How mice get into your house, and how to seal every gap
How we checked this. Signs, trapping and sealing advice from Health Canada; cleanup steps from the Public Health Agency of Canada; licence rules from the Government of Ontario; frost nights from Environment and Climate Change Canada. Prices are ProScore estimates, figures as of March 20, 2026.
The short answer
Mice get into houses through five common gaps: foundation cracks, holes around pipes and cables, worn garage door seals, dryer vents, and damaged soffits. Health Canada says they can squeeze through cracks as small as a dime. Seal every gap with steel wool or copper mesh and caulk, or a proper cover. If signs keep coming back, call a professional.
“Mice can squeeze through cracks as small as a dime.”
— Health Canada. The quick test: if a pencil fits into a gap, a mouse can use it.How do I know if I have mice?
Most people notice what mice leave behind before they ever see one. Health Canada lists these signs:
- Sounds at nightScratching and scampering in the walls after dark.
- GnawingChewed food packages and gnaw marks on wood or boxes.
- Droppings and urineSmall dark droppings, and a stale smell where they gather.
- Holes near the foundationBurrows or holes in and around the foundation walls.
- TracksSmall footprints on dusty surfaces, like a basement floor.
A gap the size of a dime is all it takes.Health Canada: mice can squeeze through cracks as small as a dime.
What should I do tonight?
Two things: set traps where you saw the signs, and take away the food. Sealing comes next, but traps work tonight.
Set traps along the walls
- Put traps along the walls, where mice travel: about one per metre, or 5 to 10 near a hole you can see.
- Set open snap traps at a right angle to the wall, with the bait end against it. Covered and electronic traps go flat against the wall.
- Bait with something strong-smelling and sticky, like peanut butter. Check traps every day, move any that catch nothing, and reset them after two or three weeks.
Source: Health Canada, rats and mice.
Take away the food
- Move dry food and pet food into metal or glass containers.
- Keep garbage in bins with tight-fitting lids, and keep meat, dairy and oily scraps out of the composter.
- Fix dripping taps and sweating pipes: mice need water too.
Children or pets in the house?
- Poison bait must go in a tamper-proof bait station anywhere children or pets can reach.
- Poison should never be the only plan: it does nothing about the gap they came in through.
Where do mice get into a house?
Almost always where two materials meet. These are the five gaps to check outside. For each one: what to look for, how mice get through, and the fix.
Foundation cracks
Look for: Hairline or wider cracks in the concrete above the soil line, often hidden behind shrubs and mulch.
The way in: A crack above the soil line runs straight into the wall, and plants and mulch hide it.
Fix: Patch cracks with a masonry sealant or patching mortar. Keep soil, mulch and plants back from the wall so you can see the foundation.
Gaps around pipes and cables
Look for: Where gas, water, cable and air-conditioner lines come through the wall. The hole is almost always wider than the pipe.
The way in: The ring of open space left around the pipe, between the pipe and the edge of the hole.
Fix: Stuff steel wool or copper mesh firmly around the pipe, then seal over it with exterior caulk.
The garage door seal
Look for: Daylight at the bottom corners of the closed garage door, or a rubber seal that is torn or flattened.
The way in: The daylight under the corners. If a pencil fits, a mouse can use it.
Fix: Replace the bottom seal, and add brush or rubber strips along the sides wherever daylight shows.
The dryer vent
Look for: A vent flap that sticks open, or a hood with the flap missing.
The way in: A flap that hangs open leaves the duct open to the outside.
Fix: Fit a pest-guard vent cover or fine metal mesh screening, as Health Canada advises, and keep it clear of lint so the dryer still vents.
Soffit and roof gaps
Look for: Loose or cracked soffit panels, or gaps where the soffit meets the wall. Mice climb siding easily.
The way in: The space where the soffit panel stops short of the wall, reached by climbing the siding.
Fix: Repair or replace damaged panels and cover soffit and attic vents with fine metal mesh screening. Work at height is the part most homeowners hand to a professional.
Then check inside
- Doors: fit metal weather stripping under exterior doors, including the door from the garage into the house.
- Windows: weather-strip any that do not close tight.
- Pipes: where pipes come through walls and floors (under sinks, behind the stove and washer), stuff steel wool around them before caulking.
- Clutter: clear stored boxes and clutter in the house and garage, where mice nest.
Source: Health Canada.
When should I mouse-proof my house?
Before the cold. In Toronto, nights at or below freezing go from almost none in October to about 8 in November and 20 in December, according to Environment and Climate Change Canada's climate normals. Do the outside work in September and October, before the first hard frosts and while it is still comfortable to work on a ladder. Our fall home prep checklist covers the other outside jobs to finish before winter.
How do I seal the gaps myself?
Most gaps at ground level take an afternoon. Tick each one off as you go, or print the list and take it outside.
- Flashlight
- Caulking gun
- Copper mesh or steel wool
- Exterior caulk or masonry sealant
- Garage door bottom seal
- Pest-guard dryer vent cover
How do I clean up mouse droppings safely?
Rodent droppings and urine can spread disease, and the advice is built around one thing: not breathing in dust from them. The Public Health Agency of Canada recommends:
- Gloves onWear rubber or plastic gloves. In a tight space like a crawlspace or attic, add an N95 respirator.
- Spray it firstSpray the droppings, nesting material and the area around them with 1 part bleach to 9 parts water, or a disinfectant approved by Health Canada.
- Wait 10 minutesLet it soak for 10 minutes before you touch it.
- Wipe it up wetPick everything up with paper towels or a wet mop.
- Bag it twiceDouble-bag the waste and the towels, seal the bag, and put it in the garbage.
- Clean yourself upWash your gloves in disinfectant and hot soapy water before taking them off, then wash your hands with soap and water.
When should I call a professional?
Call one when signs keep coming back after you have sealed what you can reach, when you find signs in more than one room or the attic, or when the gaps are at roof height. Before you hire, the five questions to ask any contractor apply to pest control too, and our guide to hiring a pest control company covers how to compare them.
- 1Do it yourselfA few droppings in one placeSet traps along the wall where you found them and seal the gaps you can reach. Check again in a week.
- 2Watch closelyNew signs after a weekFresh droppings, gnawing, or scratching in the walls at night means there is a way in you have not found.
- 3Call a professionalSeveral rooms, or the atticThey are nesting. You need exclusion, typically $300–$1,000: every entry point found and sealed from outside.
What a good visit looks like
- An inspection inside and out, including the roofline and the garage.
- A written list of entry points, with what each one will be sealed with.
- Traps, or bait in tamper-proof stations if there are children or pets, and an explanation of where they are.
- A follow-up visit and a warranty, both in writing.
How much does pest control cost in Ontario?
Mouse exclusion is priced by the job, because the work is finding and sealing every gap: a house with two gaps is a different job from one with twelve. We explain what moves a pest control quote up or down, and our Ontario pest control cost guide has the full breakdown.
| Service | Typical price | Charged |
|---|---|---|
| General pest treatment visit | $200–$400 | per visit |
| Mouse or rat exclusion | $300–$1,000 | per job |
| Annual prevention plan | $400–$800 | per year |
What should a pest control company show me?
Ask for these before any work starts.
- Their Ontario licences, if they use pesticides. Most mouse baits are pesticides. In Ontario, the company needs an operator licence and the person applying them an exterminator licence (Government of Ontario). You can look up a company's operator licence on the province's Access Environment registry.
- A written list of entry points. An exclusion quote names each gap it will seal and the material it will use.
- A warranty in writing, with the follow-up visit included in the price.
- A clean record. Search the company on Ontario's Consumer Beware List before you sign.
Questions homeowners ask
How small a gap can a mouse get through?
Very small. Health Canada says mice can squeeze through cracks as small as a dime. A quick test: if a pencil fits into a gap, treat it as a way in.
What are the signs of mice in a house?
Health Canada lists scratching and scampering in the walls at night, gnawing and chewed food packages, droppings and urine, holes or burrows around the foundation, and tracks on dusty surfaces.
When should I mouse-proof my house in Ontario?
Before the cold. In Toronto, nights at or below freezing go from almost none in October to about 8 in November, according to Environment and Climate Change Canada. Sealing in September and October gets the outside work done before the hard frosts.
How do I clean up mouse droppings safely?
The Public Health Agency of Canada advises gloves, spraying the droppings with 1 part bleach to 9 parts water, waiting 10 minutes, then wiping them up wet with paper towels and double-bagging the waste. Never sweep or vacuum dry droppings.
Are mouse poisons safe with children and pets?
Only in a tamper-proof bait station, wherever children or pets can reach, says Health Canada. Poison should not be the only method either: it does nothing about the gap the mice came through.
Does a pest control company need a licence in Ontario?
If it applies pesticides, yes. The business needs an operator licence and the person applying them an exterminator licence. Most mouse baits are pesticides, so ask to see both before work starts. You can look up a company's operator licence on Ontario's Access Environment registry.
How much does mouse exclusion cost in Ontario?
Mouse or rat exclusion typically costs $300–$1,000, depending on how many entry points the house has. A general pest treatment visit runs $200–$400. These are ProScore estimates for Ontario, figures as of March 20, 2026.
Will sealing gaps get rid of mice already inside?
Not on its own. Sealing stops new mice getting in; traps deal with the ones already inside. A good plan does both, sealing first.
Key takeaways
- Droppings, gnawing and scratching in the walls at night are the usual first signs.
- Tonight: traps along the walls and food in sealed containers.
- Mice use five common gaps outside: foundation cracks, pipe holes, garage door seals, dryer vents and soffits. If a pencil fits, seal it.
- Seal in September and October, before the frost nights arrive.
- Clean droppings wet, never dry, and call a pro if signs come back after sealing.
See licensing, insurance and reviews side by side for pest control companies in Toronto, Ottawa and every Ontario city we cover.
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