Backwater Valve — How It Works, Who Needs One, and What It Takes to Install
The one device that stops sewage from coming up your floor drain during a storm — and the most widely subsidised piece of flood protection in Ontario.
Program facts verified September 4, 2026 against toronto.ca. Programs can change at any time — confirm with the City before signing a contract.
Storm season is here: July's storms flooded 1,900+ basements in Ottawa and drenched the GTA. Find my city’s program →Quick answer
A backwater valve is a one-way valve on the pipe that carries wastewater from your house to the municipal sewer. Wastewater flows out; when the sewer surcharges and flow reverses, the flap closes. The Ontario Building Code requires one wherever a fixture sits below street level. In Toronto a mainline valve runs $1,800–$4,800 installed and the City pays 80% up to $1,600 per device (up to 2 devices) — so you pay about $360–$3,200. Every other Ontario city we track pays toward it too.
How it works
Sewer backup is sewage forced back up the lateral from a surcharged sanitary, storm or combined sewer, entering the house through its lowest fixtures — the floor drain, a basement toilet, a shower. A backwater valve sits on that lateral, usually set into the basement floor where the pipe leaves the house. Under normal conditions wastewater flows out through it. When flow reverses toward the house, the gate or flap closes and holds it back.
Two designs exist. A normally-open valve stays open in everyday use and closes only when backflow is detected; that is the design the Building Code and Toronto’s permit guide expect on a building drain serving a single home, because it does not block the air circulation the drain needs. A normally-closed valve opens only when wastewater is discharged. Both must conform to the applicable CSA standard.
Do you need one?
- Sewage has come up a drain during a storm. That is sewer backup; nothing else fixes it. Waterproofing, a sump pump or grading will not stop a surcharged sewer.
- Any fixture sits below the level of the street. The Ontario Building Code (7.4.6.4, Protection from Backflow) requires a backwater valve on every fixture drain connected to the building drain where the fixture is below the adjoining street — which describes most finished basements.
- Your city pays for it. Every Ontario program we track — Toronto, Peel, Halton, York Region, Hamilton, Ottawa — built its rebate around this device. Hamilton’s program in particular cites hundreds of identified basement-flooding hot spots in its older neighbourhoods and the lower city.
What it does not do: a valve stops water arriving through the sewer. Groundwater seeping through a crack or the wall–floor joint is a different problem — find the entry point before you buy the wrong fix.
Installation: permit, inspection, cost, who is licensed
What the job involves
The installer locates the sanitary lateral, cuts the basement slab, exposes the pipe, cuts in the valve with a cleanout so the flap can be reached, then pours the floor back. The access lid stays visible — the valve has to be inspected and cleaned for the rest of its life. A home with a separate storm lateral may need a second valve on it; Toronto’s subsidy pays for up to 2 devices per property.
Permit and inspection
In Toronto a backwater valve needs a standalone drain permit from Toronto Building (no drawings for a one- or two-unit house; the “Consent to Enter” form goes with it when you are also applying for the subsidy), and an inspection once the valve is in — do not enclose or cover the valve before the inspection is complete. Halton, Peel, Hamilton, Markham and Ottawa require a permit and inspection for their rebates as well; Ottawa reimburses the permit fee in full.
Who is licensed
A licensed plumbing professional, with the permit pulled first. For Toronto’s subsidy the installer must hold a T94 Plumbing Contractor, T92 Plumbing and Heating Contractor or T87 Drain Contractor licence. Hamilton pays its full grant only through the City’s pre-qualified list. Verify the licence on the city’s site before signing.
What it costs
| City · job | Installed | You pay after rebate |
|---|---|---|
| TorontoMainline backwater valve, installed (incl. permit) | $1,800–$4,800 | $360–$3,200 |
| TorontoComplex access / deep lateral | $4,200–$6,500 | $2,600–$4,900 |
| OttawaInterior mainline backwater valve, installed (incl. permit coordination) | $1,800–$3,200 | $800–$2,200 |
Toronto: 80% of the invoice up to $1,600 per device. Ottawa: the City pays the lesser of the invoice or the interior-valve cap. Ranges verified against contractor price guides; the permit is included in the Toronto range. Full breakdowns on the two cost pages.
What every city pays toward it
| TorontoBasement Flooding Protection Subsidy Program | 80% of cost up to $1,600 per device, maximum 2 devices ($3,200) |
| BramptonSanitary Backwater Valve Rebate | Sanitary backwater valve installation (60% of cost, up to $1,500) |
| BurlingtonEnhanced Basement Flooding Prevention Subsidy Program | Backwater valve (50% of cost, up to $1,600) |
| CaledonSanitary Backwater Valve Rebate | Sanitary backwater valve installation (60% of cost, up to $1,500) |
| HamiltonProtective Plumbing Program (3P) | Backwater valve installation, including the contractor's assessmentSump pump (installed in conjunction with a backwater valve)Building permit for the valve (required; Adapt-a-valve installations also need City pre-approval) |
| MarkhamPrivate Plumbing Protection Rebate | Indoor backwater valve, sanitary or storm lateral — up to $1,750Outdoor backwater valve, sanitary or storm lateral — up to $2,000 |
| MiltonEnhanced Basement Flooding Prevention Subsidy Program | Backwater valve (50% of cost, up to $1,600) |
| MississaugaBasement Flooding Prevention Rebate | Backwater valve on the storm lateral — up to $1,500 |
| OakvilleEnhanced Basement Flooding Prevention Subsidy Program | Backwater valve (50% of cost, up to $1,600) |
| OttawaResidential Protective Plumbing Program (RPPP) | Interior sanitary backwater valve — up to $1,000Interior storm backwater valve — up to $1,000Exterior storm backwater valve — up to $2,500 |
| Richmond HillBackwater Valve Subsidy Program | Backwater valve installation (up to $1,500) |
| VaughanSanitary Back-water Valve Subsidy | Sanitary backwater valve installation (50% of cost, up to $750) |
Read from our program registry, each program re-verified against its official page (dates in the sources below). Rules — permits, pre-approval, contractor lists — are on each city’s guide.
Living with it: maintenance
The flap sits in a sewer line, and grease and debris can stop it from seating. The Government of Canada recommends a plumber inspect the valve at least once a year, in spring, and that it be cleaned at least twice a year. Toronto’s subsidy requires it to be maintained and inspected according to the manufacturer’s directions, and the City may ask for access to verify the installation. Keep the access lid clear — a valve buried under a finished floor is a valve nobody will ever check.
Next step
A backwater valve for your home
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How does a backwater valve work?
It is a one-way valve on the pipe that carries wastewater from the house to the municipal sewer. Under normal conditions wastewater flows out through it. When the sewer surcharges and flow reverses toward the house, the gate or flap closes and blocks it. A normally-open valve sits open and closes only on backflow; a normally-closed valve opens only when wastewater is discharged.
Do I need a backwater valve?
If sewage has ever come up through a floor drain, basement toilet or shower during a storm, yes — that is sewer backup, and the valve is the fix. The Ontario Building Code also requires one on every fixture drain where the fixture sits below the level of the adjoining street, which is most finished basements. A valve does nothing for groundwater seepage through walls or the wall–floor joint; that is a sump pump or waterproofing problem.
Is a permit required?
In Toronto, yes: a standalone drain permit from Toronto Building, and an inspection once the valve is in — do not enclose or cover the valve before the inspection is complete. Halton, Peel, Hamilton, Markham and Ottawa require a permit and inspection for their rebates as well. The Toronto permit fee for a backwater valve is listed on the City’s fee schedule.
Who can install it?
A licensed plumbing professional, with the municipal permit pulled first. For Toronto’s subsidy the installer must hold a T94 Plumbing Contractor, T92 Plumbing and Heating Contractor or T87 Drain Contractor licence. Hamilton pays its full grant only through the City’s pre-qualified list.
Does it need maintenance?
Yes. The valve has a moving flap in a sewer line; grease and debris can stop it from seating. The Government of Canada recommends a plumber inspect it at least once a year — in spring — and that it be cleaned at least twice a year. Toronto’s subsidy requires the valve to be maintained and inspected according to the manufacturer’s directions, and the City may ask for access to verify the installation.
Which cities pay for it?
Every one of the twelve Ontario programs we track has a backwater valve rebate — Toronto at 80% up to a per-device maximum for up to two devices, and the other eleven at their own fixed maximums or percentages. The table on this page is read from our registry and links to each city’s guide.
Related
Sources
- Government of Canada — Get Prepared — Backwater valves. Page modified March 23, 2026. Used for: how the valve works (one-way; the gate or flap closes when backflow is detected), normally-open vs normally-closed designs, installation by a licensed plumbing professional with municipal permits, inspection at least once a year and cleaning at least twice a year.
- City of Markham — Building Standards (citing the Ontario Building Code) — Builder Tip No. 108 — Protection from Backflow (OBC 7.4.6.4). January 1, 2025, updated to the 2024 Ontario Building Code. Used for: OBC 7.4.6.4: a backwater valve is required on every fixture drain connected to the building drain where the fixture is below the level of the adjoining street; the normally-open design and CSA standards; the restriction on valves that would block air circulation in a building drain or sewer.
- City of Toronto — Basement Flooding Protection Subsidy Program. Page modified June 18, 2026 (re-read September 4 and 11, 2026). Used for: eligible sump pump work, 80% / per-item maximums, discharge to a permeable surface ≥ 2 m from the foundation, eligible licence types T94 / T92 / T87 / T85, photo requirement, two-year application window.
- City of Toronto — Toronto Building — Backwater Valve — building permit application guide (standalone plumbing, mechanical and drains). Page modified July 14, 2026; fee schedule effective January 1, 2026. Used for: a standalone drain permit is required for a backwater valve in a one- or two-unit house; the fee line for backwater valves and sumps with pumps; "a fullport (normally open) backwater valve conforming to the appropriate CSA standard may be installed in a building drain that serves only one occupancy"; the Consent to Enter form for the subsidy.
- Institute for Catastrophic Loss Reduction (ICLR) — Handbook for Reducing Basement Flooding (Dan Sandink). Handbook 2009; ICLR flood page and "Good / Better / Best" retrofit advice April 21, 2026. Used for: the three mechanisms of basement flooding (overland, infiltration, sewer backup); where infiltration enters; what each protective measure does.
- ClaimRebate.ca cost research — Backwater Valve Cost in Toronto and in Ottawa — installed price and net of the rebate. Price ranges verified August 1, 2026 against Toronto and Ottawa contractor price guides. Used for: installed price ranges for a mainline valve (Toronto and Ottawa) and the complex-access range.
- ClaimRebate.ca program registry — Ontario municipal basement flooding programs (11 cities). Each program re-verified against its official page September 4, 2026. Used for: which cities pay toward a sump pump and how much — read from the registry at render time.
ClaimRebate.ca is independent and not affiliated with any municipality or program. Amounts shown are read from our program registry at the time this page is generated; confirm current terms on the official page before relying on them.