How Mice Get Into Ontario Homes: 7 Gaps to Seal by October

Where do mice get into a house?

Mice usually get in through seven openings: gaps under exterior doors, utility penetrations, dryer vents, soffit and attic vents, foundation cracks, the sill plate gap and garage door corners.

One: the gap under an exterior door. Health Canada calls this out specifically: even the small gaps created by worn thresholds under doors will allow mice access. Garage side doors are the usual offender because nobody looks at them.

Two: utility penetrations. Every pipe, cable, and gas line entering the house passes through a drilled hole larger than the pipe. The installer sealed it with foam, or with nothing at all. Health Canada recommends stuffing steel wool around pipes before caulking or plastering.

Three: dryer vents. The flap is designed to open. Once the spring weakens or lint props it, the vent is a permanent doorway with a warm draft advertising it.

Four: soffit and attic vents. These must stay open for airflow, so they cannot be blocked, only screened. Health Canada recommends covering dryer vents, attic vents, and soffits with fine mesh metal screening.

Five: foundation cracks. Health Canada's advice is to patch cracks in foundations.

Six: the sill plate gap where the wood frame meets the concrete foundation. On many builds this is sealed with a thin gasket that shrinks. It runs the entire perimeter of the house.

Seven: garage door corners. The bottom seal compresses flat at the two corners where the door meets the frame, leaving a triangular gap. It is the single most common opening we find, and it is almost never on a homeowner's list.

Why do mice come inside in the fall in Ontario?

Mice move indoors when their food and cover outside disappear. For the deer mouse, Health Canada says this may happen in the fall, in buildings near fields and woodlands.

Properties on the edge of fields and woodlands see it hardest. Milton's newer subdivisions back onto escarpment bush and working farmland. Newmarket and Bradford both border open agricultural land. When that cover goes, the nearest heated structure is the subdivision.

The same window drives the other fall invaders. Health Canada describes cluster flies (8 to 10 mm, dark grey, slower than a house fly) as gathering in clusters after entering a house in the fall, and notes there tend to be more of them in houses surrounded by large lawns or backing onto open parks. Health Canada also says cluster flies are often attracted to the warm south and west sides of buildings.

What should I use to seal gaps against mice?

Health Canada recommends metal weather stripping under doors, steel wool stuffed around pipes before caulking or plastering, and fine mesh metal screening over dryer vents, attic vents and soffits. It also says to patch cracks in foundations. Mice gnaw, and that single fact decides which materials work.

Health Canada recommends that ultrasonic devices be used along with other pest control options. It also notes that rats and mice may adapt to the devices and return. The devices are not a substitute for closing an opening.

Outside the house, remove what makes the property attractive in the first place: cut tall grass and weeds back from the wall, secure garbage in containers with tight-fitting lids, and raise woodpiles about 30 centimetres off the ground and away from the house.

What mouse trapping mistakes let mice keep coming back?

The usual mistakes are setting traps without a 3 to 4 day warm-up, placing them away from walls, not resetting them in 2 to 3 weeks, and trapping without sealing the opening. Setting traps immediately is the most common one. Health Canada recommends a warm-up period of 3 to 4 days, baiting the traps without setting them, so the mice become comfortable taking the bait.

Placement is the second. Set 1 trap per metre along walls, or 5 to 10 traps per visible mouse hole, positioned so the baited end is closest to the wall.

Then people stop too early. Health Canada advises resetting traps in 2 to 3 weeks to catch maturing rats and mice. Stopping after the first few catches leaves the next generation in place. Every trap type needs checking daily.

The largest mistake is trapping without sealing. Removing the mice in the house does nothing about the opening that let them in, and the property that was attractive in October is still attractive in November.

How do I know if mice are already living in my home?

Health Canada lists scampering and scratching sounds in the walls at night, gnawing or chewing damage, damaged food packaging, droppings and urine, burrows or holes around foundation walls and tracks on dusty surfaces as signs of rats and mice. Some of these signs mean a population is already established rather than arriving. In Toronto, the City counted 2,523 rodent-related complaints and service requests in 2024 and says it cannot consistently tell mouse reports from rat reports, as our Toronto rat and mouse statistics page shows.

There is also a health reason not to treat this as purely a nuisance. Health Canada identifies the deer mouse as the most common carrier of hantavirus. It also says never to sweep or vacuum dry droppings, because the dust that is raised can cause illness.

If activity is inside a wall void, spread across more than one floor, or returning each year in the same season, the entry point was likely never closed. That is the point where an inspection pays for itself: finding the opening matters more than removing the animal.

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