How Small a Gap Can a Mouse Get Through?

Why the number is so small

An adult house mouse is roughly seven to ten centimetres in the body, with a tail about as long again, and weighs somewhere between 15 and 25 grams. But its length is not what decides whether it gets in.

What decides it is the skull, because that is the only rigid part. The ribcage flexes and the body has no barrel to it, so if the head passes through an opening the rest follows. In practice that puts the working figure at about the width of a pencil.

A young mouse needs less again, which matters because a house with an established population is producing young continuously.

Photographs on this page are for illustration purposes only.

Where those gaps actually are

Go round the outside of the building slowly, at knee height and below first, then look up. The list is shorter than people expect and it is much the same house to house.

• Under the doors. A door that has dropped, or a worn threshold, is the single commonest one. Hold a torch on the outside at night and look from inside.
• Pipe and cable penetrations. Waste pipes, the boiler flue, the condensate pipe, gas and water service entries, broadband and alarm cable drilled through after the house was built. Almost none of them are sealed properly around the pipe.
• Airbricks. A cracked or broken corner is a direct route into the floor void.
• Where the kitchen units meet the floor, and where the pipes come up through the boards behind them.
• The eaves and soffits. Mice climb well. A gap at the wall plate, a lifted tile or an open soffit is a front door, and it is why so much mouse noise is heard in an upstairs ceiling.
• The garage, particularly where it is integral and has a connecting door, and where the up-and-over door meets the frame.

A test you can do tonight

If you want to know whether a gap is being used rather than whether it exists, dust a thin layer of plain flour or talc across it and along the floor either side, and look in the morning. Footprints and a tail drag across flour are unambiguous, and they tell you which of six candidate gaps is the live one.

The other simple check is to clean an area completely, then look again in two or three days. Fresh droppings on a surface you know was clean is current activity; the pile you found originally could be six months old.

What to do with what you find

Do not seal everything up before anything has been treated, because a building sealed with mice already inside keeps them inside.

Note where the gaps are, photograph the ones you can, and tell us when you ring. It turns a general survey into a targeted one, and the proofing is part of the job rather than a separate quote.

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Other Mice Questions

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