Prevention
Backwater valves in Ontario: how they work and who helps pay
By ARO Restorations · Updated
Short answer
A backwater valve is a one-way flap on your sewer line that closes when sewage tries to flow back toward the house. It prevents sewer backup but not other kinds of flooding. Toronto, Mississauga, Hamilton, Halton, Peel, Markham and Vaughan all run subsidy programs.
In this guide
What it does
Your home’s sewer lateral normally carries wastewater out to the municipal sewer. When that sewer is overloaded in a storm, flow can reverse. A backwater valve has a flap that floats shut when water comes from the wrong direction, sealing the line.
What it does not do
- It does not stop groundwater seeping through the foundation.
- It does not stop surface water coming in through windows or doors.
- While it is closed, your own wastewater cannot leave either. Using toilets, showers or the washing machine during a storm can flood your basement from the inside.
Installation
A valve is installed on the main sanitary line, usually by breaking out a section of basement floor near where the line exits. It requires a plumbing permit and a licensed plumber. Weeping tile and downspout connections may need to be changed at the same time so they do not discharge on the house side of the valve.
Maintenance
Valves fail when debris stops the flap seating. Check and clean yours at least once a year through the clear access cover, and after any backup event.
Subsidy programs we have checked
Program details were checked in October 2026 and change often. Confirm on the official page, and note that some programs need approval before work starts.
| Municipality | Program | What we found |
|---|---|---|
| Toronto | Basement Flooding Protection Subsidy Program | Up to $6,650 per property across eligible work |
| Mississauga | Basement Flooding Prevention Rebate Program (City) | Up to $7,500; pre-approval required |
| Hamilton | Protective Plumbing Program (3P) | Listed as up to $2,000 for eligible homes |
| Oakville, Burlington, Milton | Halton Region Enhanced Basement Flooding Prevention Subsidy | 50% of a backwater valve up to $1,600 |
| Brampton, Mississauga, Caledon | Region of Peel sanitary backwater valve rebate | 60% of the invoiced cost up to $1,500 |
| Markham | Private Plumbing Protection Rebate Program | Up to $1,750 indoors, $2,000 outdoors for a valve |
| Vaughan | Sanitary Back-Water Valve Installation Subsidy | Listed as 50% up to $750 |
Our service area pages link to each official program.
Does it lower insurance costs?
Some insurers offer a discount or require a valve before they will sell sewer backup coverage. Ask your broker.
Do you need one?
A backwater valve makes most sense if any of these apply:
- Your basement has a floor drain, shower, toilet or laundry below street level
- Your street or neighbourhood has had sewer backups before
- You are in an older area with combined sewers
- Your insurer requires one for sewer backup coverage
- You are finishing a basement and want to protect the investment
Homes on high ground with no plumbing fixtures in the basement have less to gain.
Types of valve
Normally open valves are the common residential type for the main sanitary line. A flap lies flat and lets wastewater and sewer gas pass until water flows the wrong way and lifts it shut.
Normally closed valves stay shut until flow from the house pushes them open. They are used on some branch lines.
Inline or branch valves protect a single fixture, such as a floor drain, instead of the whole house.
Which one suits your house depends on the layout of your drains. Your plumber and the municipal inspector decide what is acceptable, and subsidy programs usually specify which installations qualify.
What installation involves
- A plumbing permit from the municipality
- A camera inspection of the lateral, in many cases, to check its condition and find the right location
- Breaking out a section of basement floor, usually near where the line leaves the house
- Cutting in the valve with a clear access cover at floor level
- Inspection by the municipal building department
- Reinstating the concrete
It is normally a one-day job. The access cover must stay reachable, so do not build a wall or lay fixed flooring over it.
Living with a backwater valve
During a storm that closes the valve, nothing can drain out. Do not run the washing machine, the dishwasher or a bath in heavy rain. Many valves have a clear lid so that you can see whether the flap is down. Some homeowners add an alarm that sounds when the valve closes.
Once a year, open the cover, flush the body with a hose and check that the flap moves freely and the seal is intact. Debris on the seat is the usual reason a valve fails when it is needed.
Questions people ask
What does a backwater valve do?
It lets wastewater flow out of your house and closes automatically if sewage starts to flow back in from the municipal sewer, protecting the basement from a sewer backup.
Is there a subsidy for a backwater valve in Ontario?
Several municipalities offer one, including Toronto, Mississauga, Hamilton, Halton Region, Peel Region, Markham and Vaughan. Amounts and rules differ and change, and some require approval before work starts, so check the official program page.
Can I use water while the valve is closed?
No. While the valve is shut, wastewater from the house has nowhere to go and will come up in the basement. Avoid using water-heavy appliances during intense rain.
Does a backwater valve stop all basement flooding?
No. It stops sewage backing up through the sanitary line. It does nothing about groundwater seepage, surface water or a failed sump pump.
How often does a backwater valve need maintenance?
Check and clean it at least once a year, and after any backup. A flap held open by debris will not seal.