How to Keep Mice Out of Your House: A Canadian Prevention Guide

TL;DR

• Seal every gap bigger than a dime with metal: steel wool, metal mesh and metal door sweeps. It is the most reliable way to keep mice out.
• Mice gnaw through foam, rubber, vinyl, wood and plastic screening (UC IPM). Back any foam or caulk with steel wool or metal mesh so the mouse hits metal first.
• Do it in late summer or early fall. Mice move indoors mostly in autumn.
• Inside, keep food and dry pet food in metal or glass containers, fix leaky taps and sweating pipes, and clear clutter mice can nest in (Health Canada).
• Outside, cut tall grass and weeds back from the house, and raise woodpiles about 30 centimetres (1 foot) off the ground, away from the walls (Health Canada).
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

Working out how to keep mice out of your house often starts with a can of expanding foam, and that is where many attempts quietly fail. Mice gnaw through foam, rubber, vinyl, wood and plastic screening, so a hole that looks sealed can still be a doorway.

What actually keeps them out is metal plus habit: steel wool, metal mesh and door sweeps on the gaps, with food, water and clutter taken away indoors and along the foundation. Peppermint oil, ultrasonic plug-ins and the family cat do far less than their reputation suggests.

Timing matters as much as materials, since the cold nights of autumn are when mice start testing every crack in the walls for a way inside.

Why Do Mice Get Into Your House?

Mice get into your house looking for food, water, warmth and a safe place to nest, and in Canada most of them arrive in the fall. A house offers all four at once, which is why a few small gaps and an open bag of birdseed are often all it takes. For a closer look at the food smells and habits that draw them in, see our guide to what attracts mice to your house.

When Do Mice Move Indoors?

Mice move indoors mostly in autumn. According to UC IPM, house mice frequently enter homes when nighttime temperatures get colder, and Health Canada notes that deer mice, the brown or grey mice with white bellies and white feet, may invade buildings near fields and woodlands in the fall. Mice do not hibernate, and once inside they can breed all year. That makes late summer and early fall the best time to rodent-proof your home in most of Canada.

How Small a Gap Can a Mouse Fit Through?

A mouse can fit through a gap as small as a dime, according to Health Canada. UC IPM puts it at about 1/4 inch (about 6 mm). Even the small gap left by a worn threshold under a door is enough. Mice are also excellent climbers that can run up rough walls and jump up to 12 inches (about 30 cm), so gaps higher up, around vents, siding and the roofline, count too.

How Fast Do Mice Multiply?

Mice multiply fast, which is why prevention beats cure. UC IPM reports that a female house mouse may have 5 to 10 litters a year, with about 5 or 6 pups per litter. Health Canada advises dealing with occasional invaders right away for this reason. House mice also seldom travel more than about 30 feet (about 9 metres) from their nest, so a mouse you see in the kitchen is usually nesting close by.

How to Keep Mice Out of Your House by Sealing Entry Points

Sealing entry points is the most reliable way to keep mice out of your house. UC IPM notes that good sanitation alone seldom eliminates mice, and Health Canada calls getting rid of easy entry points the first line of defence. If you are wondering how to prevent mice for good, start here: walk around the outside of your home, then check the basement, garage and attic from the inside.

Where Do Mice Get In?

Mice get in through the gaps you rarely look at. The most common entry points are:

• Cracks in the foundation and holes in and around foundation walls
• Gaps around water pipes, gas lines, vents and utility cables where they enter the house
• Dryer vents, attic vents and soffits without metal screening
• Worn thresholds and missing sweeps under exterior doors, including the door from the garage
• Gaps around windows and damaged screens
• Openings near the roofline, eaves and chimney

Our step-by-step guide on how to find mice entry points shows how to inspect each spot, and how mice get in your house explains the routes they use.

Steel Wool

Steel wool is the classic fix for small holes around pipes and cables because mice cannot chew through it. Health Canada recommends stuffing steel wool around pipes before caulking or plastering over it. Pack it in tightly with a screwdriver, then seal the surface. On its own, steel wool is only a temporary plug: UC IPM notes it can rust over time, so check it every year and replace it with a permanent material where you can.

Metal Mesh for Vents

Metal mesh keeps mice out of openings that must stay open for airflow. Health Canada advises covering dryer vents, attic vents and soffits with fine mesh metal screening, and HealthLinkBC recommends 6 mm (1/4 inch) metal screening or steel mesh over crawl space and attic vents. Fasten the mesh with screws or staples so it cannot be pushed aside.

Door Sweeps

Door sweeps close the gap under exterior doors, one of the easiest ways in for a mouse. Health Canada recommends metal weather stripping under doors and weather stripping around windows. Check the door between the garage and the house, and the seal at the bottom of the garage door itself, since both wear out over time. If you can see daylight under a closed door, a mouse can get through.

Foundation Cracks

Foundation cracks give mice direct access to basements and wall voids. Health Canada advises patching cracks in foundations, and the Public Health Agency of Canada lists cement, metal flashing, wire screening and steel wool as materials for sealing holes. For larger cracks, use a cement or mortar patch rather than a soft filler.

Materials Mice Chew Through

Some popular fillers do not stop mice at all. UC IPM warns that plastic screening, rubber, vinyl, insulating (expanding) foam and wood are ineffective for plugging holes because mice gnaw through them. If you use spray foam or caulk to fill a gap, back it with steel wool or cover it with metal mesh or flashing so the mouse hits metal first.

Mouse Prevention Indoors: Food, Water and Clutter

Mouse prevention indoors means taking away the food, water and nesting spots that let a few mice settle in. These habits will not stop a determined mouse on their own, but poor sanitation lets mice thrive, and a clean home makes any mouse that does get in much easier to catch.

Food Storage Containers

Food storage containers made of metal or glass are the safest choice, according to Health Canada. Move flour, cereal, rice, crackers, snacks and baking supplies out of paper and cardboard packaging. Plastic bins and bags only slow mice down because they can chew through them. Wipe up crumbs and spills right away, and do not leave food on the counter overnight.

Pet Food

Pet food is one of the easiest meals a mouse can find. Health Canada recommends storing dry pet food in metal or glass containers, and UC IPM suggests feeding pets only the amount they will eat at a single feeding. Pick up bowls at night, and keep bags of kibble and birdseed in sealed metal bins in the garage or basement.

Garbage

Garbage needs a container with a tight-fitting lid, inside and outside. Health Canada and UC IPM both advise keeping garbage sealed. Empty kitchen bins often, rinse recycling before it goes in the blue bin, and clean up any spills around the containers.

Water Sources

Water sources matter as much as food. Health Canada lists leaky taps, sweating pipes and open drains as water sources to eliminate. Fix drips under sinks, insulate pipes that sweat in the basement, and keep drain covers in place.

Clutter

Clutter gives mice quiet places to nest. Health Canada advises removing cosy nesting sites in unused clutter around your house and garage. Store items in sealed bins rather than cardboard boxes, keep basement and attic corners tidy, and get rid of stacks of paper, old clothing and fabric that mice can shred into nests.

Mice Prevention Outside the House

Mice prevention outside the house removes the cover and food that let mice live right next to your walls. The fewer mice living in your yard, the fewer will test your foundation when the weather turns cold.

Vegetation Near the House

Vegetation that touches your house gives mice cover and a ladder. Health Canada recommends cutting tall grass and weeds back from your house. HealthLinkBC suggests trimming plants near buildings so that 15 to 20 cm (6 to 8 inches) above the ground is clear, and trimming tree and shrub branches so they cannot reach the roof or balconies. UC IPM adds that plants climbing up walls should be thinned or removed, since house mice are excellent climbers.

Woodpiles

Woodpiles are favourite nesting spots. Health Canada advises raising woodpiles about 30 centimetres (1 foot) off the ground and placing them away from your house. HealthLinkBC recommends storing lumber and firewood on stands 30 cm to 45 cm (12 to 18 inches) off the ground, and removing unused piles of lumber altogether.

Outdoor Food Sources

Outdoor food sources feed the mice that later move inside. HealthLinkBC recommends removing fallen fruit and nuts from your yard, using bird feeders with trays and cleaning up spilled seed often, and never leaving pet food outside overnight. Harvest garden produce as it ripens rather than leaving it on the ground.

Compost

Compost can become a mouse buffet if it is not managed. Health Canada advises never putting fatty or oily food waste, eggs or milk products in the composter, and placing a layer of heavy metal mesh between the soil and the bottom of the bin. Keep the lid closed, and lock your green bin if your city provides one, as the City of Toronto recommends.

Garages

Garages are often the weak spot in mice prevention, because the big door, stored birdseed and clutter all invite them in. Check the bottom seal of the garage door, keep pet food and seed in metal bins, and seal the gaps where the garage meets the house. Our guide on how to keep mice out of your garage covers this space step by step, and the same steps apply to sheds.

Do Mouse Repellents Keep Mice Away?

Mouse repellents are the weakest part of mouse prevention: some may help for a short time, but none replaces sealing and sanitation. Treat them as an extra, never as your main defence.

Peppermint Oil

Peppermint oil is the most popular home remedy, but the evidence behind it is thin. Ask Extension (University of New Hampshire) says repellents, whether mothballs, essential oils or other home remedies, have not been shown to be very effective, partly because their volatile ingredients disappear quickly. Cotton balls soaked in peppermint or clove oil may make a spot less appealing for a few days, but they will not keep mice out of a house with open gaps.

Ultrasonic Repellers

Ultrasonic repellers give off sounds that mice dislike, but mice get used to them. Health Canada notes that rats and mice may adapt to these devices and return, and recommends using them only along with other control methods. UC IPM explains that the sound is directional and does not reach behind furniture or into wall voids, and HealthLinkBC calls ultrasound repellers expensive with no long-term success. If you still want to try one, see our review of the best mouse repellers.

Cats

Cats catch some mice, but they rarely solve the problem. UC IPM notes that some dogs and cats will catch and kill house mice, but there are few situations in which they control a rodent population. Do not rely on a pet as your prevention plan.

What Should You Do if Mice Still Get In?

If mice still get into your house, act quickly: trap the ones inside, find the gap they used, and clean up safely. Mice breed so fast that a week of waiting can turn one mouse into a nest.

Signs Prevention Is Failing

Signs that mice have found a way in, according to Health Canada, include:

• Scampering and scratching sounds in the walls at night
• Gnawing or chewing marks, or damaged food packages
• Droppings and urine
• Burrows or holes in and around foundation walls
• Tracks on dusty surfaces

A musky smell or nests of shredded paper and fabric are other clues.

Traps and Bait Stations as Backup

Traps are the safest backup when prevention fails. Health Canada says snap and electronic traps are easy to use and very effective when they are well placed and set properly, and peanut butter makes a good bait. The CDC advises against glue traps and live traps, because they can scare mice into urinating and raise the risk of disease, and UC IPM notes that releasing live-caught mice elsewhere does not work. Compare tested options in our best mouse traps review, and keep every trap out of reach of children and pets.

Poison should never be your only tool. Health Canada advises checking for a PCP number on the label, which shows the product is approved in Canada, and says poison should always be combined with rodent-proofing and sanitation. Under PMRA rules, rodenticides sold for home use must always be used in a bait station, and home-use products containing second-generation anticoagulants are prohibited. For the full removal plan, read our guide on how to get rid of mice.

Safe Cleanup

Safe cleanup protects you from the germs mice leave behind, including hantavirus, which the deer mouse is the most common carrier of in Canada. Never sweep or vacuum dry droppings. Open doors and windows and leave the area for at least 30 minutes, as Alberta advises, then wear rubber or plastic gloves, spray droppings and nests with a disinfectant or 1 part bleach to 9 parts water, let it soak for 10 minutes, and wipe up with paper towels, following the Public Health Agency of Canada.

When to Call a Pest Control Professional

Calling a licensed pest control professional makes sense when mice keep coming back after you have sealed and trapped, when the entry points are out of reach on the roofline or soffits, or when you find nests in the walls or attic. Professional rodent exclusion usually includes an inspection inside and out, sealing with rodent-resistant materials such as steel mesh, and follow-up monitoring. Ask each company what its warranty covers and for how long, since terms vary. Our guide to rodent control and prevention services explains what to expect, and you can get a quote from a recommended local exterminator through Pest Agent's referral form. For more mouse guides, visit our mice hub.