltration occurs when groundwater enters sewer pipes through cracks, defective joints, damaged connections or other openings.

Inflow is water entering the sewer system more directly. For example, through improperly connected downspouts, foundation drainage connections or other openings that allow rainwater or snowmelt into the system.

The distinction matters because both can add enormous volumes of water to a sewer network during a major storm. And Toronto has plenty of places where that additional water matters.

The city has a mixture of old and new underground infrastructure, different drainage systems and neighbourhoods with different topography, soil conditions and sewer configurations. In some older parts of Toronto, stormwater and sanitary sewage share the same pipe. These are combined sewers. Approximately 23% of Toronto is served by combined sewers, primarily in older areas.

During dry weather, the system can function normally. But during intense rainfall, enormous quantities of stormwater can enter the combined sewer system. When flows become excessive, the sewer network can become surcharged. Meaning water levels and hydraulic pressure inside the system rise. And when the system is under enough stress, basement flooding can become a very real possibility.

But You Don't Need to Live Beside a River

This is where many homeowners get caught off guard. You don't have to live beside a ravine, river or obvious floodplain to experience basement flooding. Groundwater can rise. Soil can become saturated. Foundation drainage systems can become overwhelmed or fail. Sewer connections can deteriorate. And water can enter through foundation cracks, floor drains, plumbing fixtures or other vulnerable points.

The water doesn't care what the property line says. It follows the path of least resistance. That's why a home that looks completely safe from the street can still have a serious flood vulnerability underground.

Toronto Has Seen What Can Happen

The August 19, 2005, storm remains one of Toronto's clearest warnings.

The City received approximately 4,200 basement-flooding complaints following the event. Across the storm, insured losses exceeded $500 million, including approximately $247 million associated with sewer-backup claims. And 2005 wasn't the end of the story.

The catastrophic GTA flooding of 2013 produced approximately $1.024 billion in insured losses, measured in 2019 dollars. These aren't just numbers on a spreadsheet. Behind them are homeowners dealing with ruined belongings, contaminated basements, damaged finishes, mould concerns, repairs, insurance claims and months of disruption.

Here's the Part Homeowners Often Miss

Toronto's sewer system isn't the only thing affecting flood risk. Your property is part of the drainage system too. In one Toronto engineering analysis, computer modelling found that at least 70% of homes within a sewer shed would need to disconnect their downspouts from the storm sewer to achieve a significant reduction in sewer surcharging.

Think about that.

Flood prevention isn't simply a matter of making the city's pipes bigger. What happens on individual properties can affect what happens throughout the drainage system.

That's why measures such as: disconnecting downspouts where permitted, maintaining sump pumps, keeping sump pump discharge systems functioning properly maintaining foundation drainage, sealing foundation cracks, correcting negative grading, maintaining catch basins and drainage systems, inspecting aging sewer connections, and installing appropriate backwater protection can all play a role in reducing flood risk.

But here's the important part:

Not every house needs the same solution. A backwater valve addresses one type of risk. A sump pump addresses another. Foundation waterproofing addresses another. A sewer camera inspection can reveal problems that aren't visible from the basement. The right protection depends on where the water is coming from and where it is trying to go.

Toronto Is Fighting the Problem from Both Sides

The City isn't ignoring the problem. Toronto has conducted basement-flooding investigations across 67 study areas between 2006 and 2024, identifying infrastructure and drainage improvements intended to reduce flood risk.

Those solutions can include sewer upgrades, drainage improvements, stormwater-management measures and other infrastructure changes tailored to individual areas. But municipal infrastructure is only one side of the equation.

The other side is your property. Your foundation. Your sewer connection. Your sump system. Your downspouts. Your backwater protection. And the condition of the drainage system underneath your home.

The Silent Siege

Flooding doesn't always announce itself. Sometimes there is no overflowing river. No dramatic wave. No obvious warning. Just saturated soil. A rising water table. A sewer under pressure. A failed pump. A defective pipe joint. A blocked drain. A foundation crack. And eventually, water where you absolutely don't want it.

Your basement doesn't need to be in a flood zone to flood.

That's why flood prevention starts with understanding the pathways water can take before the storm arrives.

At Wane Drain, that's the question we focus on:

Where can water get into your home, and what can we do about it before it becomes a $30,000 problem? Because insurance may help after the damage. Prevention starts before the water gets through the door.