How to Keep Mice Out of Your House This Fall: An Ontario Checklist

Quick answer: Mice move into Ontario homes when overnight temperatures start dropping below about 10°C — typically late August in cottage country and mid-September in the GTA. To keep them out, seal every exterior gap larger than 6 mm (a pencil’s width) with steel wool + sealant, mesh, or metal flashing — not foam alone — before that window. The big five entry points are the garage door sweep, the AC/utility line penetration, dryer and exhaust vents, brick weep holes, and the gap where decks and additions meet the foundation. Do it in August and you’re sealing mice out; do it in October and you may be sealing them in.

Every fall, the same story: the first cold snap hits, and within two weeks the calls start — scratching in the ceiling, droppings in the pantry, something in the wall behind the headboard. None of it is bad luck. Fall mouse invasions are a scheduled event, and the homes that skip it are the ones that did the boring work in August. Here’s the checklist, with the details that generic “seal your gaps” articles leave out.

Why fall, and why is late August the deadline?

Outdoor mice — both house mice and the deer mice common around rural properties and cottages — spend summer living outside, where food is everywhere. When nights cool, two things happen at once: their food supply collapses, and your home becomes the warmest object in their territory. A mouse exploring a foundation on a cool September night isn’t wandering — it’s following the warm air leaking out of every gap you haven’t sealed. Seed-heavy landscaping, bird feeders, and unpicked garden produce all shorten the distance between their summer range and your foundation.

The deadline logic is simple: exclusion done before the migration keeps mice out. Exclusion done after some have already moved in traps them inside — which converts a prevention job into a removal job. In most of southern Ontario that puts the smart window in August to mid-September.

The exclusion checklist: the big five entry points

A mouse needs 6 mm — the width of a pencil. Check these in order; between them they account for most of the entries we find on real inspections:

1. The garage door. The rubber sweep at the bottom corners is the single most common mouse entrance in Ontario suburbia. If daylight shows at either corner, that’s a door. Fix: replace the sweep, add rodent-resistant corner seals. If the garage connects to the house, the door between them needs a sweep too.

2. Utility penetrations. Where the AC lineset, gas line, cables, or outdoor faucets pass through the wall, the hole is almost always bigger than the pipe. Fix: stuff the gap with coarse steel wool or copper mesh, then seal over it. Foam alone fails — mice chew through cured foam in a night; the metal is what stops them.

3. Vents. Dryer vents, bathroom exhausts, soffit vents with torn screens. A dryer vent with a broken flap is a heated mouse door with a food smell coming out of it. Fix: metal-louvred covers or 6 mm (1/4”) hardware cloth over the opening — never block airflow, screen it.

4. Brick weep holes. Those vertical gaps between bricks are structural drainage — you must not caulk them shut. Fix: purpose-made stainless weep-hole covers (or corrugated inserts), a 20-minute job on most facades.

5. Deck, porch, and addition junctions. Wherever a structure was added to the original envelope, the joint moves and opens. Crouch and follow the foundation line behind every deck and step with a flashlight. Fix: flashing or hardware cloth, screwed, not glued.

Then do the ground rules: firewood off the foundation, bird feeder spill managed, garbage in bins with lids that actually close, and tall grass cut back from the wall — cover is half the invitation.

The materials that work (and the ones that don’t)

• Works: coarse steel wool or copper mesh backed with sealant; 1/4” hardware cloth; metal flashing; concrete patch for foundation cracks; new door sweeps.
• Fails: expanding foam on its own (chewed through), plastic mesh (chewed through), caulk alone in gaps over 6 mm, and every plug-in ultrasonic repeller we have ever been called in behind. Peppermint oil is a pleasant-smelling way to spend three weeks losing.

Signs you’re already too late for prevention

Do the checklist anyway — but add a trapping/removal step first if you find droppings (dark grains of rice, 3–6 mm) along walls or in the garage, gnaw marks on stored food or wiring, a musky ammonia smell in enclosed spaces, or you hear scratching at night. An established infestation means exclusion alone will seal mice in, and you want the sequence a professional program runs: knock down the population first, then seal on the follow-up visit. That’s exactly how our two-visit Complete Mice Protection program is structured — and why it costs $495 instead of a $200 patch job (see what mouse control costs in Ontario for the full pricing breakdown).

When to bring in a professional

Get a pro involved if the house has a history of mice (last year’s entry points are still there), if it’s a century home or fieldstone foundation, if you’re in a semi or townhouse (your exclusion is only as good as the neighbour’s), or if you simply want the inspection done by someone who finds entry points for a living. A residential protection plan covers rodents year-round alongside the seasonal insects; for one-off help, our team does inspection, exclusion, and guaranteed removal across the GTA, York Region, and Simcoe County — including a lot of fall exclusion work in Whitby and Newmarket, where newer subdivisions back directly onto farmland and the fall migration is dramatic.

Frequently asked questions

What temperature makes mice come inside? Sustained overnight lows below ~10°C trigger the indoor migration. In southern Ontario that’s typically mid-September; in Simcoe County and Muskoka it can be late August.

What smells keep mice away? None reliably. Peppermint, dryer sheets, and ultrasonic devices all fail against a mouse choosing between a scented gap and freezing. Physical exclusion is the only prevention that holds.

Can mice climb brick walls? Yes — mice climb rough surfaces easily and jump about 30 cm, which is why second-storey vents and soffits are on the checklist, not just ground level.

How do I mouse-proof a garage? New bottom sweep, corner seals, screened vents, no pet food or birdseed in open bags, and check where the garage’s electrical or water lines pass into the house. The garage is the airlock — treat it like part of the envelope.

Want the inspection done properly before the season turns? Call (705) 302-1887 — August books out fast once the first cool nights hit.