Plumbing vent (évent de plomberie): what it is, how serious, what it costs

The plumbing vent is the pipe you see sticking out of the roof; a French report calls it « évent de plomberie ». Inside the house, it connects to the drains. It lets air into the piping so water drains properly, and it sends sewer gas outside, above the roof.

It works with the traps, the U-shaped pipes under every sink, basin and shower. The trap holds a little water that blocks odours; the vent keeps draining water from sucking that water seal out. Reports say things like “improperly terminated vent”, “leak at the vent flashing”, “S-trap” or “missing trap”.

It depends on what is noted. A rubber or non-standard trap is a cheap upgrade. A blocked or poorly terminated vent, or a missing trap, can let sewer gas into the house: it smells, and it is not air you want to breathe.

On the roof, the weak spot is the flashing around the pipe: when it leaks, water gets into the attic. That is why it shows up in several lists: “Can wait” for a trap to replace, “Fix first” for a vent that lets odours in or an active roof leak.

Replacing an S-trap under a sink with a compliant trap: $300 to $750. Installing a compliant trap under a bathroom sink: $200 to $450. Insulating the vent pipe where it crosses the attic, to prevent frost and condensation: $150 to $350. Fixing a roof leak around the vent: $400 to $1,500.

When the installation looks homemade, start with a plumber’s evaluation: $250 to $600.

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

1. What exactly is the problem: the vent on the roof, a trap, or both?
2. Did you notice a sewer smell in the house, and where?
3. Did you see water marks in the attic around the vent pipe?

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

Often it is simpler: a trap that dried out because the fixture is rarely used, like a floor drain or a spare shower. Pour water into it. If the smell comes back, have a plumber check the vent and the traps.

It is an air admittance valve: it lets air in without letting odours out. It is allowed in some cases, must stay accessible and gets replaced when it no longer closes properly. A plumber can tell you whether it suits that spot.