How Mice Get In: The 5 Entry Points We Seal Every Day (and 7 More Homeowners Miss)

Quick answer: A house mouse can squeeze through a gap about the width of a pencil — roughly 6 mm. On most Ontario homes the same five openings do the damage: the hole where the AC or gas line enters the wall, the bottom corners of the garage door, the open J-channel under the siding, a dryer vent flap that no longer closes, and unscreened brick weep holes. Foam alone does not stop them — the fix that holds is steel mesh packed tight, then sealed over. One caution before you grab the caulk gun: if mice are already inside, sealing first traps them in your walls. Treat first, confirm they’re gone, then seal. Sani IQ bills every visit after it’s done — no monthly fees. Mice inside? The Sani IQ Rodent Program is $445 at the first visit, then $150 after each quarterly visit, with a 12-month warranty. Want the whole house covered? Sani IQ Protect is $249 a visit, four visits a year, rodent prevention included. Compare plans →

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 5 entry points we seal almost every day

Ask any of our technicians which gaps they seal most and you’ll get the same five, in roughly this order. If you only have twenty minutes and a flashlight this weekend, check these first.

1. The AC line set and gas line penetration. Where the refrigerant lines or gas pipe enter the wall, the installer drilled an oversized hole and either foamed it or left it open. This is the single most common route we find on homes built after 1990, and it feeds straight into the wall cavity — which is why you hear mice on the main floor or upstairs, not just in the basement. Look behind the condenser unit and around the gas meter.

2. The two bottom corners of the garage door. The rubber seal compresses and curls at the corners, leaving a small triangular gap that opens wider as the cold stiffens it. Lie on the garage floor and look toward daylight. If you can see a triangle, a mouse sees a door — and from the garage, the house is one worn sweep away.

3. The J-channel at the bottom of vinyl siding. The bottom edge of the siding sits in an open channel just above the foundation. It looks finished; it isn’t sealed. Mice run the foundation line, find the channel, and go straight up behind the siding. This one is missed by almost every homeowner and most contractors.

4. The dryer vent. The exterior flap is supposed to drop closed by gravity. Lint, bent louvres and age leave it standing open a few millimetres — with warm, food-scented air blowing out of it. Replace the hood with a louvred cover backed by 6 mm hardware cloth. Never tape or foam a dryer vent shut.

5. Brick weep holes. 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 caulk them shut. You screen them. Unscreened weep holes are the most common entry point we find on GTA subdivision homes built from the 1980s onward, and stainless steel weep-hole covers solve it permanently without blocking drainage.

The order matters more than the material. Sealing is the last step of a mouse job, not the first. If there’s an active population inside and you close the exits, you don’t have fewer mice — you have the same mice with nowhere to go but deeper into the structure, and eventually a dead one in a wall. Our Rodent Program is sequenced for exactly this reason: treat and place stations on day one, leave exit routes open by design so mice leave and die outside, verify at about three weeks, and only then seal every entry point. That’s what the clip above shows — the final step, on a house that has already been cleared.

Those five cover most jobs. The next seven are the ones that catch the homes that “did everything right.”

7 more entry points homeowners miss

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

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

3. 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.

4. 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.

5. 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.

6. 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.

7. The roofline and the deck ledger. 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. Where a deck or porch ledger was bolted on, flashing is often incomplete, and the deck gives a mouse cover to work in.

Steel mesh 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 or copper mesh — tight enough that you can’t pull it out with your fingers — 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 hole, 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 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.

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.

The Sani IQ Rodent Program is built so that doesn’t happen: $445 at the first visit — treatment, stations and the inspection that finds every route — then $150 after each quarterly visit, with every entry point sealed once the house is verified clear and a 12-month warranty behind it. Billed after each visit, no monthly fees, cancel any time. If ants, wasps and spiders are in the mix too, Sani IQ Protect is $249 a visit, four visits a year, rodent prevention included. 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 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 or copper mesh first, then seal over it.

Can I just stuff steel wool in the hole and call it done? Steel or copper mesh is the right material, but pack it tight and seal over it or it works loose. And only do it once you’re sure nothing is still inside.

Should I seal the entry points before or after getting rid of the mice inside? After. Treat and station first, confirm at about three weeks that activity has stopped, then seal. Sealing with mice still inside traps them in the structure — which is why the Rodent Program leaves exit routes open until verification, then closes everything.

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.