How Do Mice Get in Your House? Entry Routes, Abilities and How to Stop Them

TL;DR

• Mice get in through small gaps in the outer shell of your home. They find them by following warm air and food smells, and can widen them by gnawing.
• A small gap is enough: mice fit through cracks as small as a dime (Health Canada) and openings of about 6 mm (1/4 inch) (UC IPM).
• Check where two building materials meet loosely and where pipes, wires or vents pass through the wall, plus foundation cracks, worn door thresholds and weep holes.
• Don't forget the roofline. House mice are excellent climbers that can run up any rough vertical surface (UC IPM).
• Remove the mice inside first, then seal with copper or stainless steel wool, hardware cloth, sheet metal or concrete. Spray foam alone won't stop them: they chew straight through it.
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

Mice get in your house through openings you would walk past without a second look: a worn threshold under the back door, a weep hole in the brick, the gap around a gas line, a torn vent screen up near the soffit. Many of those openings are nowhere near the ground, since mice climb brick and siding easily, and pest controllers in Ottawa report mice often entering along the roofline as well as at ground level.

Closing the wrong gap, or closing it with the wrong material, changes nothing. Rubber and vinyl weather stops give way to gnawing quickly, and a sealed hole does little good if the mice are still using another one.

A mouse does not find these openings by luck. Its body, its teeth and its sense of smell all work together to turn a small flaw in the outer shell of a house into a doorway.

How Do Mice Get in Your House?

Mice get in your house through small gaps in the outer shell of the building, which they find by following warm air and food smells and can widen by gnawing. They are small, flexible, good climbers and persistent, so a single overlooked opening is often enough.

How Small a Gap Can a Mouse Fit Through?

A mouse can fit through a gap about the size of a dime. Health Canada says mice can squeeze through cracks as small as a dime, and UC IPM puts it at openings of about 6 mm (1/4 inch), adding that house mice can slip under gaps 1/4 inch tall and through openings 3/8 inch (about 1 cm) wide. Rats need a little more room: slightly more than 1/2 inch (about 1.3 cm), or a quarter-sized hole according to Health Canada.

How Do Mice Gnaw Their Way In?

Mice gnaw their way in by widening cracks that are almost big enough. Their front teeth grow continuously, so they gnaw often to keep them worn down. The University of Nebraska-Lincoln warns that rodent gnawing can quickly enlarge small crevices, and that rats and mice can chew through rubber or vinyl weather stops. Mouse entry holes are often about 1 1/2 inches (3.8 cm) across or less, with rough, chewed edges.

How Do Mice Reach Higher Openings?

Mice reach higher openings by climbing. UC IPM describes house mice as excellent climbers that can run up any rough vertical surface, and the University of Nebraska-Lincoln lists wood, brick and concrete among the surfaces they can often climb. They also run along electrical wires, cables, vines, shrubs and trees to reach a building. This is why an opening under your eaves or around a second-floor window can be just as useful to a mouse as a crack near the ground. Learn more in our article on whether mice can climb walls.

How High Can a Mouse Jump?

A mouse can jump about 30 cm (12 inches) straight up onto a flat surface, according to UC IPM and Oregon State University Extension. Oregon State adds that a mouse can often survive a fall of several feet.

How Do Mice Find the Way In?

Mice find the way in by following air currents and smells. Oregon State University Extension explains that a mouse running along the outside of a building is drawn inside by warm air and food odours coming from under doors and through holes in the wall. The University of Nebraska-Lincoln notes that mice may be attracted to buildings when they detect openings and escaping heat. Food, water and clutter then make them want to stay; our guide on what attracts mice to your house covers those triggers.

Can Mice Be Carried Inside?

Mice can be carried inside. The University of Nebraska-Lincoln points out that rodents can sometimes arrive in a box or an appliance, which is why even a well-sealed home may get a mouse. Check boxes, outdoor furniture and stored items before you bring them in from a garage, shed or storage unit.

Where Do Mice Get Into Homes?

Mice get into homes wherever two building materials meet loosely or a pipe, wire or vent passes through the wall. Skedaddle Humane Wildlife Control, an Ontario wildlife company, notes that these weak spots are often where two different building materials meet, and that mice are as likely to enter along the roofline as at ground level. Here are the most common routes.

Foundation Cracks

Foundation cracks are a classic way in. Health Canada lists burrows or holes in and around foundation walls as a sign of rodents and advises patching foundation cracks. Look closely at basement windows and along the top of the foundation wall, where it meets the rest of the house.

Where Siding Meets the Foundation

The joint where siding meets the foundation is one of the most overlooked entry points. HealthLinkBC tells homeowners to pay close attention to gaps between the siding and the house foundation, and Skedaddle lists it among the common mouse entry points on Ottawa homes.

Utility Lines and Pipes

Utility lines and pipes create openings wherever they pass through a wall: water and gas pipes, electrical cables and air conditioning lines. Oregon State University Extension calls gaps created for plumbing and wiring one of the ways house mice get into buildings, and notes that once inside, mice follow these lines up through walls and floors. UC IPM generally recommends sealing around water pipes, vents and utility cables with metal or concrete.

Gaps Under Doors

Gaps under doors are an easy entrance, because warm air and food smells flow out right there. Health Canada warns that even the small gaps created by worn thresholds under doors let mice in. The University of Nebraska-Lincoln recommends that the space between the bottom of a door and the threshold be less than about 1/4 inch (about 6 mm), and Oregon State recommends brush-type door sweeps on exterior doors.

Garage Doors

Garage doors often leave a gap at the bottom or sides, especially when the rubber seal is worn or chewed. Pest control companies list garage door seals among the entry points they check, and the same rule as for any exterior door generally applies: no gap wider than about 6 mm (1/4 inch). Since rubber and vinyl can be gnawed, a garage that mice already use needs extra attention.

Windows

Windows let mice in when the frame, the screen or the weather stripping no longer fits tightly. Orkin Canada lists worn door and window screens among possible entry points, and UC IPM advises that doors, windows and screens should fit tightly. Health Canada recommends weather stripping your windows.

Vents

Vents are open by design, so they need a screen or a cover that mice cannot chew. Health Canada recommends covering attic vents and soffits with fine mesh metal screening, and HealthLinkBC suggests about 6 mm (1/4 inch) metal screening for crawl space, fresh air and attic vents. Health Canada lists dryer vents too, but the wrong screen on a dryer vent is a fire risk. Be careful with dryer vents: the University of Nebraska-Lincoln warns that the wrong screen traps lint, and UC IPM notes that a buildup of dryer lint can cause fires. Seal the gap around the dryer pipe instead and consider a dryer vent cover with a floating ball that closes the vent when the dryer stops.

Weep Holes in Brick

Weep holes are the small openings at the bottom of brick walls that let water drain out and air dry the wall. Skedaddle explains that mice may make the short climb up the brick and tuck themselves into the wall, and that plastic weep vent covers often cannot stand up to mouse chewing. Never fill weep holes with caulk, mortar or cement, because the wall needs them to drain. Use steel inserts made for weep holes, which let water and air through but help keep mice out.

The Roofline

The roofline is a common way in for mice that climb. Skedaddle lists gaps where the brick meets the soffit, and the University of Nebraska-Lincoln notes that mice can use trees, shrubs and wires to reach a building. HealthLinkBC advises trimming branches of trees and shrubs so they are less likely to give access to roofs or balconies, and blocking holes around the roof.

When Do Mice Get in Your House?

Mice often get in your house in the fall, when nighttime temperatures drop and food outside can become harder to find. A mouse can still come in at any time of year if it finds an opening.

Fall

Fall is peak season for mice moving indoors. UC IPM says house mice frequently enter homes in autumn when nighttime temperatures become colder, and the ICWDM notes that the onset of cold weather each fall can push mice into buildings in search of shelter and food. Health Canada adds that deer mice, widespread in parts of Canada according to the Public Health Agency of Canada, may invade buildings near fields and woodlands in the fall. That makes the weeks before the first cold nights the best time to check your home for gaps.

Year Round

Mice can still get in during summer and winter. House mice can be active all year and generally do not hibernate, and once inside, they may breed all year round (Orkin Canada, Skedaddle).

What Happens Once Mice Get Into Your Home?

Mice that get into your home look for a hidden, quiet spot to nest and start feeding and gnawing nearby. They move into wall voids, attics, basements and kitchen cabinets, often close to food. Our guide to where mice hide in a house shows the usual spots, and these are the two main problems they cause.

Chewed Wires

Chewed wires are the most serious damage mice cause. UC IPM notes that mice may gnaw on electrical wires and gas lines, and that many fires of unknown origin are believed to be caused by rodents gnawing on wires. Insulation in walls and attics is another frequent target.

Health Risks

Health risks come from droppings, urine and nesting material. Health Canada notes that mice carry disease and that the deer mouse is the most common carrier of hantavirus. When you clean up, the Public Health Agency of Canada advises wearing rubber or plastic gloves and never sweeping or vacuuming droppings. If you are not sure how big the problem is, check the signs of a mouse infestation.

How Do You Block the Ways Mice Get In?

Blocking the ways mice get in means finding the gaps they use, removing the mice already inside, and then sealing every opening with materials mice cannot chew. UC IPM calls this exclusion the most successful and permanent form of house mouse control.

Find the Active Entry Points

Finding the active entry points starts with the clues mice leave along their routes. Look for dark, greasy rub marks left by their fur along walls and pipes, fresh gnaw marks or shavings, droppings about 3 to 6 mm (1/8 to 1/4 inch) long near a gap, and scratching sounds in the walls at night. Then walk around the outside of your house from the foundation to the roofline. Our guides on how to find mice entry points and how to find mice in your house walk you through a full inspection.

Remove the Mice Already Inside

Removing the mice already inside comes before sealing. The University of Nebraska-Lincoln advises against closing an opening while an animal may still be using it, because rodents shut inside may cause further damage and gnaw new entrances. Snap traps and electronic traps work well when placed correctly, according to Health Canada. If you use poison, choose a product with a PCP number and, as UC IPM advises, use it in a tamper-proof bait station, out of reach of children, pets and wildlife. See our step-by-step guide on how to get rid of mice and our picks for the best mouse traps.

Seal Gaps With Gnaw-Proof Materials

Sealing gaps with gnaw-proof materials is what keeps mice out for good. Use these, based on the size of the gap:

• Small cracks and gaps around pipes: often copper or stainless steel wool packed tightly, then covered with a sealant (University of Nebraska-Lincoln, Oregon State)
• Vents and larger openings: usually metal hardware cloth with 6 mm (1/4 inch) mesh, metal flashing or sheet metal
• Foundation cracks and holes in masonry: concrete or mortar
• Door bottoms: a metal threshold or flashing plus a brush-type door sweep

Plain steel wool pads work as a temporary plug, but UC IPM notes they can rust over time. For a complete prevention plan, read how to keep mice out of your house.

Why Foam Does Not Stop Mice

Spray foam does not stop mice because they can chew straight through it. UC IPM lists plastic screening, rubber, vinyl, expanding foam and wood as ineffective against mice, and Oregon State does not recommend foam for larger holes. The ICWDM notes that expanding foam helps reduce heat loss but may not prevent rodent entry, so if you use it, put a gnaw-resistant barrier such as metal mesh or flashing over the gap.

Trim Plants and Branches

Trimming plants and branches removes the bridges mice use to reach your walls and roof. HealthLinkBC suggests keeping the first 15 to 20 cm (6 to 8 inches) above the ground clear around buildings and trimming branches that touch the roof. Health Canada also advises cutting back tall grass and weeds and raising woodpiles about 30 cm (1 foot) off the ground, away from the house.

When to Call a Pest Control Professional

Calling a pest control professional makes sense when you cannot find how mice are getting in, when the gaps are high up along the roofline, or when mice keep coming back after you have sealed and trapped. HealthLinkBC suggests a licensed company if you cannot control rodents on your own. A professional typically inspects the house inside and out, seals openings with rodent-resistant materials, removes the mice and comes back to check the results. You can get a quote from a recommended local exterminator through Pest Agent's referral form, and our mice hub has more guides on every step.