Backwater valve not located (clapet antiretour): what it is, how serious, what it costs

The backwater valve, « clapet antiretour » or « clapet antirefoulement » in a French report, sits on the main drain in the basement. It lets wastewater leave the house, but closes when the street sewer fills up, during a heavy storm for example. Without it, sewage can come back up through the floor drain, the toilet or the shower in the basement.

Many Quebec municipalities require one for basement fixtures, and an insurer may refuse to cover a sewer backup if there is none.

In a report, it often appears as “not located” or “not verified”: the inspector did not find it, or it was hidden. That is why, in analyses, it usually lands under “To check”.

It matters before buying: a sewer backup means redoing a finished basement. You want to know whether there is one, whether it can be reached for maintenance and whether it protects every basement fixture.

Having a plumber evaluate the valve and its access: $250 to $600. Installing a valve, or opening up access to an existing one: $1,200 to $3,000.

Some cities offer a subsidy for installation: check with your municipality.

These are ballpark figures for Quebec, to be confirmed with quotes.

1. Did you find a backwater valve, and where is it?
2. Could it be reached, with a cover that opens for maintenance?
3. Are the floor drain, toilet and shower in the basement all protected?

Ask the seller whether there has ever been a sewer backup, and an insurance claim.

Does your report mention the backwater valve? Upload it: in five minutes, I tell you what comes first, what to have checked and what to ask the seller.

Yes. It is opened and cleaned about once a year, so debris does not keep it from closing. A plumber can do it, or you can if the access is clear.

No. The sump pump has its own small check valve, on the pipe that sends water outside. The backwater valve protects against the street sewer.