How Do You Find Mouse Entry Points Around a Property?

The Size You Are Looking For

A mouse gets through roughly 6mm — about the width of a pencil, or a gap you can slide a biro into. Young mice manage less. That is a completely different search to a rat, which needs something nearer thumb-width and leaves obvious damage getting there.

It also means you cannot do this by eye from three feet away. You need to be close, low, and using a torch held at an angle to the surface so that gaps throw a shadow.

The Difference That Catches People Out

Rats generally come into the building. Mice are frequently already in the building — living in a cavity wall, under a suspended floor, or in the void between floors — and the gap that matters is the one between that void and your kitchen.

So a property can be sealed beautifully on the outside and still have mice, because the mice are not coming from outside. That is why we look at the internal penetrations before we look at the brickwork.

Find the Run First, Then the Gap

Mice run along edges and rarely cross open floor. So look along the base of walls, behind and under the kitchen units, along the back of cupboards, and at the perimeter of a room, and follow the evidence rather than searching randomly.

The evidence: droppings collected along a line rather than scattered, a greasy smudge at a pinch point, gnawed packaging in a low cupboard, and shredded nesting material. Where the droppings stop is usually within a foot of the gap.

The Gaps That Are Almost Always There

Where the waste and supply pipes pass through the wall or the unit carcass under the sink. Around the gas or water service entry. Behind and beneath the boiler where the pipework goes through. Where the cooker hood duct passes through the wall. At the top of a stud wall into the ceiling void. Where a floorboard meets a wall and the skirting does not quite close it. Around a cable entry for the meter or telecoms.

Outside: air bricks with damaged grilles, the gap under a garage or side door, the joint where an extension meets the original brickwork, and eroded mortar at ground level.

What to Do Once You Have Found Them

Not expanding foam, and not steel wool on its own — mice chew through the first and it rusts away from the second. Gaps need closing with something they cannot gnaw: proprietary metal mesh, mortar, or a plate, correctly fitted to the material around it.

And not while there is activity. Proofing an occupied property leaves mice sealed in with you. Treatment first — £190 for a flat, £240 for a house, with the follow-up included — and then rodent proofing at £180 for up to an hour, which covers rats, mice and squirrels.