A Dime Is the Whole Problem
Most people go looking for a hole. There usually is not one. Health Canada puts the threshold plainly: mice can squeeze through cracks as small as a dime, and rats through a quarter-sized hole (Health Canada, "Rats and mice", canada.ca/en/health-canada/services/pest-control-tips/rats-mice.html). A gap that small is not damage. It is a feature of how houses are built.
That is why sealing works and why it so often fails. Homeowners patch the one opening they can see and miss the six the house came with. Every entry point below is a standard part of residential construction in Ontario, not a defect, and each one is dime-sized or larger on a typical home.
The deadline matters too. Health Canada notes that the deer mouse may invade buildings near fields and woodlands in the fall — which describes a large share of housing across Halton, Milton, and the escarpment edge. Sealing in November is remediation. Sealing in September is prevention.
The Seven Openings Mice Actually Use
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. Foam does not stop a mouse.
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. Ontario freeze-thaw cycles widen hairline cracks every winter. A crack that was cosmetic in 2024 is an entry point by 2026.
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 October Is the Deadline in Ontario
Mice do not decide to move indoors because it is cold. They move because their outdoor food and cover disappear. Across the GTA that happens when fields are cut and harvested and landscaping dies back, which in most years falls through late September and October.
Properties on the edge of that transition see it hardest. Milton's newer subdivisions back onto escarpment bush and working farmland. Bradford sits in the Holland Marsh. 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, "Clustering flies"). If you are getting sluggish flies at south-facing windows in October, the same gaps are letting mice in.
What Seals a Gap, and What Does Not
Mice gnaw. That single fact decides which materials work. Expanding foam, caulk on its own, steel-free weather stripping, and plastic vent covers all fail because a mouse can chew straight through them, usually within a season.
Health Canada's guidance is specific about the alternatives: use metal weather stripping under doors, patch cracks in foundations, stuff steel wool around pipes before caulking or plastering, and cover dryer vents, attic vents, and soffits with fine mesh metal screening. The pattern is consistent — metal does the work, and the sealant only holds the metal in place.
Ultrasonic repellers are the most common thing homeowners try first and the least likely to help. Health Canada's own assessment is that rats and mice may adapt to the devices and return. They 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.
The Mistakes That Undo the Work
Setting traps immediately is the most common one. Health Canada recommends a warm-up period of three to four days, baiting the traps without setting them, so the mice become comfortable taking the bait. Traps set on day one catch the boldest mouse and teach the rest to avoid them.
Placement is the second. Set one trap per metre along walls, or five to ten traps per visible mouse hole, positioned so the baited end is closest to the wall. Mice travel along edges, not across open floor.
Then people stop too early. Health Canada advises resetting traps in two to three weeks to catch maturing mice — the ones that were still in the nest during the first round. 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.
When Sealing Is No Longer the Answer
Some signs mean a population is already established rather than arriving. Health Canada lists scampering and scratching sounds inside walls at night, gnawing or chewing damage, damaged food packaging, droppings and urine, burrows or holes around foundation walls, and tracks on dusty surfaces.
There is also a health reason not to treat this as purely a nuisance. Health Canada identifies the deer mouse — the species most likely to move in from fields and woodlands in the fall — as the most common carrier of hantavirus. Droppings in an enclosed space such as a crawl space or attic should not be swept or vacuumed dry.
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 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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Common Questions
How small a gap can a mouse fit through? Health Canada's guidance is a crack the size of a dime. Rats need roughly a quarter-sized hole. If you can push a pencil into an opening, treat it as an entry point.
Does steel wool actually stop mice? Yes, when it is installed correctly. Health Canada recommends packing steel wool around pipes before caulking or plastering over it. The steel wool is what mice will not chew; the caulk holds it in position and seals the remainder.
Do ultrasonic pest repellers work on mice? Not reliably. Health Canada notes that rats and mice may adapt to the devices and return. Use them alongside sealing if you like, but not instead of it.
Why do I get mice every single October? Because the entry point is still open. Recurring activity in the same season year after year almost always means a previous population was removed without the structural opening being closed, so the property is re-colonised as soon as outdoor cover disappears.
Is it too late to seal in November? No, but the job changes. Sealing before the migration keeps mice out. Sealing after it can trap them inside, so it needs to be paired with removal rather than done on its own.
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