Toronto's Combined Sewer Problem: What It Means for Older Home Buyers

If you are buying a home in one of Toronto's older neighbourhoods — anywhere built before the 1960s — there is a good chance the property sits above a combined sewer system. It is one of the most practical infrastructure details a buyer can know, and one of the least discussed during the typical home purchase process.

What Is a Combined Sewer System?

A combined sewer system carries both stormwater (rain and snowmelt) and sanitary sewage (wastewater from sinks, toilets, and drains) in a single pipe. This design was standard practice across North American cities built before the mid-20th century.

The alternative — a separated sewer system — uses two independent pipes: one for stormwater and one for sanitary sewage. Separated systems became the standard for new construction in Toronto from roughly the 1960s onward.

The distinction matters because combined systems are inherently more vulnerable to surcharging during heavy rainfall events. When the combined pipe reaches capacity, the excess has to go somewhere — and that somewhere is often a basement floor drain.

Which Parts of Toronto Have Combined Sewers?

Combined sewer infrastructure is concentrated in Toronto's pre-1960 core, including:

• Trinity-Bellwoods and Dufferin Grove
• Kensington Market and Chinatown
• The Annex and Seaton Village
• Cabbagetown and Regent Park
• Large parts of old East York
• Leslieville and Riverdale
• Bloor West Village and High Park

Suburban areas built after 1960 — including most of Scarborough, North York (north of Lawrence), and Etobicoke — were built with separated sewer systems and generally do not face the same combined sewer surcharge risk.

Property Proof classifies sewer type using official City of Toronto manhole records. Each manhole record includes a flow medium attribute that indicates whether it serves a combined or separated system. By analyzing manholes in proximity to the subject property, the report determines the likely sewer infrastructure type.

Why Combined Sewers Matter When Buying a Home

Combined sewer areas are not inherently dangerous, and the presence of combined infrastructure does not mean a basement will flood. But it does mean the risk is elevated relative to a separated system, particularly during the intense summer rainstorms that have become more frequent in recent years.

For buyers, the practical implications include:

• Finished basements — If the property has a finished basement in a combined sewer area, the cost of a sewer backup event is significantly higher than in an unfinished space.
• Backwater valve — Many Toronto homes in combined sewer areas have had backwater valves installed (sometimes subsidized by the city's Basement Flooding Protection Program). Verifying whether one is present is part of good due diligence.
• Insurance — Sewer backup coverage is typically available as an add-on to a standard homeowner's policy, but premiums may vary based on the property's sewer infrastructure and claims history in the area.

Basement Flooding Risk and Combined Sewer Areas

Toronto has experienced several major basement flooding events in recent years, including the July 2013 storm that caused over $1 billion in insured damage across the GTA. Many of the hardest-hit areas were in combined sewer zones where the system was overwhelmed by rainfall intensity that exceeded design capacity.

The City of Toronto has invested in infrastructure upgrades, including sewer separation projects, increased pipe capacity, and the Basement Flooding Protection Program. But the pace of replacement is measured in decades, and much of the combined system remains in service.

What Toronto Is Doing About Combined Sewer Infrastructure

The city's long-term infrastructure plan includes gradual sewer separation in priority areas, construction of underground storage tanks to reduce combined sewer overflow during storms, and continued funding of the Basement Flooding Protection Program, which subsidizes the installation of backwater valves and sump pumps for eligible homeowners.

These are positive developments, but they do not change the current reality for buyers. If you are purchasing a home in a combined sewer area today, the infrastructure is what it is. Knowing this before you close — rather than discovering it during the next major storm — is the point.