Mice in Ottawa homes each fall: entry points, sealing and why bait alone fails
Updated
Mice get in through gaps the size of a dime. Where to look in an Ottawa house, how to seal the gaps so they stay sealed, and why Health Canada says bait should never be the only tool.
Why fall brings them in
When the nights turn cold in September and October, mice look for warmth and food. A house has both. Many Ottawa homeowners first notice them in the fall: droppings in a kitchen drawer, chewed packaging in the pantry, or scratching in a wall at night.
Older homes in neighbourhoods like Centretown and Vanier tend to have more ways in. Stone and block foundations, old mortar joints, and decades of added pipes, wires and vents all leave small openings. Newer houses are not immune. Gaps around utility lines, garage doors and dryer vents are common in any age of house.
How small a gap is too small
Health Canada says mice can squeeze through cracks as small as a dime. Rats need a hole about the size of a quarter. Health Canada also notes that even a worn door threshold is enough for them to get in.
The places to check, from the outside in: where the gas line, water line, air conditioner lines and cables enter the house. The dryer vent and other wall vents. Garage door bottom seals and the corners of the garage door. Door thresholds and sweeps. Cracks in the foundation and gaps where the wood framing sits on top of it. Holes around pipes under sinks and behind appliances inside the house.
Sealing that holds
Mice chew. A gap filled only with something soft may not stay closed. Health Canada recommends stuffing steel wool around pipes before caulking or plastering over it, using metal weather stripping around doors and windows, and covering vents with fine metal mesh screening.
Replace worn door sweeps and garage door bottom seals, and fill the gap at the corners of the garage door, where mice often get in. Keep firewood, bins and bird feeders away from the walls of the house.
Seal from the outside where possible, and seal inside as a second layer. Do a full walk-around, since mice will use the one gap that was missed. Seal in early fall, before they move in, and check again in spring.
Why bait alone fails
Health Canada says baits or poisons should not be used as the only method. There are practical reasons for that.
Bait does nothing about the gaps, so new mice keep coming in to replace the ones that die. A poisoned mouse can die inside a wall or ceiling, where the smell lingers and it cannot be removed easily. And any poison in a home is a risk to children and pets if it is placed badly.
Health Canada describes snap traps and electronic traps as easy to use and very effective when they are placed and set properly. Placed along walls where droppings are found, they also tell you when the problem is over.
Signs and what they tell you
Droppings are the clearest sign. Mouse droppings are small, dark and pointed, and fresh ones are soft and shiny. Where you find them tells you where mice travel. Look along walls, under the sink, behind the stove and fridge, and in cupboards.
Other signs include gnawed food packaging, shredded paper or insulation used for nests, and faint greasy marks along walls where they run. Scratching in walls or ceilings, mostly at night, is common in the fall. If the droppings keep appearing after trapping, the entry points are still open.
Integrated pest management in practice
Integrated pest management means using several methods together and checking the results. For mice in a house, it looks like this:
Inspect to find where they are getting in and where they are active. Seal the entry points. Remove food and water sources by keeping food in hard containers, cleaning up crumbs and pet food, and fixing leaks. Trap the mice already inside. Then monitor and return to check.
Clean up safely. Deer mice are the main carrier of hantavirus in Canada, according to Health Canada. Wear protective equipment, and never sweep or vacuum dry droppings, because the dust raised can cause illness.
If a company uses rodenticides, they must hold the proper Ontario pesticide licences. Indicative 2026 range from the price guide on this site, before HST: mouse control with basic exclusion runs $200 to $500. Larger sealing jobs are quoted after inspection.