Why Mice Enter Montreal Homes in Winter (And How to Stop Them)

Every autumn in Montreal, the same thing happens: temperatures drop below 10°C and mice start looking for a warm place to spend the winter. Your home is exactly what they are looking for. A single mouse can squeeze through a gap the size of a dime — about 6 mm — making most homes surprisingly easy to enter. And once inside, they do not leave on their own.

Montreal's climate makes this problem particularly acute. The city's extreme temperature swings — from -30°C winters to +35°C summers — create a powerful biological drive for rodents to seek shelter indoors. Combined with Montreal's dense housing stock, aging building infrastructure, and the prevalence of older brick and stone construction, the city has some of the highest rodent activity rates in Canada.

Why Montreal's Climate Drives Mice Indoors

Mice are warm-blooded animals that cannot survive prolonged exposure to sub-zero temperatures. As outdoor temperatures drop in October and November, their food sources — seeds, insects, plant material — disappear under snow. The combination of cold and food scarcity creates an urgent need to find shelter.

Your home offers everything a mouse needs: warmth, water, food scraps, and nesting material. Once a mouse finds a way in, it releases pheromones that attract other mice to the same entry point. A single entry point can allow dozens of mice to enter over the course of a winter.

Entry Points Specific to Montreal Housing

Montreal's housing stock is unique in Canada. The city has a high proportion of older brick and stone buildings, duplexes and triplexes with shared walls, and basement apartments with ground-level access. These features create specific entry point vulnerabilities:

• Foundation cracks in older brick and stone buildings — common in homes built before 1970
• Gaps around utility penetrations (gas lines, electrical conduits, water pipes) where they enter through the foundation
• Deteriorated mortar joints in brick foundations — mice can enlarge these gaps easily
• Gaps under exterior doors — especially in older buildings where door frames have settled
• Basement window frames that have warped or deteriorated over decades
• Gaps around dryer vents and bathroom exhaust fans
• Shared wall voids in duplexes and triplexes — mice move freely between units
• Roof vents and attic access points in older homes with wood-frame construction

DIY Prevention Tips That Actually Work

Before calling a professional, there are several effective steps you can take to reduce the risk of mice entering your home:

• Walk the perimeter of your home with a flashlight and a pencil — any gap a pencil fits through is large enough for a mouse
• Seal gaps under 1 inch with steel wool packed tightly, then covered with caulk — mice cannot chew through steel wool
• Install door sweeps on all exterior doors, including the garage door
• Cover dryer vents and exhaust fans with hardware cloth (1/4 inch mesh)
• Store all food — including pet food and birdseed — in airtight metal or glass containers
• Keep garbage in bins with tight-fitting lids and empty them regularly
• Eliminate clutter in basements, garages, and storage areas — mice nest in cardboard boxes and fabric
• Keep firewood stacked at least 6 metres from the house
• Trim tree branches that overhang the roof — mice use them as bridges

When to Call a Professional

If you have found droppings, heard scratching in walls or ceilings at night, seen a mouse, or noticed gnaw marks on food packaging or structural materials, the infestation has already begun. At this point, DIY traps can help reduce the population but rarely solve the problem completely.

A professional exterminator will do three things that DIY cannot: identify every entry point (not just the obvious ones), set a strategic trap network in the right locations, and seal the home against re-entry. Without sealing entry points, new mice will continue to enter even after the current population is eliminated.

What Blackline's Rodent Exclusion Process Looks Like

Our rodent exclusion service is a two-phase process. In the first phase, we conduct a thorough inspection of the entire property — interior and exterior — to identify all active entry points and assess the severity of the infestation. We then set a strategic trap network and apply rodenticide in tamper-resistant bait stations in appropriate locations.

In the second phase, typically 1 to 2 weeks later, we return to check traps, remove any deceased rodents, and seal all identified entry points using professional-grade materials: steel wool, hardware cloth, expanding foam, and metal flashing. We also provide a written report of all work completed and recommendations for ongoing prevention.