Mice are determined and opportunistic pests. Most people don’t appreciate just how adept mice are at squeezing through narrow gaps to get into their homes. One of the reasons mice are so good at getting into places they aren’t wanted is because they can compress their bodies, bones and all, enabling them to pass under doors, even when there’s only a small gap for them to fit through. Young mice can pass through gaps and openings as narrow as 5 mm. In other words, if a regular pencil can fit through a gap, so can a small mouse.
If you know there are mice in your neighbourhood and want to keep them out of your house, you probably already know that you should keep all your doors and windows closed, so you don’t provide them with easy access to your property. But mice are cunning creatures. Just as they will exploit any gap in your home’s exterior to get inside, they can use every opening to their advantage once they are in your home. Like other rodents, mice can flatten their rib cages and make themselves as flat as possible to squeeze through gaps under doors; just keeping them closed isn’t always enough.
To be absolutely sure that a mouse won’t be able to get through the gap between your doors and the floor, you need to install some kind of physical barrier. Draft excluders work very well for this purpose; mice don’t have any tricks for getting past those.