Sewer backup

A backup happens when the municipal sewer — often a combined system in the older central boroughs, where storm water and sewage share the same pipe — takes on more water than it can carry, and the excess comes back up through the lowest point in the house, almost always the basement floor drain.

A backup shows itself first by smell, a clear sewage smell rather than dampness, often before the water is even visible, and by the fact that it rises through the floor drain instead of running down from a wall or ceiling. That's what sets it apart from a pipe leak or an overflowing water heater: those have a single source you can shut off, while a backup keeps rising as long as the municipal system stays overloaded, no matter what you close inside the house.

While it's happening, stop using toilets, sinks, the washing machine and the shower: every flush adds to the volume trying to come back out. Move whatever you can up out of the basement and avoid direct contact with the water, which contains sewage.

Once the water is out, a plumber can install or check a backwater valve, the device that actually stops this from happening again. If the backup is coming from the municipal system rather than your own plumbing, the City may be involved too: we point you to 311 in parallel when that's the case.

In the central boroughs like the Plateau and much of Rosemont, the sewer is still combined, storm water and sewage in the same pipe, which means a summer storm can overwhelm the system before it even reaches a street a block over that's already been separated. In a triplex, the basement floor drain usually serves all three units at once, so a backup rarely hits just one tenant; it usually means coordinating with the other units, and sometimes the landlord, before the plumber even arrives. A basement finished as a rental unit, common in Saint-Michel and elsewhere, is especially exposed since it's often the lowest point in the building.

How much cleanup is involved mostly comes down to how long the backup ran before water use in the house stopped, and whether the basement is finished or not: a bare concrete basement clears in one visit, a finished basement with carpet and drywall adds a separate restoration job. Installing a backwater valve afterward also varies with access to the floor drain and whether the existing plumbing allows it to go in without breaking the slab.

Before you call, note how long the water has been backing up, whether it's still rising or has settled, and whether the basement is finished or not. If more than one unit is affected, try to find out whether the neighbours have also stopped using their plumbing: that helps confirm whether the problem is in the municipal system or a blockage in the building's own stack.

A backwater valve works with a simple flap that closes under the pressure of water trying to come back up, blocking the backup from continuing inward while still letting the house's own wastewater flow out normally to the sewer. Like any mechanism, it can get stuck open by debris or built-up grease, which makes it useless at the exact moment it's needed most; an occasional check, especially after a backup, confirms it's still closing properly.

After the water is out, proper cleanup uses a disinfectant rather than a plain water rinse, since backup water carries sewage even when it looks clear on the surface. A porous material that sat in contact with it for a while, carpet, drywall, baseboards, usually gets replaced rather than disinfected, a call that belongs to the restoration specialist rather than the plumber.