BuzzSkito Mosquito & Tick Control Specialists · Published August 22, 2026
Why the Garage Door Is the Biggest Hole in Your House
Mice get into an Ontario house through the garage more often than through any single opening people actually think about, and they do it at the bottom corners of the overhead door. The reason is mechanical rather than mysterious. A garage door is a heavy panel that seals against the floor with a compressible rubber astragal, and it compresses that rubber hardest where the weight is — along the middle. At each end, where the astragal runs out and meets the vertical jamb weatherstripping, the two seals meet at right angles and never quite marry. What is left is a small triangular void, open to the outside, at ground level, on both sides of an opening that is otherwise several metres wide.
Then Canadian conditions make it worse. Slabs are usually poured sloping toward the door for drainage, so the door contacts the high side first and leaves daylight on the low side. Freeze-thaw and road salt spall the concrete under the seal into pits a rubber lip cannot bridge. And twenty winters of cycling leave a hardened, shrunken astragal that has lost height it cannot give back.
A mouse needs about 6 mm. That is the whole equation. Close the door on a bright afternoon, walk outside, and look along the bottom from ground level with the sun behind the building: any daylight is an opening a mouse has already found. Do the same at the two vertical edges. It is the fastest inspection on the entire house, and it takes thirty seconds.
The stakes are higher than the garage itself. An attached garage is a staging area, not a destination: warm relative to outdoors, sheltered, often storing birdseed, pet food or bulk dry goods, and sharing a wall and a doorway with your living space. A great many autumn calls that start “we heard something in the wall” began with an animal that got into the garage in October. Sealing this door is upstream of most of the rest of the work.
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Threshold Seal or Bottom Seal? Diagnose Before You Buy
These two get shelved next to each other and solve different problems; buying the wrong one is the most common way people spend money without closing the gap.
A bottom seal — the astragal — lives in a retainer channel on the underside of the door and travels with it. Replace it when the rubber has gone hard, cracked, torn, or flattened, which on a Canadian door is usually somewhere between year eight and year fifteen. If your concrete is flat and true and the only problem is perished rubber, this is the entire repair, it costs very little, and it is installable in any weather because no adhesive is involved.
A threshold seal is a separate rubber ramp bonded to the garage floor, which the door then closes onto. It raises the sealing plane up to meet the door instead of asking the door to reach down to an uneven floor. This is the fix when the slab is sloped, spalled, cracked, or worn low in the middle — conditions no astragal can compensate for, because the gap is not on the door. It has the useful side effect of stopping meltwater, drifting snow, and blown leaves, which on a driveway that slopes toward the house is worth as much as the rodent exclusion.
The diagnostic is simple. Close the door and slide a sheet of paper under it at five points across the width. Where the paper slides freely, the door is not touching. If the failures are at the corners only, you need corner treatment. If the paper slides at several points across the middle, you have a floor problem and you want the threshold. If nothing slides anywhere but the rubber is visibly cracked, replace the astragal on general principle before it fails in January.
The Three Measurements That Decide Whether It Fits
More garage door seals get returned for fitment than for quality, and it is always one of the same three numbers.
Retainer profile. Look at the end of the seal currently in your door and note its cross-section shape. The common families are single-T, double-T (two T-shaped ribs that slide into two channels), and bead or bulb (a round rib in a C-shaped channel). A T-end will not go into a bead channel, and no amount of persuasion changes that. Photograph the end of the existing seal before you shop — a straight-on photo tells you everything a product listing needs to be matched against.
Channel width. Measure across the opening in the aluminum retainer. Seals are sold to fit specific channel widths, and a seal slightly too narrow will pull out under its own weight over a winter of thermal cycling.
Door width, plus extra. Single doors in Canadian housing are typically 8 or 9 ft, doubles 16 ft. Always buy 15 to 30 cm longer than the door and trim to fit; trimming is thirty seconds with a utility knife, whereas coming up short means the ends — the exact place mice enter — are unsealed. If your retainer itself is bent, corroded, or missing sections, replace the retainer first with an aluminum retainer track; a new seal in a warped channel will sit crooked and leak at the ends by design.
The Corner Fix Nobody Sells as a Product
Here is the part most guides skip, and it is the part that actually matters for rodents. Even a brand-new astragal on a perfect floor leaves a void at each end where it meets the vertical jamb weatherstripping. It is small, it is triangular, and it is exactly the shape and size a house mouse is looking for.
Close it in two layers. First, pack the void with copper mesh or stainless wool, pushed in tight with a screwdriver so it fills the full depth rather than plugging the face. Copper is the right choice outdoors in Canada because ordinary steel wool rusts through in a season, shrinks as it corrodes, stains the concrete, and eventually falls out and reopens the hole. Second, cover the packed void with a corner seal insert or a wedge of rubber so weather and UV are kept off the mesh and there is no exposed edge for a rodent to get its teeth around. Where the gap is larger — a chunk missing from the slab edge, a void where the frame meets a block wall — go to quarter-inch galvanized hardware cloth cut oversize and fastened into solid material.
What not to do: do not fill the corner with spray foam and call it sealed. Cured polyurethane offers a rodent almost no resistance, and a foamed corner is worse than an open one because you can no longer see the tunnel through the middle of it. Foam is a weather layer applied over a metal core, never a barrier. That rule is the same everywhere on the building, and it is the backbone of our rodent exclusion kit guide.
Finish with the sides. Vertical jamb weatherstripping shrinks, tears at the bottom, and pulls away from a settled frame — and a mouse that cannot get under the door will happily walk around it. Brush weatherstrip is worth the upgrade over rigid vinyl on any older Ontario garage: the bristles conform to a jamb that is no longer plumb, and they do not stiffen in the cold. Then treat the door between the garage and the house like the exterior door it functionally is — a proper door sweep and intact weatherstripping turn the garage into an airlock rather than a corridor.
Cold-Weather Materials: What Ontario Does to a Cheap Seal
Buy the rubber compound, not the picture on the package. EPDM and thermoplastic rubber stay pliable far below freezing, which means the seal is still conforming to your slab in February. Rigid PVC and cheap vinyl stiffen as temperatures drop, stop following the small irregularities in the concrete, and eventually crack along a fold. The failure mode is quietly perfect: the seal looks intact all winter while the corners have lifted off the floor by a few millimetres — and a few millimetres is the entire specification a mouse cares about.
Adhesives carry the same constraint from the other direction. Threshold seals bond to concrete with construction adhesive or a supplied tape system, and those generally want a clean, dry slab above roughly 10 °C to cure. A cold, damp, salt-dusted January floor is why so many threshold seals are peeling by spring. If the slab is spalled, fill and cure it with a concrete crack filler first, then bond to a flat surface with polyurethane construction adhesive, and where the frame needs mechanical fixing into masonry use proper concrete anchor screws rather than hoping tape holds through a freeze-thaw cycle.
A garage that runs damp accelerates all of this — corroding retainers, softening concrete, rotting wood jambs. If yours sweats through shoulder season, the companion problem is covered in our guide to dehumidifiers for a cold garage.
When the Cheap Fix Is the Right Fix
We would rather you spent less and closed the gap than spent more and did not. If the paper test shows the door making contact across its full width, the concrete is sound, and the jambs are straight, then a replacement bottom seal in the correct profile plus a few dollars of copper mesh in the two corners is the complete, correct, permanent job. Skip the threshold kit. It would sit on top of a floor that did not need it, add a lip your snow shovel will find every January, and close nothing that was open.
The threshold kit earns its money in three situations: a slab that slopes or has worn low, a driveway that runs water toward the door, and a bottom rail racked out of straight. Two of those are as much a water problem as a rodent problem, which makes the spend easy to justify. If the door itself is bowed or the panels no longer close square, no seal is the answer — that is a door service call. Our Canadian pest control cost guide is the useful check against the alternative: paying a pest company to keep trapping what a one-time repair excludes for good.
Do It in September, Not November
Rodent entry into Canadian buildings is sharply seasonal. Through the summer, mice live outdoors on abundant food. As overnight temperatures start holding below about 10 °C and that food collapses, they probe structures for warmth, and the indoor migration runs hard from early October into December.
That gives the garage door a deadline. Sealing in late summer means bonding adhesive to a warm dry slab, working in daylight after supper, buying from an aisle not yet converted to Christmas stock, and — most importantly — closing a garage that is still empty. Sealing in December risks trapping animals inside, and locks you out of the threshold option until spring.
If you are already hearing something in the garage, reverse the order: trap first, seal second. Run a snap-trap line from our best mouse trap guide until you get five to seven quiet nights, then do the sealing pass. And if vehicles live in that garage, know that a parked car is its own nesting site with its own damage bill — the wiring and cabin-filter side of that story is in how to keep mice out of your car. The rest of the building envelope, gap by gap, is walked through in how to keep mice out of your house this winter.
One regulatory note, since it is a pleasant one: none of this hardware is a pest control product. Seals, mesh, brush strip and adhesive need no Health Canada PMRA registration and can be installed by anyone, in any building, rentals included. Unlike anything you spray, the only homework is measuring correctly.
Frequently Asked Questions
How are mice getting into my garage?+
Under the garage door, almost always, and almost always at the two bottom corners rather than the middle. A garage door seal compresses hardest where the door is heaviest and least where it meets the vertical track weatherstripping, so the corners hold a small triangular void even when the door looks shut. Add a chipped or sloped concrete slab and that void becomes a permanent open doorway. Close the door on a bright day and look along the bottom from outside at ground level: any daylight you can see is an opening a mouse can walk through, because a mouse needs only about 6 mm.
What is the difference between a garage door bottom seal and a threshold seal?+
A bottom seal is the rubber astragal that slides into a retainer channel on the underside of the door and travels with it. A threshold seal is a separate rubber ramp bonded to the garage floor that the door closes onto, raising the sealing plane. They solve different problems. If the floor is flat and only the rubber has perished, a replacement bottom seal is the whole job and costs very little. If the slab is sloped, cracked, or worn so the door cannot make contact across its width, the threshold seal is what actually closes the gap — and it is the fix that also stops meltwater and drifting snow running in.
What size garage door bottom seal do I need?+
Three measurements, and getting any of them wrong is the most common return in this category. First, the retainer profile on the underside of your door: single-T, double-T, or bead/bulb — look at the end of the existing seal and match the shape exactly, because a T-end will not slide into a bead channel. Second, the retainer channel width, usually measured across the opening in the aluminum track. Third, the door width, plus 15 to 30 cm of extra length so you can trim rather than come up short — single garage doors typically run 8 to 9 ft and doubles 16 ft. Photograph the existing seal end-on before you shop.
Can mice chew through a rubber garage door seal?+
Yes, and they routinely do once they have found an edge. Standard EPDM or vinyl astragal is soft enough for a rodent to work through in a night or two, but the important detail is that mice rarely chew a fresh hole in the middle of a flat seal — they enlarge an existing corner gap where they can get teeth around an edge. That is why the durable fix is geometric rather than material: close the corner void with mesh or a corner insert so there is no edge to start on. Copper mesh packed into the corner before the insert goes on is the professional version of that repair.
Can I install a garage door threshold seal in winter in Canada?+
Not well. Threshold seals bond to concrete with an adhesive that generally needs a dry slab above roughly 10 degrees Celsius to cure properly, and a cold, damp, or salt-dusted floor is the single most common reason a threshold seal peels up by February. If the mice are already coming in and it is December, do the reversible fixes now — replacement bottom seal, corner inserts, brush weatherstrip on the jambs, copper mesh in the voids — and schedule the threshold bonding for a dry stretch in spring or late summer. That timing constraint is exactly why the work belongs in September, not November.
Is spray foam enough to seal a gap under my garage door frame?+
No. Bare cured polyurethane foam takes a rodent minutes, and a foamed gap is worse than an open one because it hides the hole from you while the tunnel through the middle stays invisible. Foam has exactly one legitimate role: as a weatherproofing carrier applied over a metal core. Stuff the void with copper mesh or stainless wool first, then foam or seal over it. The metal stops the teeth, the foam stops the draft. The same rule applies everywhere on a house, which is why our exclusion guide leads with metal.
Do I need to seal the sides and top of the garage door too?+
The sides matter, the top rarely does for rodents. Vertical jamb weatherstripping — the vinyl or brush strip on the door stops — is what closes the gap between the door face and the frame, and when it has shrunk or torn near the bottom it hands a mouse a route around the astragal entirely. Brush weatherstrip is worth knowing about here: the bristles conform to an uneven or out-of-plumb jamb far better than a rigid vinyl flap, which is common on older Ontario garages that have settled. The top seal is mostly a weather and energy item, though it is worth checking if you have squirrels or birds in the neighbourhood.
What temperature rating should a garage door seal have for Ontario?+
Look for EPDM or a thermoplastic rubber rather than rigid PVC. The practical difference shows up in January: quality rubber compounds stay flexible well below minus 30 degrees Celsius, while cheap vinyl stiffens, stops conforming to the floor, and cracks along the fold. A stiff seal in February is a seal that no longer touches the slab at the corners — which is the same gap you set out to close, arriving by a different route. Packaging that quotes a cold-temperature rating is telling you something useful; packaging that says only "all weather" is telling you nothing.
Related Rodent Guides
- Rodent Exclusion Kit Canada — Seal Mice Out Before October
- How to Keep Mice Out of Your House This Winter
- How to Keep Mice Out of Your Car
- Best Mouse Trap Canada — Snap vs Electronic vs Catch-and-Release
- How to Get Rid of Mice in Canada — Full Playbook
- Best Dehumidifier for a Cold Garage in Canada
- Pest Control Cost Canada — What Professionals Charge