What backwater valve installation involves
The mainline backwater valve is set into the sewer pipe beneath the basement slab, usually just inside the front wall. It has a flap that lets your wastewater out and swings shut if water tries to come back in. Installing one is a concrete job as much as a plumbing job.
- Camera the main drain to find the right spot and check the pipe.
- Get the permit in place before any digging.
- Cut and remove the floor slab over the pipe.
- Dig down, cut in the valve level and in the right direction of flow.
- Have the work inspected before backfilling.
- Set an access box so the valve can be opened and cleaned, then pour new concrete.
Why North York houses need it
Large parts of North York drain into sewers that can run full during heavy summer storms. When that happens, sewage looks for the lowest opening, which is often a basement floor drain. A finished basement with a rec room or a rental suite has a lot to lose. Toronto's Basement Flooding Protection Subsidy Program has, at times, paid back part of the price of a valve; check the City's current rules and amounts before you rely on it.
Plumber's tip: A valve closed during a storm means nothing drains out either. When the flap is shut, do not run the washer, dishwasher or showers until the storm passes.
How we write up the job
The order lists the valve model, its location measured from a wall, the permit number and the inspection date. The City's subsidy application has asked for items such as the permit and an itemized invoice, so we make sure those line items are clear. We also note whether the house has a sump or a weeping tile connection, because that affects where your foundation water goes once the valve is in.
Signs you should call
- You have had sewage or grey water come up the floor drain before.
- Neighbours on your street flooded in the last big storm.
- You are about to finish the basement.
- Your insurer has asked whether you have a backwater valve.
For a wider view of keeping a basement dry, read our page on basement flood prevention, and once the valve is in, keep the backwater valve maintenance leaflet handy.
Costs and what affects them
Some houses also need the foundation drain separated from the sanitary line at the same time, so the valve does not trap weeping tile water inside. That adds a sump pit and pump to the job. We tell you up front if your house falls into that group.
Depth of the pipe, slab thickness, finished flooring that has to come up, and how much of the old drain needs replacing all move the number. A camera inspection beforehand removes most of the surprises. You see the quote on paper before we cut the floor.
Job at a glance
| Category | Basement |
|---|---|
| Tools | Concrete saw, jackhammer, backwater valve body, no-hub couplings |
| Final test | Water run from upper floors with the flap opening freely |
Tell-tale signs
- Basement has flooded from sewer backup before
- House sits in a low spot on the street
- Finished basement with a bathroom below grade
Steps on the job
- Camera the main to pick a straight section near the wall
- Break out the slab and expose the building drain
- Cut in a normally-open mainline valve with an access box
- Patch concrete and test with water from upstairs
Your questions on this one
Does a backwater valve need maintenance?
Yes. Open the access cover a couple of times a year, see that the flap swings easily and pick out any debris. Our leaflet walks through it.
Will a valve stop all basement water?
No. What it blocks is sewer backup through the drain. It does nothing for groundwater through cracks or water coming in a window well.
Do I need a permit?
In Toronto, installing a mainline backwater valve has required a plumbing permit. The subsidy program has also required one.