Mice in walls: what the scratching means and what to do

If you hear scratching in a wall or ceiling at night this fall, it is usually mice that came in through a gap freeze-thaw opened. Confirm it first. Trap along the run. Do not seal the inside hole yet. The real fix is the outside joint, and October is not too late.

Hearing it every night? Have us find and close the gap: mice in walls in Calgary.

Request a written quote Call 825 747 5787

Is it actually mice?

Sound alone is not proof. People hear a big noise and guess a raccoon. People hear a small one and assume mice when the pile under the soffit is something else. Before you buy anything, look for three field marks near the noise:

Droppings. Fresh ones are dark and soft. Old ones are dry and grey. Either one marks a route.
Gnawed edges. Chewed baseboard, a roughened door corner, a cable jacket with bite marks.
A rub stain. A thin dark film of dirt and oil from fur, on the exact slot they use.

Mallis's Handbook of Pest Control (8th ed., Moreland ed.) lists ten rodent signs a technician should look, smell, or listen for before any control work: droppings, gnawing, burrows, runways, tracks, grease marks, urine stains, sightings, sounds, and odors. The three marks on this page are the ones a homeowner can confirm without a flashlight in a void.

If the droppings are in the attic, name them before you crawl or seal anything. A mouse pellet holds its shape when pressed. Bat guano crumbles to powder. Bats are a different job, with different rules in Alberta. Our attic droppings note walks through the press test.

What to do tonight

Set snap traps along the run: against the wall, where you found droppings or a rub stain. A run is the path they repeat.
Put food in glass or metal. Cereal boxes and bags in the pantry are an open invitation.
If you use bait, use a labelled product, follow the label, and put it only where children and pets cannot reach.
Skip the ultrasonic plug and the kitchen fogger. Neither one closes a hole.

One mouse, one night, in an otherwise tight house may only need traps and a daylight check on your doors. If a store product is enough, that is the honest answer.

Why you should not seal the inside hole first

It feels right to stuff the hole behind the stove. Don't, yet.

Sealing first and hoping the mice leave is how people close the house and still hear the ceiling for two weeks. The mice already inside stay inside. They chew a new way through, or they die in the wall.

The same chapter treats exclusion as one of five pillars of a rodent program (inspection, exclusion, sanitation, non-chemical control, rodenticides). Sealing the interior face of a wall before the exterior joint is closed leaves the population in the void the book describes as structural harborage.

The order that works: knock down the population on the living-room side, and close the outside joint that let them in. Sequence, not religion. The longer version is in Mice are a building problem, not a trap problem.

Where they got in: look up, not just down

A house mouse fits a hole about 6 mm across, roughly the size of a dime. Penn State Extension notes that size, and Pest Control Technology documented mice passing it in a live trial. If a pencil fits, a mouse is considering it.

Mice move with the first cold nights, often late August into September. The usual Calgary entries:

The garage weatherseal and the door from the garage into the house
Pipe and cable sleeves at grade
The rim joist and the deck ledger
Unscreened dryer, bath and HRV vents
The brick–soffit seam under the roofline

That last one is the one most people miss. If you have been trapping at floor level and the ceiling still runs at night, the hole may be twenty feet up. Read the brick–soffit seam before you buy a ladder.

The full walk-around, garage to roofline, is mice entry points into a house. Prevention habits for the garage and kitchen are in how to keep mice out of your house and garage.

Cleaning up droppings safely

Do not sweep or vacuum droppings. That puts dust in the air.

Alberta's hantavirus guidance sets out the steps: open doors and windows and stay out for at least 30 minutes, wear rubber gloves, soak droppings, nests and dead mice with one part bleach to nine parts water (or a household disinfectant), let it sit five minutes, then wipe up, bag, seal and bin it. Wash your gloves, then your hands. For a heavy infestation or a closed space with poor air, the province says to call Health Link 811.

Read the full steps on alberta.ca: Reducing the risk of hantavirus. If someone in the house is unwell, that is a question for a clinician, not a pest page.

When it is a visit, not a trap

Book an inspection when:

You find fresh droppings every morning, even after trapping
You hear traffic in the ceiling, not only the kitchen
This is the second winter in a row
The gap is up high, in a rain-screen, or in the rim joist, and you cannot reach it or name the assembly

Not sure where you sit? The free envelope score sorts it into four outcomes:

Watch: one night, tight house, no ceiling traffic. Sanitation and a daylight test.
Survey: repeat droppings or a nest you can name. Book an inspection.
Close: two winters running, or a joint you can see at grade. Inspect, treat what is inside, close the gap (about 6 mm).
Urgent: occupancy, food, clinical, or a nest over a door or play area. Call 825 747 5787.

The paid visit is mice in walls in Calgary, part of mouse-proofing: we inspect, treat what is there, and close the joints. A typical visit is $100–$1,500 depending on the severity of the infestation and the scope of exclusion, GST extra. Inspection is complimentary when you proceed with the recommended service. See what it costs. The Gap Guarantee covers re-entry through a joint we closed and documented, during the first freeze-thaw season after the visit. It is not a whole-house seal.

Questions

It's already frosty. Is it too late to mouse-proof?

No. October is not too late. January is still worth doing. Mice start testing the house in late August and September, so in October the job is two things at once: treat what is already inside and close the gaps they used.

I trapped a few, but I keep finding droppings. Why do they keep coming back?

Traps only catch the mice already inside. They do nothing about the hole that delivered the next one. If a joint of about 6 mm is still open at the garage weatherseal, a pipe sleeve, the rim joist or the roofline, the house is still recruiting. Treat what is inside, then close the joint.

Is it safe to clean up mouse droppings myself?

Alberta's hantavirus guidance is written for people cleaning up themselves. Follow it exactly: ventilate first, wear rubber gloves, soak with one part bleach to nine parts water or a household disinfectant, wait five minutes, then wipe up and bag it. Never sweep or vacuum. For a heavy infestation or a closed space with poor air, call Health Link 811. Full steps: alberta.ca.

Exclusion or spraying: which actually works for mice?

Spray does not keep a mouse out of an open hole. Traps and labelled bait clear the population inside. Exclusion closes the way in. Most homes need both, in that order. One mouse in a tight house may only need snap traps. See how we compare and why we lead with exclusion.

Sources: Mallis's Handbook of Pest Control, 8th ed. (Moreland ed.); Penn State Extension and Pest Control Technology (gap size, linked above); Alberta hantavirus guidance (linked above).

Not sure what you are hearing? Text a photo of what you found to 825 747 5787, or Book an inspection. We'll tell you if this is a nest, a joint, or a watch-and-wait.

Service area: Calgary, Airdrie, Cochrane, Chestermere, Okotoks, High River.