Key takeaways
- A mouse only needs a gap about a quarter of an inch wide; a rat needs half an inch.
- Five routes cover almost every infestation: door line, frame corners, vents, pipe holes and food left out.
- Seal to the actual gap size - a universal strip either holds the door up or leaves the gap open.
- Mice chew foam and expanding foam, but they will not chew copper mesh or coarse steel wool.
- Deal with residents before you seal, and keep poison out of a garage where pets and children go.
A mouse does not need an open door; it needs a gap about the width of a pencil, and a garage door that sits a quarter of an inch off the slab gives it one. Autumn is when they move, looking for somewhere warmer than a hedgerow, and a garage with a bag of birdseed in the corner and a stack of cardboard boxes along the wall is exactly what they are looking for. The good news is that the same sealing job that keeps the cold out keeps the mice out, and it is a weekend afternoon's work.
How they get in
Five routes account for almost every garage infestation. The gap under the door is the first and the largest. The corners where the door frame meets the wall are second, because trim hides a gap you cannot see. Vents and dryer outlets are third, since a louvred vent with a broken blade is an open window. Pipe and cable penetrations through the wall are fourth - the hole is always cut bigger than the pipe. And the fifth is not a gap at all: an open bag of birdseed, a bin without a lid, or pet food left in a bowl overnight is an invitation that no amount of sealing will cancel out.
The gap test, and what to seal each gap with
Slide a piece of cardboard under the closed garage door. If it moves freely, that is your gap, and it is usually a quarter of an inch or more on an older door. A quarter inch is enough for a mouse; half an inch lets a rat through. Then check the other four with a torch: frame corners, vents, pipe holes, and the edges of the door itself. For the floor line, fit a threshold seal sized to the actual gap rather than a universal strip, because a seal that is too thick holds the door off the floor and one that is too thin leaves the gap. For pipe holes, push coarse steel wool or copper mesh into the hole and cover it with sealant - mice chew through foam and expanding foam alone, but they will not chew through copper. For vents, fit a galvanised mesh screen with a quarter inch grid. For frame corners, foam backer rod plus a bead of sealant, then trim.
| Route | The sign | What to seal it with |
|---|---|---|
| Under the garage door | Cardboard slides under the closed door | Threshold seal sized to the gap |
| Frame corners | Trim hides a gap you cannot see | Backer rod plus sealant, then trim |
| Vents and dryer outlets | A broken louvre blade, or open mesh | Galvanised screen with a quarter inch grid |
| Pipe and cable holes | A hole cut bigger than the pipe | Copper mesh or coarse steel wool, then sealant |
| Food and shelter left out | Birdseed, bins, cardboard boxes | Lidded containers, plastic totes off the floor |
Take away the welcome mat
Sealing stops them getting in; tidying stops them wanting to. Store birdseed, grass seed and pet food in plastic containers with lids rather than bags, sweep up spilled seed under the feeder, and take food waste out of the garage rather than leaving it overnight. Cardboard boxes are nesting material - use plastic totes with lids, and keep them off the floor on shelves or pallets. Move leaf piles and firewood away from the garage wall, since both are dry shelter, and check drip trays and buckets for standing water.
If they are already inside
Deal with the residents before you seal, or you are locking them in. Snap traps are the reliable tool: place them along the wall rather than in the open, because mice travel with one flank against a wall, bait with peanut butter and check them daily. Avoid poison in a garage, for two reasons that matter more than efficiency - pets and children can reach it, and a poisoned mouse usually dies inside a wall, where the smell lasts for weeks. Glue traps are a poor choice in a space where birds or pets might blunder into them. Once the activity stops for a week, seal the entry points, because a sealed garage with a resident mouse is a smell waiting to happen.
Why the door line matters most
The same quarter inch gap that lets a mouse in also lets in cold air, blown snow, leaves and road grit, and it is the gap that makes a garage feel colder than it is. Sealing it is one job with four benefits, and it is the reason we say the floor line is the part of a garage that is worth doing first: the opening gets the attention, but the line at the bottom does the work.
The same gap lets in cold air, blown snow and leaves, which is why we treat it as one job: keeping snow out of a garage and fall garage prep both start at the same line.
The bottom line
Find the five routes, seal the floor line and the pipe holes, screen the vents, and put the food away. Mice need a gap the width of a pencil - close the gaps and the garage stops being a winter address.
That one comes up often, and Garage Bug Screens: Mesh, Magnets & What Keeps Flies Out answers it properly.
Related reading
- How to keep snow out of your garage (without blocking airflow)
- Fall garage prep: keep leaves, rain and pests out
- What size garage screen door do I need?
The screen we make is our 16x7 ft sliding garage screen door - it keeps the insects and leaves out of the opening, and we will say plainly that the floor line and the pipe holes are what keep the mice out.