How Mice Get Into Your House (and How to Seal Them Out)

You found droppings under the sink, or heard something scurrying above the ceiling, and the first thing you want to know is how mice get into a house that looks perfectly closed up. The answer is a little unsettling. An adult mouse fits through a hole the width of a pencil, about 6 mm, or a quarter inch. If the head goes through, the rest follows.

This guide walks through the entry points we see over and over on South Shore homes, shows you how to find them yourself at dusk with a flashlight, and explains how to seal them properly (spray foam on its own does not work). It ends with what a professional exclusion involves and what it costs in Quebec.

How small a hole can a mouse fit through?

A mouse's skull is the only rigid part of its body. Everything else is muscle and fur that compresses. So an adult mouse squeezes through a 6 mm opening, roughly the diameter of a pencil, or a slot the width of a dime. A young mouse gets through something smaller still.

That is why "I looked and there are no holes" does not mean much. People look for a hole they can see from the driveway. What you are actually looking for is a few millimetres of daylight under a threshold, a split bead of caulk around a pipe, or a lifted corner of screen on a dryer vent.

Two more things to know. A mouse climbs a rough vertical surface (brick, vinyl siding, a downspout) without effort, so openings up high count as much as the ones at ground level. And it jumps about 30 cm, which puts the bottom edge of your siding within reach from a bed of mulch.

How do mice get into a house on the South Shore?

Homes in Brossard, Saint-Hubert, Longueuil and Greenfield Park have a lot in common: bungalows from the 60s and 70s, brick on the front, an attached garage or carport, and often an addition tacked on later. Each element has its weak spots. Here is the tour, in the order we find them most often.

The garage door weatherstrip

This is entry point number one, by a wide margin. The rubber weatherstrip along the bottom of the garage door compresses over the years, cracks in the cold, and the corners stop touching the floor. The garage becomes a heated waiting room for rodents, and from there the door between the garage and the house rarely seals tight. Look from inside with the door closed and the lights off: if you see daylight at the corners, a mouse fits.

Dryer and range hood vents

The dryer vent and the range hood vent exit through the exterior wall with a plastic flap. That flap gets stuck open with lint, or it breaks off. The duct then becomes a straight tunnel into the laundry room or the space above the kitchen cabinets. The same applies to the air exchanger outlet on newer homes.

Where pipes and AC lines enter

Anywhere something passes through the wall or the foundation, there is a hole drilled bigger than what runs through it: water service, heat pump condensate drain, refrigerant lines for the central air, cable TV, electrical service, gas. The installer usually plugs the gap with a bit of foam or caulk that does not last ten years. AC line sets are a frequent culprit, because the foam insulation around them degrades and leaves a soft gap that is easy to chew.

Foundation cracks and the sill plate

The top of the foundation, where the wood framing sits on the concrete (the sill plate), is a joint between two materials that move differently. On older homes you find gaps between the concrete and the wood, especially at the corners. Shrinkage cracks in the concrete itself are rarely wide enough for a mouse, but one that frost has widened can be. This spot is hard to see from outside when a flower bed runs up against the wall: you have to pull the mulch back.

Door thresholds and patio doors

A front door or side door with a worn sweep leaves a few millimetres of gap along its whole width. Sliding patio doors are worse: the bottom track fills with debris, the door no longer closes all the way, and the frame corners often show daylight. Do the flashlight test at night: someone outside with the light, you inside with the lights off.

Soffit and roofline (for the attic)

When the noises come from the ceiling rather than the basement, the entry point is up top. Mice climb the siding or a downspout to the soffit. There, a soffit panel that is not clipped in properly, the gap between the soffit and the top of the brick, or an attic vent with a rusted-out screen gives direct access to the attic insulation, the warmest and quietest place in the house. The spots where a lower roof (garage, addition) meets a taller wall are the first places to check.

Unscreened weep holes in brick

Look at the bottom of a brick wall: about every 60 cm there is a vertical joint left open. Those are weep holes, and they are necessary. They let out the water that gets behind the brick. The problem is that they are about 10 mm wide, plenty for a mouse, which then ends up in the cavity between the brick and the wall. You never plug them. You fit them with a screened insert that lets water and air through but not rodents.

Old bungalow additions

Plenty of bungalows around here got an addition in the 80s or 90s: a sunroom, a bedroom over the garage, a kitchen bump-out. The joint between the old building and the new one is almost always the weak link: different foundations, siding that does not quite meet, a former exterior wall now inside with vents nobody ever closed off. If you have an addition, spend twice as long on it.

Recognized three or four of these while reading? That is normal, and it is exactly what we find on every inspection. Extermination BETA checks every entry point on homes in Brossard, Saint-Hubert, Longueuil and across the South Shore. The first inspection is free. Call 514-553-2070 or reach us through the contact page.

How do you find the entry points yourself?

A proper inspection takes an hour and is done at the right time of day. Here is the method.

Pick dusk. Low-angle light throws shadows into joints and cracks that full sun washes out. It is also the hour mice start moving, so you may see one going in or out.

Bring a strong flashlight and hold it off to the side. A beam at an angle to the wall shows gaps and rub marks. A headlamp works too, but the angle is worse.

Walk the whole exterior with your nose 30 cm from the wall. Start at the garage door, follow the bottom edge of the siding in one direction, and stop at every pipe, every vent, every corner. Pull back the mulch to see the top of the foundation. Look up at every downspout to check the soffit.

Look for signs, not just holes. A mouse that uses an opening regularly leaves a dark, greasy smudge on the edge (its fur rubbing), droppings right beside it, and sometimes a few hairs caught on the surface. An opening with those signs is active. A clean opening is a risk.

Repeat inside, in the basement. Lights off, a helper outside running the flashlight along the wall. Every point of light you see from the basement is an entry. Check behind the dryer, under the kitchen sink and inside the bathroom vanity too, where pipes come up through the floor.

Record everything on your phone. A photo of each spot plus a quick note. You will need it for the sealing job, or to show the technician.

If you are not sure you actually have mice and are still at the suspicion stage, our post on the 10 signs of mice in your house helps you confirm before you pull out the flashlight.

How do you seal the entry points properly?

The principle is simple: mice chew through anything softer than their teeth. Spray foam, caulk alone, plastic, soft wood and rubber do not hold. You need metal in every hole, then something over it to block air and water.

Steel wool plus caulk

For small holes (up to 2 or 3 cm): pack coarse steel wool tightly into the hole with a screwdriver, then cover it with a good exterior caulk (polyurethane or silicone). Steel wool alone eventually rusts and crumbles, and caulk alone gets chewed. Together they last for years. Where the opening sees water, use copper wool instead, which does not rust.

Hardware cloth

For larger openings, or ones that need to keep breathing: attic vents, the underside of a deck, a gap under the soffit. Use galvanized hardware cloth with 6 mm (quarter inch) mesh, never ordinary window screen, which a mouse bites through in one go. Fasten it with screws and washers or construction staples, not office staples.

Copper mesh

Copper mesh sold in rolls packs into irregular shapes better than steel wool: around a pipe, into a weep hole, into the corner of a door frame. It does not rust and does not stain brick.

Door sweeps and weatherstrips

For doors, you replace, you do not patch. An aluminum door sweep with a dense rubber blade for entry and side doors. For the garage door, a new weatherstrip of the right width, and if the floor is uneven, a rubber garage threshold glued to the slab to fill the corners. Both parts cost a few tens of dollars and solve most garage entries.

Vent covers

For the dryer and range hood, an exterior vent cover with a spring-loaded or magnetic flap, designed not to trap lint. Check that it actually closes when the dryer is off. Never put fine screen directly over a dryer vent: lint builds up and becomes a fire hazard.

What does not work

Spray foam on its own: mice go through it in a night. Duct tape or plumber's tape: a few days. Ultrasonic repellers: no demonstrated effect on a mouse that has already found warmth and food. Mothballs and peppermint oil: they smell strong, and the mice keep coming.

What order should you eliminate and seal in?

This is where most people go wrong, and the mistake is expensive. If you seal every entry on a Saturday afternoon while mice are already living in your walls, you have just locked them inside. They can no longer get out to forage, so they chew inward to find food: cabinets, pantry, electrical wiring. And the ones that die inside a wall, you will smell for two weeks.

The right order:

1. Confirm and eliminate first. Traps or bait stations placed along the walls where mice travel, for one to three weeks, until activity stops. Our rodent control service does exactly that, with a follow-up visit to confirm it is over.
2. Seal the main entries during the treatment, leaving one or two active openings with a trap or station right in front of them. Whatever mice remain get funnelled to that point.
3. Close the last openings once the traps have stayed empty for several days in a row.
4. Re-inspect the following fall. Materials move, caulk splits, a new pipe gets installed. A ten-minute walk-around every September saves you from starting over.

One exception: if an opening is clearly unused (no smudges, no droppings), you can seal it right away. The lock-in risk applies to active entries.

What is a professional exclusion, and why does it cost more than a treatment?

A mouse treatment aims to eliminate the mice that are there. An exclusion aims to make sure no others get in. They are two different services, and the second costs more because it takes more time, more materials and a solid understanding of how the building is put together.

In practice, a professional exclusion includes:

• A full inspection of the building, exterior, basement, garage and attic, with a photographed list of every entry point, active or potential.
• Sealing every point with the right material for each situation: wool or mesh plus caulk, screwed-down hardware cloth, weep hole inserts, metal flashing at roof junctions, vent covers.
• Replacing weatherstrips and door sweeps where needed.
• A coordinated treatment plan, so the mice already inside are eliminated before the final close-up.
• A written guarantee on the sealing work.

What separates it from a DIY job is mostly the part you cannot see: the attic, the top of the brick walls, the roof junctions that need a ladder and familiarity with how houses around here were built. A technician who knows South Shore bungalows knows where to look before getting out of the truck.

What does it cost in Quebec?

The ranges below are what you see on the Quebec market. They vary with the size of the house, the number of entry points and how easy the attic is to reach.

• Mouse treatment only (inspection, traps or stations, one or two follow-ups): generally $250 to $500.
• Partial sealing (the five to ten main points at ground level and the garage): $300 to $800.
• Full exclusion (the whole house, including soffit, roofline and attic, with guarantee): $800 to $2,500, sometimes more for a large home with several additions.
• Treatment and exclusion combined: often a bit cheaper than the two separately, because the inspection is done once.

These figures give you an order of magnitude, not a quote. Every house is different, and a serious price is given after seeing the building. Call for an exact quote.

Frequently asked questions

Can mice get in through the toilet or a drain?

Rats can, in rare cases, through the sewer. Mice almost never do: they are weaker swimmers and the traps in the plumbing are a real obstacle. Mice found in a bathroom came in through the pipe hole under the sink or behind the toilet, not through the water.

Does spray foam work if I put steel wool in it?

Yes, that is an acceptable combination for large cavities: steel wool or copper mesh at the back, foam over it for the air seal, then caulk or a metal plate to protect the foam from sunlight. What does not work is foam by itself.

How long does a sealing job last?

A job done right with metal and a quality exterior caulk easily lasts five to ten years. The first things to give out are the moving parts: the garage weatherstrip (two to five years depending on use) and door sweeps. A quick check every fall keeps everything in order.

I rent. Who pays for the sealing?

In Quebec, maintaining the building envelope (foundation, doors, vents, soffits) is generally the landlord's responsibility. Notify them in writing at the first sign, with your photos. In most cases the landlord hires the exterminator and has the repairs done.

Should I seal even if I have not seen a mouse yet?

It is the best time to do it. Preventive sealing done in September, before the first cold nights, costs less than a treatment plus a sealing job in December. And you skip the smell, the droppings and the chewed wires as a bonus.

Block them before the first cold night

Mice do not need an open door, just a 6 mm hole and a reason to come in. On the South Shore, that reason arrives in late September with the first nights below 10 degrees. Do the walk-around now, at dusk, with your flashlight, and seal what you find with metal.

If you would rather have someone do it for you, or the mice are already in, Extermination BETA is based in Brossard and covers Longueuil, Saint-Hubert, Saint-Lambert, Greenfield Park, La Prairie, Chambly and Greater Montreal. Our certified technicians (Quebec CD pesticide permit) inspect for free, eliminate, then seal in the right order. Same-day response. Call 514-553-2070 or write to us through the contact page.