Mice Entry Points: How to Find and Seal Them in Your Home

TL;DR

• Look for gaps about 1/4 inch (about 6 mm) wide or larger. A mouse entry point can be as small as a dime (Health Canada).
• Start inside: note where droppings, gnaw marks and other signs are thickest, then check the walls and pipes close to those spots.
• Use a flashlight after dark, look for daylight under exterior doors, and leave a thin flour patch overnight to see where mice travel.
• Check where two materials meet and where pipes, wires or vents pass through a wall, from the foundation up to the roofline.
• Trap the mice inside first, then seal, typically with metal wool, hardware cloth, sheet metal or concrete. Expanding foam alone usually won't stop mice: they can chew straight through it.
• Need help? Get a quote from a recommended local exterminator through Pest Agent's referral form.

Mice entry points explain the most discouraging part of a mouse problem: you set traps, catch a few mice, and a week later fresh droppings are back on the counter. As long as the gap they used stays open, new mice can follow the same route inside.

Finding that gap is harder than it sounds. The opening is often smaller than people expect, it can sit anywhere from the foundation to the roofline, and an old crack full of cobwebs looks a lot like the one mice slipped through last night.

What Are Mice Entry Points?

Mice entry points are gaps, cracks and holes about 1/4 inch (about 6 mm) wide or larger that lead from outside into your home. If you want the full picture of how mice climb, squeeze and gnaw their way inside, read our guide on how mice get in your house. The practical job is finding the gaps mice are using in your home and closing them.

How Small Can a Mouse Entry Point Be?

A mouse entry point can be as small as a dime. Health Canada notes that mice can squeeze through cracks that size, while rats need a quarter-sized hole. UC IPM puts it in measurements: house mice can squeeze under gaps 1/4 inch (about 6 mm) tall and may get through openings about 3/8 inch (about 1 cm) wide. The University of Kentucky gives an easy reference: a 1/4 inch gap is about the diameter of a pencil. That is why HealthLinkBC advises sealing any opening in the outside of your home that is larger than 6 mm (1/4 inch).

What Does an Active Entry Point Look Like?

An active entry point usually has signs of traffic around it. According to the Internet Center for Wildlife Damage Management (ICWDM), look for:

• Dark, greasy smudges (rub marks) on the edge of the hole, left by oil and dirt from mouse fur
• Fresh gnawing, with small piles of wood shavings, insulation or other chewed material
• Droppings nearby, about 1/4 inch (0.6 cm) long
• A round hole, often 1 1/2 inches (3.8 cm) across or less for mice, compared with 2 inches (5 cm) or more for rats

A gap with cobwebs, dust and no marks is a possible entry point. A gap with smudges, fresh shavings and droppings is one mice are using now.

Why Do Small Gaps Matter?

Small gaps matter because mice can make them bigger. The University of Nebraska-Lincoln Extension warns not to ignore crevices smaller than 1/4 inch, and UC IPM notes that mice will gnaw to enlarge openings to gain entry. Treat every gap near the size of a pencil as a candidate.

How to Find Mouse Entry Points

Finding mouse entry points starts with the signs mice leave inside, then traces them back to the wall, floor or door they come through. You need only a flashlight and some flour.

Follow the Signs Inside

The signs mice leave inside point toward the places they come in. Health Canada lists droppings, urine, holes around foundation walls and tracks on dusty surfaces as signs of rodents, and UC IPM adds signs such as fresh gnaw marks, nests of shredded paper and a musky odour. HealthLinkBC notes that when your house is quiet you may hear them moving in walls, attics and under floors. Note where the signs are thickest: in a basement corner, under the kitchen sink, behind the stove. Entry points are usually close to those spots, along walls or around pipes. For help reading the evidence, see our guide to the signs of mice.

Use a Flashlight After Dark

A flashlight after dark is the simplest inspection tool, because mice are most active at night. ICWDM notes that mice may be seen after dark with the help of a flashlight or spotlight. Check basements, attics, crawl spaces and the garage, and shine the light along the bottom of walls, behind appliances and around pipes.

Look for Light Under Doors

Light under doors shows you gaps that mice can use. The University of Kentucky suggests lying on the floor and checking for light coming in under exterior doors, with particular attention to the bottom corners, where rodents may get in. Do this during the day with the inside lights off, at every exterior door, including the door between the garage and the house. If you can see daylight, a mouse may be able to fit.

Set Flour Tracking Patches

Flour tracking patches show you exactly where mice travel. ICWDM describes a simple test that can help: sprinkle a thin patch of flour (or talc), smooth it with a rolling pin or a can, and leave it overnight along a wall or beside a suspected hole. Check it after about 24 hours. Tiny footprints and tail marks mean mice are using that path, and the direction of the tracks can lead you toward the gap. In a larger space like an unfinished basement, ICWDM suggests spacing patches roughly every 6 to 9 metres (20 to 30 feet) to see which areas mice use most.

Check for Rub Marks

Rub marks are dark, greasy smudges that mice leave on surfaces they brush past again and again. ICWDM explains that they come from oil and dirt rubbing off mouse fur along regular travel routes, on beams, pipes, walls and around holes. Mouse rub marks are usually smaller and fainter than rat marks, so look closely at the edges of holes and where pipes enter.

Inspect in Fall

Fall is the best time to inspect for mice entry points. UC IPM notes that house mice may enter homes in autumn as nighttime temperatures drop, and Skedaddle Humane Wildlife Control (commercial) recommends checking for new cracks and gaps before fall and winter. In much of Canada, a walk-around in early fall lets you close gaps before the cold pushes mice indoors. Mice can be active year round, though, so check again any time you see new signs.

Where Should You Check for Mice Entry Points?

Mice entry points cluster where two materials meet or where a pipe, wire or vent passes through a wall, so check those spots first, from the foundation up to the roofline.

Foundation Cracks

Foundation cracks are a common entry point. Health Canada advises patching cracks in foundations, and holes in and around foundation walls can be a sign of rodents. Check basement walls from the inside too.

Where Siding Meets the Foundation

The line where siding meets the foundation is one of the most overlooked mice entry points. HealthLinkBC tells homeowners to pay close attention to gaps between the siding and the foundation (the Government of British Columbia stresses eliminating rodents' access), and Nebraska Extension notes that gaps can exist where wall framing or siding meets the foundation.

Utility Penetrations

Utility penetrations are the holes made for pipes and wires to pass through your walls. The University of Kentucky lists examples such as outdoor faucets, electrical receptacles, gas meters, clothes dryer vents and telephone or cable TV wires. Oregon State University Extension notes that gaps around plumbing and wiring can be one of the main ways mice find their way into a building.

Exterior Doors

Exterior doors let mice in through worn seals and thresholds. Health Canada warns that even the small gaps created by worn thresholds under doors will allow mice access, and HealthLinkBC points to door jambs. Nebraska Extension recommends that the gap between the door bottom and the threshold be less than 1/4 inch.

Garage Doors

Garage doors are a frequent weak spot, because the bottom seal wears, cracks and pulls away at the corners. Check the full length of the seal, especially the bottom corners, and the gaps at the sides. Our guide on keeping mice out of your garage covers the garage in more detail.

Windows

Windows become entry points when frames, screens or weather stripping no longer fit tightly. UC IPM advises that doors, windows and screens should fit tightly, and Health Canada recommends weather stripping windows. Check basement windows and window wells closely.

Vents

Vents connect the inside of your home directly to the outside. Health Canada names dryer vents, attic vents and soffits, and HealthLinkBC adds crawl space and fresh air vents. Look for missing, bent or plastic covers, torn screens and gaps between the vent and the wall.

Weep Holes

Weep holes are the small openings left at the bottom of brick walls so moisture can drain out. Skedaddle Humane Wildlife Control (commercial) lists weep holes between bricks, and gaps where brick meets the soffit, as possible mouse entry points on Ottawa homes. If your house has brick, check each weep hole for chewed or missing covers.

The Roofline

The roofline is easy to forget, but mice are excellent climbers that can run up most rough vertical surfaces, according to UC IPM. Skedaddle (commercial) notes that openings high along the roofline are just as likely to be used as those at ground level. From the ground or a safe ladder, check eaves, soffits, gaps under the eaves, roof vents and the spots where gutters meet the roof. HealthLinkBC also advises trimming tree and shrub branches so they cannot give rodents access to roofs or balconies. Read more about whether mice can climb walls.

Inside Your Home

Inside your home, mice entry points are usually the openings where pipes and wires pass through floors and walls. Oregon State University Extension notes that once mice are in a crawl space, they can come up through the floor along gaps around plumbing and utility lines. Check under kitchen and bathroom sinks, behind the stove, fridge and washer, around the furnace and water lines, and in the attic, basement and crawl space.

How to Seal Mouse Entry Points

Sealing mouse entry points usually works when you use materials mice cannot chew, and when the mice already inside are dealt with first. UC IPM describes exclusion as among the most successful and permanent forms of house mouse control.

Remove the Mice Inside First

Removing the mice inside comes before sealing their last way out. Nebraska Extension suggests that if there is any doubt whether a hole is being used by an animal, you should not close it until you have a control plan. Otherwise, mice trapped inside may chew new holes. Set snap traps along the walls near active signs (Health Canada says snap and electronic traps are very effective when well placed), and seal the active hole once you stop catching mice. Professionals sometimes install one-way doors that can let mice leave but not come back. See how to get rid of mice and how to set a mouse trap.

Metal Wool

Metal wool is the go-to filler for small gaps and cracks. Nebraska Extension suggests tightly wedging copper or stainless steel wool into the gap. Plain steel wool works as a temporary plug, but UC IPM and Nebraska Extension both note that it can rust over time. Copper mesh products sold for rodent exclusion are often woven to stay in place, as long as you pack them tight and snug. Push the wool in firmly so a mouse cannot pull it out, then cover it with caulk to hold it in place.

Gaps Around Pipes

Gaps around pipes and wires need a filler and a seal. Health Canada advises stuffing steel wool around pipes before caulking or plastering. Oregon State University Extension suggests siliconized acrylic latex or polyurethane sealants for gaps of 1/4 inch or more, because they can stretch as the house expands and contracts with temperature changes. Nebraska Extension frames it the other way round for larger voids: seal the gap to stop airflow, then cover it with a metal barrier that mice generally cannot chew. Either way, the metal stops the mice and the caulk stops drafts and holds the metal in place. Take care not to crush or cut cables while you work.

Hardware Cloth

Hardware cloth is a stiff metal mesh used to cover larger openings. To exclude mice, Nebraska Extension suggests hardware cloth, typically 24 gauge with 1/4 inch mesh. Cut it larger than the opening, fasten it firmly to solid material with screws or staples, and seal the edges. Use it for crawl space openings and large holes that wool cannot fill.

Sheet Metal

Sheet metal covers holes and edges that mice gnaw. Nebraska Extension suggests galvanized sheet metal, typically of 24 gauge or heavier, and UC IPM notes that you may need to cover the edges of doors and windows with metal to stop gnawing. Nebraska Extension adds that rodents can gnaw through aluminum sheeting, so choose galvanized steel for spots mice are actively chewing.

Concrete Patching

Concrete patching is the lasting fix for foundation cracks and holes in masonry. UC IPM recommends sealing foundation cracks with metal or concrete, and Oregon State University Extension advises filling larger holes with good quality concrete rather than steel wool or foam.

Door Sweeps

Door sweeps close the gap at the bottom of exterior doors. Oregon State University Extension suggests high-quality, brush-type door sweeps, and Health Canada recommends metal weather stripping under doors. Nebraska Extension adds that rodents can gnaw through rubber or vinyl weather stops, so a metal channel on the bottom edge of the door can help where mice are chewing. For garage doors, the University of Kentucky suggests a rubber bottom seal, because vinyl tends to seal poorly in cold weather, a real concern in a Canadian winter.

Vent Screens

Vent screens keep mice out while letting air through. HealthLinkBC suggests covering crawl space, fresh air and attic vents with metal screening or steel mesh of about 6 mm (1/4 inch). Clothes dryer vents are the exception: Nebraska Extension warns that improper screening can cause lint build-up, and UC IPM notes that a buildup of dryer lint can cause fires. Instead, seal the gap around the dryer pipe and consider a dryer vent cover with a floating ball that closes the vent when the dryer stops.

Weep Hole Inserts

Weep hole inserts block mice without blocking drainage. Weep holes generally need to stay open so water can drain and the wall can dry, so do not fill them with caulk, mortar or foam. Skedaddle (commercial) notes that plastic weep vent covers may not stand up to mouse chewing, while steel screen inserts let air and water through but generally cannot be chewed.

Why Expanding Foam Does Not Stop Mice

Expanding foam does not stop mice because they chew straight through it. UC IPM lists insulating (expanding) foam with plastic, rubber, vinyl and wood as generally ineffective materials, and ICWDM notes that expanding foam reduces heat loss but may not prevent rodent entry. Nebraska Extension adds that sunlight can break down foam outdoors. Foam is fine as a draft seal over metal wool or mesh, but never as the only barrier.

How Do You Know the Seal Worked?

A sealed entry point has worked when, over the following weeks, you find no fresh droppings, gnawing or tracks near it. Nebraska Extension notes that rodent-proofing will not guarantee a rodent-free building, so a few simple checks can help you catch a missed gap early.

Recheck Sealed Spots

Recheck each sealed spot a few days after the work, then again every few weeks. Look for fresh gnaw marks on the edges, metal wool pulled out of place, new droppings and new rub marks. If mice have chewed around a patch, the gap was probably bigger than it looked: cover it with hardware cloth or sheet metal instead.

Keep Traps as Monitors

Traps left in place act as an early warning system. Keep a few snap traps along walls in the basement, under sinks and in the garage, where children and pets cannot reach them, and check them regularly. Health Canada also notes that poison baits should never be the only method and must be in a tamper-proof bait station where children and pets have access, with a PCP number on the label. If you need a trap, see our picks for the best mouse trap. It also helps to remove what attracts mice to your house.

Clean Up Droppings Safely

Cleaning up droppings near entry points needs care. Health Canada warns never to sweep or vacuum dry droppings, because the dust raised can cause illness, and the deer mouse is the most common carrier of hantavirus in Canada. The Public Health Agency of Canada advises wearing rubber or plastic gloves. Follow our step-by-step guide on how to clean mouse droppings before and after sealing.

When to Call a Pest Control Professional

A pest control professional is worth calling when you cannot find where mice get in, when mice keep coming back after you seal, or when gaps are high on the roofline or behind finished walls. HealthLinkBC advises contacting a licensed company with certified staff if you cannot control rodents on your own. A professional exclusion visit typically includes an inspection inside and out, sealing with rodent-resistant materials and often follow-up monitoring. Read about what a mouse exterminator does, or get a quote from a recommended local exterminator through Pest Agent's referral form.