How Are Mice Getting In? The 12 Entry Points Ontario Homeowners Miss

Quick answer: A house mouse can squeeze through a gap about the width of a pencil — roughly 6 mm. In Ontario homes, the openings that let them in are almost never the dramatic hole you’re picturing. They’re unscreened brick weep vents, curled garage door seals, the unsealed gap around an air conditioner line, and the sill-plate seam in your basement. Mice come in from late September through November as night temperatures drop, which means sealing now costs a fraction of trapping in December. Every Sani IQ mouse job starts with a $95 inspection, credited when you book the treatment — and the honest fix is exclusion plus year-round protection, not a handful of traps.

If you’ve found droppings in a kitchen drawer or heard something moving behind the baseboard at 2 a.m., the first question is always the same: how did they get in?

Most of the advice answering that question was written for homes in Ohio, Georgia or the UK. That matters more than it sounds. Ontario housing stock has its own weak points — brick veneer with unscreened weep vents, garage door seals that stiffen in the cold, vinyl siding with an open J-channel at grade, and basements with a sill plate that was never properly sealed. Those are the openings our technicians actually find, on homes from Etobicoke to Whitby to Richmond Hill.

First, the rule that reframes everything: 6 mm

An adult house mouse needs a gap roughly 6 mm wide — about the width of a pencil, or a dime-sized hole. Juveniles get through less. The skull is the limiting factor, and if the skull fits, the body follows.

This is why homeowners miss entry points. You’re scanning for a hole you could put a finger through; you need to be scanning for a gap you could slide a pencil into.

Two more things before you start looking. Mice climb brick, downspouts, stucco, rough siding and corner trim without difficulty, so “it’s too high up” isn’t a defence. And mice follow warm air — if you can feel a draft there in January, a mouse could find it in October.

Not certain what you’re dealing with? Start with identification in our house mouse profile, then come back to this list.

The 12 entry points, in inspection order

1. Brick weep vents. The small vertical gaps in the mortar at the bottom course of brick veneer. They’re there on purpose — they drain moisture out of the wall cavity — so you never seal them shut. You screen them. Unscreened weep vents are the single most common entry point we find on GTA subdivision homes built from the 1980s onward, and almost nobody looks at them. Stainless steel weep vent covers solve it permanently without blocking drainage.

2. The two bottom corners of the garage door. In most homes the rubber seal has compressed or curled at the corners, leaving a small triangular gap that opens wider as the cold stiffens it. This is the entry point that puts mice in your garage, and from the garage, into the house.

3. The garage side door threshold. Same story, different door — and it gets less attention because it’s used less. Check the sweep and the gap at the bottom of the frame.

4. Air conditioner line set and gas line penetrations. Both come through a drilled hole that the installer foamed or left open. Foam alone is not a mouse barrier. This accounts for a surprising share of main-floor and second-floor activity, because the wall cavity behind it is a highway.

5. The dryer vent. The exterior flap is supposed to close by gravity; lint, age and bent louvres stop it. A vent standing open a few millimetres is an open door with a warm breeze coming out of it.

6. Kitchen and bathroom exhaust hoods. Same problem, higher up, where nobody looks. Both are solved with a vented cover backed by 6 mm hardware cloth — never by taping them shut.

7. The sill plate seam. In your basement, follow the line where concrete foundation meets wood framing with a flashlight. Gaps, missing insulation, daylight — all of it is a route into your wall cavities. On older Oshawa, Whitby and east-end Toronto homes this seam is often the main event.

8. Basement window frames and wells. Rotted wood, failed caulk, and wells holding leaf litter against the frame. If a business card slides between frame and masonry, that’s a mouse-sized entry.

9. Utility service entries. Electrical mast, cable, internet and irrigation lines each pass through a drilled hole, each sealed by a different trade on a different day.

10. Sump pump discharge and plumbing penetrations. Where the discharge line exits the foundation and where plumbing passes through below grade. Also check for a dried-out floor drain.

11. The roofline — soffit, fascia, and roof-to-roof transitions. Where a lower roof meets an upper wall there’s usually a gap in the flashing or soffit return, and mice climb the downspout to get there. If you hear activity in a second-floor ceiling rather than a basement wall, look up here first.

12. The garage-to-house door and the deck ledger. The interior door between an attached garage and the house usually has a worn sweep, and the framing cavity beside it connects to the wall system. Where a deck or porch ledger was bolted on, flashing is often incomplete — and the deck gives a mouse cover to work in.

Steel wool works. Foam alone does not.

The most common DIY failure we see isn’t laziness — it’s expanding foam used on its own. Mice chew through cured foam easily, and because it hides the hole, the homeowner believes the job is done while the mice keep using the route.

The method that holds:

• Gaps up to about 25 mm: pack them tight with stainless steel wool or copper mesh, then seal over top with exterior-grade sealant or mortar. The metal is the barrier; the sealant just keeps it in place and keeps water out.
• Larger openings: 6 mm galvanized hardware cloth, cut oversize and fastened down, then sealed or mortared at the edges.
• Vents that must keep working: purpose-made screened covers. Never block a weep vent, dryer vent or exhaust hood.
• Doors: replace the sweep or seal rather than patching it. A new seal from any hardware store fixes a recurring problem permanently.

And one myth to retire: peppermint oil, dryer sheets and ultrasonic plug-ins do not exclude mice. There is no scent or sound that convinces a mouse to ignore a warm, food-scented building in November. Physical exclusion is the only thing that works, which is genuinely good news — it means the fix is permanent once it’s done properly.

Why sealing in October beats trapping in December

This is the part that decides how much the whole problem costs you.

Ontario mouse activity inside homes climbs sharply from late September through November, as night temperatures fall and outdoor food thins out. Seal the envelope during that window and you’re keeping a small number of mice out of an empty house.

Wait until December and the arithmetic turns against you. House mice breed year-round indoors, with litters arriving every few weeks — so what was an exclusion job in October becomes a population-control job in January, plus droppings and urine in insulation and drawers, chewed wiring, and possibly a dead mouse somewhere you can’t reach. Health Canada’s guidance on rats and mice is blunt about the contamination side, and Public Health Ontario covers why you never sweep or vacuum droppings dry.

Weighing DIY against a call? Our guide on whether you need an exterminator for mice walks through where the line falls. Short version: sealing accessible exterior gaps is reasonable DIY. Finding the ones you can’t see — inside garage framing, above a drop ceiling, behind soffit — and confirming nothing is still inside, is not. That second part matters more than people expect: sealing a house that already has mice in it doesn’t solve anything, it just changes where they go.

The honest recommendation on cost

A One-Time Mice Treatment is $345, and Complete Mice Protection is $495 — the interior-and-exterior job with exclusion work included, carrying our 60-day guarantee. Every service starts with a $95 inspection, credited in full when you book the treatment.

But a one-time treatment is the entry point, not the answer. It clears what’s in the house now. It doesn’t stop next October, because the pressure on your building envelope is seasonal and permanent — new gaps open as caulk fails, seals compress and mortar cracks.

Complete Mice Protection at $895 per year is what we actually recommend for any Ontario home that has had mice once, and it’s worth comparing honestly against the alternative. One winter with mice in the walls can mean contaminated insulation, chewed wiring and repeated call-outs in a single season — damage that routinely runs past a thousand dollars before anyone counts their time. A year of scheduled monitoring, seasonal exclusion checks and covered call-backs is $895, and it’s built so the problem doesn’t come back. If ants, wasps and spiders are in the mix too, Essentials at $1,195 covers the lot on one plan.

No initiation fee. No setup fee. No monthly billing. See the full breakdown on plans and pricing, or tell us what you’re seeing through the free quote quiz and we’ll tell you which tier actually fits.

Local mouse work: Mice Control Whitby · Mice Control Etobicoke · full-service pest control in Whitby and Richmond Hill · our residential pest control programs. Or call (705) 302-1887.

Frequently asked questions

How small a hole can a mouse really get through? About 6 mm — the width of a pencil, or a dime-sized hole. If you can push a pencil into a gap, treat it as an entry point.

Does expanding foam stop mice? Not on its own. Mice chew through cured foam. Pack the gap with stainless steel wool or copper mesh first, then seal over it.

Should I seal the entry points before or after getting rid of the mice inside? Inspect first. If there’s an active interior population, sealing alone can push mice deeper into the structure. The right order is: confirm what’s inside, treat it, then exclude — which is how Complete Mice Protection is sequenced.

Is it too late to seal if it’s already November? No — but at that point assume you may also have mice inside and plan for both. The cost difference between October and January is mostly the difference between exclusion and population control.

Do you inspect the whole house or just where I’ve seen droppings? The whole envelope. Droppings tell us there’s activity; they don’t tell us where the route is. The $95 inspection covers the full perimeter and interior, and it’s credited when you book the treatment.