A sound of water in the bowl when no one has flushed the toilet. A sewage smell that intermittently returns to the basement. A customer in Longueuil called us last year after finding a rat in his bathroom drains, literally sitting on the outlet of the floor drain. He thought it was an isolated accident. It was the third building on the street. The Ville de Montréal fact sheet on check valves also names rodent damage as one of the causes of failure of these devices, which says a lot about the frequency of the phenomenon.
Where a rat enters the plumbing of a building
Norway rats live in sewer systems because they find everything there: water, food, warmth in winter, and predator-free traffic. They don’t try to climb up to your home. They look for an exit when the system becomes inhospitable, and your private hookup is the closest exit.
Three defects open the door. A pipe cracked by frost or ground subsidence, first, often on the section that passes under the slab. Secondly, a poorly sealed connection, typically where an old cast iron pipe has been rebuilt with PVC during a renovation. And finally an abandoned drain that has never been properly plugged, that of an old washing tank or a disused sump.
On municipal construction sites, the problem is getting worse. A pipe that is open for weeks moves an entire population to neighbouring buildings. Our calls are systematically increasing in the streets under construction, and this is no coincidence.
The trap plays a role that is underestimated. That U-bend under each appliance permanently holds a water plug that prevents sewage gases from coming back. It also blocks rodents, as long as it remains full. A floor drain that was unused for a summer dries up by evaporation, and the barrier disappears without anyone noticing. Pouring a liter of water into each rarely used drain, two or three times a year, costs zero dollars and closes a door. This is the least spectacular advice we give, and probably the most cost-effective.
The non-return valve, mandatory since 2011 and often out of service
Montreal has required a non-return valve in all new buildings since 2011, and the installation becomes mandatory for buildings that have already suffered a backup. The device is primarily used to block sewer backups during heavy rains. It also prevents rodents, provided they are in good condition.
That’s where the problem lies. The city recommends a check twice a year, and the list of common failures can be summed up in four points: poor location, deterioration, rodent damage, and blocked access because a floor was laid on top. The last case is the most common in finished basements, and it is the one that costs the most to correct.
The Rénoplex program subsidizes the installation to the tune of $80 for a standard valve, $560 per unit for a valve under an existing foundation, and $1,500 for a retaining pit with a pump. The City recommends using a plumber who is a member of the Corporation of Master Pipe Mechanics of Quebec and requiring a certificate of compliance.
One thing that few homeowners know: a valve in good condition stops the water, but a determined rat ends up damaging a tired seal. The valve is a barrier, not a guarantee. It must be accompanied by a diagnosis of the actual condition of the pipe.
What a Smoke Test Shows in Fifteen Minutes
Looking for a crack in a blindly buried network is like breaking through concrete at random. The method that avoids this is called the smoke test, and it comes from the municipal world, where it has been used for decades to identify cross-connections and leaks in sewer systems.
The principle is straightforward. The roof vent is plugged, the siphons are filled with water, and then pressurized smoke is blown into the network for about fifteen minutes. The smoke follows the path of the air, and therefore that of the leaks. It comes out through the crack, through the open joint, through the forgotten drain. A plume behind a baseboard or under a basement step, and the entry point is identified to within a few centimetres.
The smoke used is manufactured for these tests. It leaves no residue or stains, dissipates in a few minutes in a ventilated room, and does not clog the pipes. Our rodent infiltration smoke testing service follows this protocol, supplemented if necessary by a camera inspection when the leak is located under the slab.
My rule on this type of case is simple: no repairs before proof. Breaking a floor on an intuition is twice the price and often the wrong place.
The test also provides a service that no one anticipates: it documents. The photos of the plume, the exact location, the date, all this constitutes a file that can be used with an insurer, a municipality or a syndicate of co-ownership. On buildings where liability is discussed between several parties, this file is worth more than any argument.
Rat, mouse or field mouse, don’t get the wrong file
Misdiagnosis is expensive, because the three species do not enter by the same paths and do not treat each other in the same way. A mouse passes through a wall, a threshold, a conduit passage. A Norway rat passes through the ground, the drain, the foundation. The field mouse, on the other hand, comes from the ground and generally stays on the ground floor.
| Observed sign | Probable species | Where to look Verification | Logical sequence | |
|---|---|---|---|---|
| 15 to 20 mm droppings in the basement | Norway rat | Floor drain, foundation base | Smoke test | Pipe repair |
| Droppings 4 to 7 mm high | Mice | Cabinets, attics, ducts | Visual inspection | Exclusion and caulking |
| Water noise or intermittent sewer odor | Norway rat | Bowl, siphon, flapper | Smoke test | Damper and sealing |
| Holes in the lawn near the foundation | Norway rat or field mouse | Outside perimeter | Site inspection | Supervised baiting and exclusion |
| Scratching in a wall upstairs | Mouse or squirrel | Soffit, attic Roof | inspection | Access closure |
| Damper that no longer closes | Norway rat | Basement, retaining pit | Plumber and smoke test | Damper replacement |
Morphological details count as much as tracks. The difference between a rat, a mouse and a field mouse is the tail, ears and shape of the muzzle, and an owner who correctly describes what he has seen wins us a visit. For cases already installed, our page on the Norway rat details the treatment once the entrance is closed.
Questions that come up on this type of call
Can a rat really get out of the bowl?
Yes, but this is the rarest case. The vast majority of lifts are made through a floor drain, a dry siphon or a cracked pipe. A basin with a siphon that remains full is already a water barrier.
Does poison in the drain solve the problem?
No, and it is even contraindicated. A rodent that dies in a pipe or under a slab creates an odor that is impossible to locate without demolition. The treatment involves physical closure.
How long does it take to identify the entrance?
A smoke test on a single-family residence is done in one visit. On an apartment building with several connections, count two passes.
My neighbour is under construction, should I be worried?
Watch for signs for the next six weeks. A sewer site displaces people, and the most porous buildings on the street are visited first.
Who pays, the owner or the city?
Private driving is the responsibility of the owner up to the property line. Beyond that, it’s the municipality. A smoke test that locates the leak on the public side completely changes the conversation with the city.
Should you notify your insurer?
Yes, and as soon as possible. Many policies cover damage resulting from a backflow, but require proof of reasonable maintenance of the valve. A dated test report works in your favor in this case.
And in an apartment building?
Responsibility is shared according to the declaration of co-ownership or the lease, but the main pipe is almost always a common portion. Settling the matter on a unit-by-unit basis leads nowhere: the entire network is handled, otherwise the population moves from one floor to another.
A rat that comes up is never a coincidence: it is a symptom of a physical defect somewhere on the course. As long as this defect exists, baiting only saves time. Our teams work on this type of file on the South Shore, from Longueuil to Saint-Hyacinthe, and the first step is always the same: find the hole before treating. The rest of the rodent cycle is detailed in our pest calendar.
