How to Keep Mice Out of Your House This Fall (Ontario, 2026)

Quick answer: Mice start entering Ontario homes in late August and early September, not in November when you first hear them. They need a gap about 6 mm wide — roughly a pencil’s diameter. The work that actually keeps them out is exclusion: sealing the garage threshold, utility penetrations, weeping tile gaps, dryer vents and foundation cracks before the nights turn cold. A single mice visit is $495; Complete Mice Protection, which includes the exclusion work, is $495. Call (705) 302-1887.

The nights have started cooling. That’s the trigger — not snow, not the first frost. Mice respond to falling overnight temperatures by moving their search for shelter from fields and hedgerows toward buildings, and in most of Ontario that shift begins in the last two weeks of August.

Which means the mouse you’ll hear in the ceiling in November found its way in about ten weeks earlier, through a gap that is still open today.

The entry-point checklist

A mouse needs roughly 6 mm. It can flatten its ribcage and it can climb brick, siding and downspouts, so height doesn’t protect you. Work these in order of how often they turn out to be the actual route:

Garage threshold and door seals. The single most common entry into Ontario homes. The rubber sweep at the bottom of an overhead door compresses and cracks; the corners where the seal meets the frame open up first. Check by closing the door and looking for daylight from inside.

Utility penetrations. Where the gas line, water service, electrical conduit, AC line set and cable enter the foundation. The original installer’s foam has usually shrunk or crumbled. Copper mesh packed into the gap, then sealed, holds up far better than foam alone — mice chew straight through expanding foam.

Weeping tile and foundation cracks. Older homes in Durham and York Region — Whitby, Oshawa, Newmarket — commonly have hairline foundation cracks from decades of freeze-thaw. Anything you can slide a pencil into.

Dryer and bathroom exhaust vents. Dampers stick open or break off entirely. From outside, confirm the flap closes and sits flush.

Soffit and roofline. Mice climb. Gaps at the soffit-fascia joint and around roof penetrations are real routes, particularly where a lower roof meets an upper wall.

Under decks and porches. Not an entry point in itself, but harbourage that puts a population right against your foundation. If mice are living under the deck, they will eventually find the rim joist.

Garage-to-house door. Often the last unsealed door in the building. Check its sweep too.

House mice and deer mice behave differently once indoors, and deer mice carry a hantavirus caution that house mice don’t — our pest library covers how to tell them apart.

When to book fall exclusion

The useful window in Ontario is mid-August to late September. Earlier than that and you’re sealing before the pressure exists; later and you’re sealing mice inside rather than out.

The ordering matters, and it’s the thing people get backwards. If mice are already active indoors, exclusion has to come with or after removal, not before — sealing a building with a resident population traps them in the walls, where they die in inaccessible cavities. If there’s no current activity, exclusion alone is the whole job and it’s the cheapest rodent work you will ever buy.

That’s why the two-visit service is the only mice service we sell. $495 — Complete Mice Protection covers treatment plus the exclusion work that closes the routes so you’re not paying again next autumn. If this is your first mouse ever, the $495 visit may be all you need. If it’s your second or third consecutive fall, you don’t have a mouse problem, you have an open building, and $150 more solves the actual issue.

What doesn’t work

Peppermint oil, dryer sheets, ultrasonic repellers. No credible evidence any of these keep mice out of a structure. They’re popular because they’re cheap and because mouse activity fluctuates naturally, so anything you happen to do before a quiet week gets the credit.

Poison as a first move. Bait kills mice wherever they happen to be when it takes effect, which is often inside a wall. It also doesn’t touch the entry route, so you get a fresh population next month. In homes with pets or children it introduces a risk that exclusion simply doesn’t carry.

Snap traps alone. Fine for confirming activity and knocking down a small number. They do nothing about the gap that let them in.

Expanding foam by itself. Mice chew through it. Foam is a sealant, not a barrier — it needs mesh, steel wool or hardware cloth behind it.

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.