The One-Way Gate in Your Main Drain

In a nutshell

The valve sits in the main drain under the basement floor, past every fixture in the house. Water from your sinks and toilets pushes its flap open and carries on to the street. If the City sewer fills and starts pushing back, the flap closes and seals. A removable lid set into the floor gives you a way in for checks.

Working through it

1. The mechanism

Most residential models use a hinged flap, sometimes with a float that helps it close firmly.

2. The location

Downstream of all drains, including the floor drain, so everything in the house is protected.

3. The install

A section of slab is cut out, the drain is opened, the valve is set level and tested, and the floor is patched with an access box left in place.

4. The paperwork

A plumbing permit and an inspection are normally needed.

5. The upkeep

Twice a year, pop the lid, swing the flap with a gloved hand and clear off anything clinging to it.

Clues worth noticing

• Water appearing at the floor drain during heavy storms.
• Neighbours on your street who have flooded.
• A finished rec room, rental unit or bedroom downstairs.
• No access cover in the basement floor, which usually means no valve.

Signals that it needs a licensed plumber

Toronto has offered homeowners money toward these valves in past years; ask what the program looks like now before you book. Our backwater valve installation page covers the job itself.