A backwater valve is a one-way valve in your home's main sewer line. It closes when the municipal sewer backs up, so sewage cannot come up through your basement floor drain. For a finished basement in Woodbridge, Thornhill or anywhere else in Vaughan, it is one of the more useful protections a plumber can install. It usually needs a plumbing permit, and before you book you should ask your city whether a subsidy program is running for your address, because some have to be applied for before the work starts.
What a backwater valve does, and what it does not
In a heavy storm, the municipal sewer can fill faster than it drains. When that happens, sewage can be pushed back up the sewer lateral towards your house and out of the lowest opening, which is usually the basement floor drain. A backwater valve sits in the main drain, normally near the front wall of the basement, and stops that flow.
- It does let your drains work normally day to day. The common type stays open and closes only when water flows the wrong way.
- It does not stop groundwater seeping in through the walls or floor. That is a sump pump's job, and many basements need both.
- It does not stop surface water coming in through windows, doors or cracks.
- While it is closed, water you use in the house cannot leave either. During a backup, avoid running taps, flushing, showering or doing laundry, or that water has nowhere to go but your own floor drain.
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Why it matters in Woodbridge, Thornhill and Vaughan
Most of Woodbridge is subdivisions from the 1970s through the 1990s, and many of those homes have large finished basements, some now used as second suites. A sewer backup through the floor drain in a basement like that means flooring, drywall and contents, not just a mop.
Thornhill has one of the older housing stocks in the area, from the old village around Yonge and Centre Street to 1950s through 1980s streets. Many of those homes were built before backwater valves were common, and older clay or cast-iron sewer laterals under mature trees add their own backup risk.
Thornhill also straddles Yonge Street. West of Yonge is in the City of Vaughan and east of Yonge is in the City of Markham, and that decides which city handles your permit and any basement flooding program. Woodbridge, Maple and the rest of Vaughan go through the City of Vaughan.
Installation, permits, subsidies and what drives the cost
Installing a backwater valve usually means cutting into the basement floor at the main drain, fitting the valve into the line, adding an access cover so it can be inspected and cleaned, and restoring the floor. Many plumbers run a camera through the drain first to see its condition and depth.
One detail matters before any cutting starts. In some older homes, the foundation drains (weeping tile) connect into the sewer line inside the house. Where the valve goes in relation to that connection makes a difference, and some programs require a sump pump to be added at the same time for that reason. Ask the plumber to check where your foundation drains go.
A backwater valve usually needs a plumbing permit and an inspection. Ask whether the plumber will take out the permit.
On subsidies, the only program named here is the City of Toronto's Basement Flooding Protection Subsidy Program, which applies to homes in Toronto. For Vaughan and Markham, ask the city directly whether any basement flooding or backwater valve program is running for your address, what it covers, and whether you must apply or have an inspection before the work is done. Programs and their rules change, so check before you book rather than after you pay.
Prices vary too much from house to house to quote a useful number. What drives the cost:
- How deep the main drain is and where it runs under the floor.
- What has to be opened and restored: bare concrete, tile, finished flooring or walls.
- Whether a sump pump or foundation drain change is needed at the same time.
- What the camera finds, such as roots or damage in the drain.
- Permit and inspection requirements, and access to the work area.
Checklist for Vaughan and Thornhill homeowners
- Confirm which city you are in, especially in Thornhill.
- Ask that city whether a subsidy program is running before you book, and whether you need approval first.
- Note whether your floor drain has ever backed up, and when.
- Find your main cleanout or floor drain, and note what is built over it.
- Ask about a camera inspection of the drain first.
- Ask where your foundation drains connect.
- Confirm who takes out the permit.
- Keep the access cover clear afterwards, and keep the permit and receipts. Your insurer may ask about them.
When to call a plumber
Call if your floor drain has backed up even once, if you are finishing or have just finished a basement, if you are adding a basement suite, or if you are buying an older home with no backwater valve. If the floor drain backs up when the washing machine empties, the main drain may be partly blocked as well. Our drain clearing page covers that side, and our sump pump and backwater valve page explains how the two protections work together.
If sewage is coming up right now, stop using water in the house, keep clear of the water if it is near anything electrical, and call your home insurer as well as a plumber.
Frequently asked questions
Does a backwater valve need maintenance?
Yes. Debris can stop the flap from sealing, so the valve should be checked and cleaned regularly. Many manufacturers suggest once a year. Keep the access cover where you can reach it, and do not build over it.
I already have a sump pump. Do I still need a backwater valve?
They solve different problems. A sump pump handles groundwater around the foundation; a backwater valve stops sewage coming back in through the floor drain. If your floor drain has ever backed up, a sump pump alone will not prevent it happening again.
Is my Thornhill home in Vaughan or Markham?
It depends on which side of Yonge Street you are on. West of Yonge is the City of Vaughan; east of Yonge is the City of Markham. Permits and any municipal programs go through that city.
If you are thinking about a backwater valve, the short form is free to use and sends your request to one local plumber who covers your part of Vaughan or Thornhill.