How to keep mice out of your house and garage

Keeping mice out comes down to two things: give them nothing to stay for and no way in. In Calgary the way in matters more, because freeze-thaw keeps reopening joints that were tight in summer. Start in the garage.

Already hearing them in a wall or ceiling? Skip to what to do tonight, or have us close it.

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Start with the garage

For most houses the garage is the first line and the door into the house is the second. Mice that get into the garage have already cleared the outer envelope.

Do the daylight test. Close the overhead door after dark, lights off inside, and look along the bottom and both corners. Any light is a way in.
Check the door from the garage into the house the same way. It needs a sweep that touches the threshold.
Keep bird seed, grass seed and pet food in lidded metal bins, not the bag they came in.
Get cardboard boxes off the floor. Cardboard is nesting material. Use lidded bins on shelves.
Keep garbage and recycling in bins with lids, and rinse what goes in them.
Do not leave the overhead door open into the evening in fall.

A car or RV parked for the winter is its own problem. We will cover that separately.

Mallis notes that a professional rodent inspection is done before control, because it is the only way to map entry points and harborages rather than treat the room where the droppings were noticed.

Inside the house

Put dry food in glass or metal. Mice chew through bags, boxes and thin plastic.
Do not leave pet food out overnight.
Clean under the stove and fridge. Crumbs there feed a mouse you never see.
Clear clutter in storage rooms and the basement so there is less cover.
Set a few snap traps along walls in the utility room through fall. A catch means a gap is open.

Outside

Stack firewood off the ground and away from the wall.
Trim shrubs back from the siding so there is no covered runway to the house.
Move bird feeders away from the house. Spilled seed feeds mice.
Clear leaves and debris piled against the foundation.

Close the way in

Habits lower the pressure. They do not close a hole. A house mouse fits through a gap of about 6 mm, and Alberta’s mouse control guidance says to seal gaps that size with gnaw-resistant material such as metal or concrete.

Olkowski, Daar and Olkowski, Common-Sense Pest Control (1991), put physical exclusion ahead of any product for exactly this reason: a deterrent or a trap does not change the opening the animal is already using.

The gap-by-gap walk-around, garage door to roofline, with what to seal each one with, is mice entry points into a house. The one most people miss is the brick–soffit seam.

What does not keep mice out

Ultrasonic plugs and smell deterrents. Neither closes a hole.
Expanding foam or caulk on its own. Mice chew through it.
Bait stations with the gap left open. You end up feeding the next mouse through the same joint.

When to do it

Mice start testing the house in late August and September, before the first stretch of cold nights. October is not too late. January is still worth doing. On a cold day, check the minimum temperature on the sealant label before you use it outside.

When it is a visit

Fresh droppings every morning, even after trapping
Traffic in the ceiling, not only the kitchen
The second winter in a row
A gap up high, behind a rain-screen or at the rim joist that you cannot reach or name

That 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.

Questions

How do I keep mice out of my garage in winter?

Close the gap under the overhead door and at its corners, keep seed, pet food and garbage in lidded metal bins, get cardboard off the floor, and keep the door into the house sealed. Leave a couple of snap traps along the garage wall as an early warning.

What smell keeps mice away?

None that we would rely on. Peppermint oil, dryer sheets and ultrasonic plugs do not close a hole. If the gap stays open, the house keeps recruiting.

Should I leave traps out all fall?

Yes, a few snap traps along the walls of the garage and utility room are a cheap early warning. A catch tells you a gap is open somewhere. Keep them where children and pets cannot reach.

What should I use to seal small gaps?

Copper mesh packed tight with a sealant that moves, and metal screens on vents. Not foam or caulk on their own; mice chew through both. Gap by gap: mice entry points into a house.

Sources: Olkowski, Daar and Olkowski, Common-Sense Pest Control (1991); Mallis's Handbook of Pest Control, 8th ed. (Moreland ed.); Alberta mouse control guidance (linked above).

Not sure where you sit? Text a photo of what you found to 825 747 5787, or request a written quote.

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