Inspect the garage door threshold, which is the strip of rubber adhered to the ground at the bottom of the door. Mice will need as little as 1/4 inch of space between the door and the ground to fit through. If the threshold is damaged or missing, put in a new one.
Thoroughly clean the bottom of the door and make sure the ground is also dry and clean.
Center the threshold on the floor between the tracks on each side of the garage door entrance. Cut the threshold with a utility knife or another straight edge to ensure that it extends the entire width of the opening, and rests securely against both sides of the garage door.
Lower the door and place the 45-degree angle section of the threshold against the inside of the bottom of the door.5
Label the inside edge of the threshold on the ground with chalk. Open the door and trace the outer edge of the threshold as well.
Remove the threshold and apply two beads of adhesive formulated for use with masonry or concrete onto each line you marked on the ground about 3/4-inch back from each line. Run a zigzag bead of adhesive between the first beads all the way along the width of the opening.
Let the adhesive cure according to the manufacturer’s directions and then lay the threshold in place over it, aligning the edges with the lines you marked on the ground.
Lower the garage door. Wipe up excess adhesive with a damp cloth.
Install metal door flashing on the interior bottom and sides of wooden garage doors. The flashing protects the edges of the door so that the mice cannot chew through the wood.
Inspect the door for holes. Temporarily block small openings by stuffing steel wool or screen wire into them tightly. For a long-term repair, mix quick-drying joint compound into a wad of copper mesh, then push the mixture into the hole. Use a putty knife to fill and smooth the patch into the door.
Repair holes larger than 3 inches in diameter by covering them with 1/4-inch woven/welded hardware cloth. Fill the hole with joint compound or foam caulk.
Stuff holes along the perimeter of the garage’s foundation and walls using a copper mesh and concrete mixture.
While you must repair areas where mice are entering your garage, do not seal these points until the mice have been eliminated or populations significantly reduced from your garage. You can find effective traps and baits at any hardware store.
Do not use hardware cloth that is not labeled as woven/welded. Other hardware cloths will break easily and will not provide a suitable barrier to prevent mice from entering the garage.