How Are Mice Getting Into Your House? The Ontario Entry-Point Audit

Quick answer

A house mouse can pass through any gap about 6 mm wide — roughly the thickness of a dime. In Ontario homes the same handful of openings are responsible for the majority of entries: brick weep vents, the garage door sweep and side jambs, gas and air-conditioning line penetrations, the sill plate gap where the frame meets the foundation, and dryer and bathroom vent hoods. Find them with a dusk flashlight audit, seal them with material mice cannot chew through, and seal in the right order — sealing while mice are already inside traps them in your walls. Sani IQ inspections start at $95, credited when you book the treatment. One-Time Mice Treatment is $345; Complete Mice Protection is $495; the year-round Mice Protection plan is $895/yr.

Every year around the third week of August, the phone starts. The first genuinely cool night drops overnight lows into single digits, and mice that spent the summer in a hedge, a woodpile or a neighbour’s compost bin start testing the outside of your house for a way in.

They are not breaking in. They are walking in — through openings that have been there for years and that you have walked past a thousand times without seeing.

The dime rule, and why it is worse than it sounds

An adult house mouse weighs about 20 grams and has a skull that is flexible at the jaw. If the head fits, the mouse fits — so any gap 6 mm across, the width of a dime on edge, is a door.

Two things make that worse than it sounds. Mice enlarge what they find: a 4 mm gap in soft mortar, wood trim or foam insulation becomes a 10 mm gap in an evening of gnawing, because their incisors grow continuously and they chew regardless of hunger. And they climb — brick, stucco, downspouts and rough-sawn wood, plus a 20–25 cm vertical jump from standing. Roughly a third of the entry points we seal on Ontario homes are above the first floor, so a ground-level inspection is half an inspection.

The Ontario entry-point map

Ontario housing stock has a recognizable set of weaknesses. Whether it’s a 1960s bungalow in Whitby, a 1990s two-storey in Barrie, or a new subdivision build, the same short list accounts for most of what we find.

Brick weep vents

Weep vents are the deliberate open slots in the bottom course of brick veneer. They exist for a good reason — they let the cavity behind the brick drain and breathe — and they are, from a mouse’s point of view, a purpose-built entrance into your wall. This is the most common entry point we find on Ontario brick homes, and the one homeowners never think of, because it looks like it belongs there.

It does. It just needs a stainless steel weep vent cover, which passes water and air and stops rodents. Never caulk a weep vent shut — you’ll trade a mouse problem for a moisture problem.

The garage door: sweep and side jambs

The rubber sweep along the bottom of a garage door compresses and tears over the years, and the two bottom corners where it meets the vertical jamb are almost always open by a centimetre or more. Attached garages are the most common staging area for mice in Ontario homes — from there, the gap around the interior door or a wall penetration gets them into the house proper. Check the corners from inside with the door closed and the lights off. If you see daylight, so does a mouse.

Gas lines, AC line sets, cable and hose bibs

Every penetration through your exterior wall was drilled oversize and then, on a good day, filled with expanding foam. Mice chew expanding foam like cotton candy. The refrigerant line set for a central air conditioner is a reliable offender, because the foam sits hidden behind a plastic line cover nobody has removed since installation.

The sill plate and rim joist

Where the wooden frame sits on the concrete foundation there’s a joint, and on older homes it has often shifted, shrunk, or was never properly sealed. This gap is frequently continuous — metres of open seam behind the siding or above the parge — and in finished basements it delivers mice straight into the joist space.

Dryer, bathroom and range vents

Vent flappers and louvres stop working the moment lint packs them or the plastic hinge cracks. A dryer vent is a warm, sheltered, straight tube into your house.

The roof line

Soffit returns, gable vents with torn screening, and gaps where a roof meets a lower roof section are all standard entries — especially where a tree, fence or downspout gives access. Older homes with mature tree cover see this constantly.

How to run a dusk flashlight audit

You need one evening, a bright flashlight, and a helper.

1. Walk the perimeter in daylight first. Note every penetration, vent and utility entry on a rough sketch. You’re building a checklist, not sealing yet.
2. At dusk, put your helper inside with the flashlight and stand outside. Have them shine it along the rim joist, into the garage corners, along basement wall/ceiling junctions, and behind the furnace and laundry. Any pinhole of light you can see from outside is an opening.
3. Reverse it — you take the flashlight outside. This catches openings that angle away from you.
4. Look for the physical tells. Mice leave greasy grey rub marks along routes they use repeatedly, at the edges of a used gap and along baseboards. Droppings the size of a grain of rice concentrate near food and nesting areas. Fresh chew marks show light-coloured wood.
5. Check the garage last, at night, lights off, door closed. Highest-yield ten minutes of the audit.

If you’d rather it be done properly, that’s what an inspection is — the same walk with a moisture meter, a borescope for wall voids, and the pattern knowledge of thousands of Ontario homes. The $95 fee is credited in full when you go ahead with the treatment.

Why steel wool alone fails

Steel wool is the internet’s favourite answer and it’s half right. Loose steel wool stops a mouse in the short term, then rusts — usually within a season on an exterior opening — and gets dragged out, because nothing is holding it in place.

What actually holds: stainless steel wool or copper mesh packed tight and capped with sealant, mortar or metal flashing so it can’t be pulled out; hardware cloth (19-gauge, 6 mm mesh) cut oversize and fastened for larger openings; sheet metal or aluminum flashing over chewed wood edges and soffit returns; and purpose-made covers for weep vents, dryer vents and gable ends.

What doesn’t hold: expanding foam alone, caulk alone, duct tape, plastic mesh, or a rag.

Seal order matters — do not seal mice in

This is the mistake that turns a small problem into an expensive one.

If mice are already active inside the structure and you seal every exterior opening on a Saturday afternoon, you haven’t removed the mice — you’ve removed their exit. They die in your wall voids, which produces an odour lasting weeks and can require opening drywall, and the survivors, cut off from their outdoor food, push harder into your living space rather than less.

The correct order: assess whether there’s an active interior population and where; reduce the interior population first; then exclude the exterior openings once the interior is clear; monitor through winter and re-treat the perimeter in spring.

Anyone selling you exclusion work without the first two steps is selling you a smell.

What full exclusion actually costs — and what it prevents

Canonical Ontario pricing as of 2026:

Service	Price
Professional inspection	$95, credited when you book the treatment
One-Time Mice Treatment	$345
Complete Mice Protection	$495
Mice Protection plan (year-round)	$895/yr
Essentials plan	$1,195
VIP plan	from $1,395

One-time services carry a 60-day guarantee.

Now the part that decides which of those numbers is right for your house.

A mouse pair that establishes indoors in September is not a September problem. Mice breed year-round in a heated house — a female can produce five to ten litters a year — so a two-mouse September becomes a real infestation by February, in a season when you can’t work frozen ground and can’t see entry points under snow. The costs that follow are rarely the treatment: they’re the chewed wiring behind a dishwasher, the contaminated insulation, the ruined pantry, the drywall opened to retrieve a dead mouse.

Which is why the annual plan is the honest recommendation for most Ontario homes, not the one-time treatment. A one-time treatment is the right entry point — it deals with what’s in your house now, with a 60-day guarantee. But the mice aren’t coming from inside your house. They’re coming from outside it, every autumn, and a treatment in September does nothing about the ones testing your weep vents next September.

The Mice Protection plan at $895/yr covers the full cycle: seasonal exterior work, monitoring, return visits when something changes, and the exclusion maintenance that keeps sealed openings sealed. Against one bad winter, it stops being a close call.

Not sure which tier fits? The free quote quiz takes a minute, or compare the tiers on plans and pricing. Our Ontario pest control price guide covers costs across the province, and we weighed the two paths directly in one-time service vs. an annual plan.

We handle rodent exclusion across the GTA, York and Durham, Simcoe County and cottage country — including Whitby and Durham, and Barrie and Simcoe. Full scope on our residential pest control page, and species background in our mouse pest library entry.

Call (705) 302-1887 to book an inspection.

Frequently asked questions

How small a hole can a mouse really get through? About 6 mm — the width of a dime. If a pencil slides into the gap, a mouse can use it. Rats need roughly double.

I sealed everything and still have mice. What did I miss? Usually an opening above the first floor, the garage door corners, or a penetration hidden behind a line cover. The other possibility is that you sealed them in — if the activity is now louder and more central to the house, that’s the likely explanation, and it needs an interior assessment.

Do ultrasonic repellers or peppermint oil work? No. Mice habituate quickly to sound and to strong odours they associate with no consequence, and neither closes a 6 mm gap. Exclusion is what works.

When should I do the exclusion work? Late August through October — the ground is workable, pressure is starting but the population isn’t established, and you can still see what you’re doing. Spring is second-best. Mid-winter is the hardest and most expensive time to do it.

Does the $95 inspection fee get added to my bill? No. It’s credited in full when you book the treatment. It only stands as a charge if you decide not to proceed.

Sani IQ is BBB Accredited with a 5.0 rating across 126 Google reviews, carries $5M liability coverage, and is licensed under the Ontario Pesticides Act and registered with MECP. Our technicians are employed, licensed and background-checked, and we use reduced-risk products wherever they’ll do the job.