Why Mice Are Hard to Keep Out
A house mouse can pass through a gap about the width of a pencil and climb almost any rough vertical surface. That is the whole difficulty: the gaps that let them in are the ones nobody would think to look at, and they are almost never the obvious hole in a wall.
Flats above shops
The parades along Finchley Road, Ballards Lane, Burnt Oak Broadway and Golders Green Road put a commercial kitchen or a food shop directly beneath somebody's living room. Service voids, riser ducts and the gaps around waste pipes connect the two, and mice use all of them. A flat treated in isolation above an untreated shop is a flat that will be treated again, which is why we ask what is downstairs.
Interwar semis
Finchley, Mill Hill, Edgware, Whetstone and Burnt Oak are largely 1920s and 1930s houses with suspended timber floors and cavity walls. Mice get under the floor at an air brick or a service entry and then move the length of the house without ever crossing a room. Activity heard under a floor but never seen is the classic version.
The newer blocks
At Colindale and on the newer developments, mice travel the warm service risers and the gaps around pipework between flats. In a block, what your neighbours are doing matters as much as what you do.
Signs of mice
- Small dark droppings, a few millimetres long, often along a wall or in a cupboard
- Gnawing on food packaging, plastic and wiring
- A musty, sharp smell in an enclosed cupboard
- Shredded paper, insulation or fabric gathered into a nest
- Scratching or scurrying behind a kickboard or under the floor at night
- Greasy smear marks where they run the same route repeatedly
Stopping them coming back
Treatment removes the mice that are present. Keeping them out is proofing work: gaps around pipework, under doors, at damaged air bricks and where a later extension meets the original wall. In a flat or a terrace it is worth accepting that your neighbours' gaps are part of your problem.
