Combined Sewer Overflow in Toronto Explained

# Combined Sewer Overflow in Toronto Explained

The phrase "combined sewer overflow" appears repeatedly in Toronto's flood-prevention literature without much explanation of what it actually means or why it matters for your basement. Combined sewer overflow (CSO) is the technical reason older Toronto neighbourhoods flood — and it is the structural problem that the new $6,650 City of Toronto Basement Flooding Subsidy is designed to address.

This post explains how Toronto's combined sewer system works, which neighbourhoods sit on combined vs separated sewers, why heavy rain causes overflow, and how the subsidy-funded backwater valve + weeping tile disconnection bundle reduces both basement flood risk AND combined sewer overflow into Lake Ontario. For full subsidy detail, see [Toronto Basement Flooding Subsidy 2026: $6,650 Program Explained](/blog/toronto-basement-flooding-subsidy-2026-6650-program). For broader context, see the pillar [Backwater Valve & Sump Pump Toronto: Complete 2026 Subsidy Guide](/blog/backwater-valve-installation-toronto-2026).

Combined vs Separated Sewers

Modern cities use separated sewer systems:

Sanitary sewer — carries household and commercial waste to the treatment plant
Storm sewer — carries rainwater runoff directly to a river, lake, or storm-water management facility

Older cities — including most of central Toronto — were built before sewer separation became standard. They have combined sewers:

Combined sewer — single pipe carrying BOTH sanitary waste and stormwater

Toronto built its earliest sewers in the 1860s–1880s as combined systems. Sewer separation has been ongoing since the 1950s but is incomplete. As of 2026, approximately:

30% of Toronto's central sewer system is still combined
60% is separated
10% is partially separated (separated for new construction but still combined at the trunk lines)

For homeowners, the key fact: most of central Toronto built before 1975 sits on combined sewers.

Combined-Sewer Neighbourhoods (Detailed Map)

The highest concentrations of combined sewers in Toronto:

Area	Coverage	Notes
Old East York	~70% combined	Pape, Donlands, O'Connor corridor
Riverdale	~80% combined	Broadview, Logan, Withrow Park area
Cabbagetown	~85% combined	Parliament, Carlton, Wellesley
Leslieville	~65% combined	Queen East corridor, Jones, Greenwood
The Annex	~75% combined	Bloor-Yonge to Bathurst
Forest Hill (parts)	~50% combined	Older Forest Hill Village section
North Etobicoke	~55% combined	Albion, Rexdale, Mount Olive
Yonge-Eglinton	~40% combined	Original midtown core
South Hill	~55% combined	Below St. Clair

For a property-specific check, request the sewer service map from Toronto Water (free, 5–10 business days).

How Overflow Happens

In a typical dry-weather day, the combined sewer carries only sanitary waste — typically running at 5–15% capacity. The pipe was designed for this base load.

When it rains, stormwater enters the combined sewer through:

Street catch basins
Roof downspouts (in homes where downspouts are connected to sanitary)
Foundation drains / weeping tile (in homes where they connect to sanitary)
Driveway drains
Surface infiltration through manhole covers

A heavy rain event can multiply the flow rate by 100x. The pipe was designed for the dry-weather peak, not the storm peak. When the pipe is full, the system has two failure modes:

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Failure Mode 1: Backup Into Homes

The combined sewer surcharges at the lowest opening — which is the basement floor drain in homes connected to that pipe. Water flows backward up the home's sewer lateral and into the basement.

Failure Mode 2: Overflow Into the Lake

At designed overflow points, the combined sewer is allowed to discharge directly into the Don River, the Humber River, Mimico Creek, or Lake Ontario. This protects the upstream piping but discharges raw sewage + stormwater into receiving waters.

Toronto has approximately 70 active combined sewer overflow points. The 2014–2026 capital plan ($1.6B) is primarily aimed at reducing the volume and frequency of these overflows.

Why Foundation Drain Disconnection Matters

The 2026 subsidy expansion added a new $3,650 category specifically for disconnecting the foundation drain (weeping tile) from the sanitary sewer and re-routing it to a sump pit. The reason:

In Toronto homes built before approximately 1975, the foundation drain (the perimeter drain around the footing) was routinely connected directly to the sanitary sewer lateral. This was standard practice at the time. The result:

Every rain event sends groundwater AND stormwater into the sanitary sewer
During a storm, foundation drain inflow contributes meaningfully to combined sewer overflow
The home's connection adds to the overload that ultimately causes the very surcharge that floods the basement

By disconnecting the foundation drain and routing it to a sump pit (which discharges to lawn or storm sewer), the home:

1. Stops contributing to combined sewer overflow
2. No longer experiences foundation-drain-driven sewer back-pressure
3. Has working drainage even if the sanitary sewer surcharges

The City's $3,650 rebate per property reflects how meaningful this is at scale. Toronto Water estimates that disconnecting all eligible foundation drains across the central city would reduce combined-sewer overflow volume by 12–18%.

How a Backwater Valve Helps

A backwater valve installed on the sanitary lateral mechanically blocks reverse flow during a surcharge event. Even when the city's combined sewer is overwhelmed and surcharging, your home is isolated.

Combined with foundation drain disconnection, the home is decoupled from the combined-sewer overflow problem in two ways:

1. Sewer surcharge cannot enter the basement (backwater valve closed)
2. Stormwater from foundation drain no longer feeds the sewer (re-routed to sump)

This is exactly why the City's subsidy structure is what it is.

Toronto Bylaw 681 (Sewer Use)

Toronto's Sewer Use Bylaw governs what may legally enter the sanitary and storm sewers. Three relevant provisions:

1. Foundation drains may not discharge to the sanitary sewer. This was technically illegal for decades but rarely enforced. The 2026 subsidy makes financial accommodation for compliance.
2. Sump pump discharge may not connect to the sanitary sewer. Must daylight to lawn/grade or to the storm sewer.
3. Roof downspouts may not connect to the sanitary sewer. A separate Toronto Water program offers a small rebate for downspout disconnection.

A licensed plumber doing a 2026 backwater valve install will check ALL three of these and address violations as part of the project.

What the Capital Plan Looks Like

The City of Toronto's 25-year sewer plan ($1.6 billion, 2014–2039) includes:

Sewer separation — converting combined sewers to separated systems in priority neighbourhoods. ~$600M.
Storage tanks — large underground reservoirs that hold combined sewer flows during storms for later treatment. ~$400M.
Green infrastructure — bioswales, permeable pavement, urban forest expansion. ~$200M.
Subsidy program — homeowner-side mitigation. ~$300M (cumulative).
Other — engineering, monitoring, contingency. ~$100M.

The subsidy is roughly 19% of the capital plan. Toronto Water has internally documented that the homeowner subsidy delivers more flood protection per dollar than any other capital category — disconnecting 90,000 homes' foundation drains is functionally equivalent to building a major new storage tank, at a fraction of the cost.

Implications for Homeowners

The combined sewer story matters for individual homeowners in three ways:

1. Your Risk Depends on Sewer Type

A home in a combined-sewer neighbourhood faces meaningfully higher basement flood risk than a comparable home on separated sewers. Verify your sewer type with Toronto Water before deciding the scope of flood protection.

2. The Subsidy Is a Public Good

The City's subsidy structure isn't just generous — it is also strategic. Each homeowner who installs the bundle reduces system-wide overflow volume. The City benefits AS MUCH as the homeowner.

3. The Window Is Now

Toronto's commitment to combined sewer remediation is multi-decade and well-funded. But the homeowner-side subsidy may shift over time. Acting in 2026 captures the current $6,650 cap — future programs may differ.

Ready to Disconnect from the Problem?

RenoHouse provides full bundle installs (backwater valve + sump pump + weeping tile disconnection) compliant with Toronto Bylaw 681. Visit our [Backwater Valve & Sump Pump Bundle Service Page](/services/plumbing/backwater-valve-sump-pump-bundle).