How do I find where mice are getting into the house?
A systematic survey of a house at rodent height, the real gap sizes involved, and the order to seal so you do not shut mice inside.
- Difficulty
- intermediate
- Time
- 3 hr
- Read
- 5 min
- Safety
- caution
Short answer
A mouse gets through a 6mm gap — the width of a pencil. Survey at floor level outside and inside with a torch, working round the whole perimeter: under the front and back doors, around every service pipe and cable, at airbricks, behind the kitchen units, around the boiler flue, and where the extension meets the original house. Seal from the outside inwards, leaving the interior last.
Trapping without sealing is a permanent trapping programme, so the survey is the part that ends the problem. Mice do not need a hole — they need a gap, and a house has dozens of them. The good news is that they are in predictable places, because they are where two building trades met.
Safety
What you'll need
- A powerful torch
- Stainless steel or copper wool, and a fine metal mesh
- Filler, mortar or an exterior sealant
- Expanding foam, only as a backing behind wire wool (optional)
- Gloves, mask and knee pads
- A mirror on a stick, for behind units (optional)
Step by step
- Do the outside first, at ground level, all the way round.Get down to the level of the wall base with a torch and work methodically. Look where the render stops, where the damp-proof course line is, at the junction of an extension and the original wall, and at every downpipe base.
- Check every service penetration.The gas supply, the water main, the electricity cable, the overflow, the waste pipes, the boiler flue and the condensate pipe. Almost every one is a hole drilled oversize and imperfectly filled. This is where most mice come in.
- Look at the airbricks and subfloor vents.Cracked, missing or with the mesh perished. They must stay open for ventilation, so the fix is a fine stainless mesh cover, never filler.
- Check under the doors and at the thresholds.A gap under an external door of more than about 5mm is a route. A brush strip is not enough on its own; a proper threshold seal or a metal kick strip is.
- Look at the roofline and where a lower roof meets a wall.Mice climb rough render, cavity wall ties and pipework readily. Gaps at the eaves, missing bird combs at the tile line, and the point where an extension roof abuts the house are all routes into a loft or a cavity.
- Move inside and go behind the kitchen units.Take the plinths off. Behind and under the units is the most productive place in the house: pipe entries through the floor, the gap where the units meet the wall, and the space behind the washing machine and the dishwasher.
- Follow the pipework and cable runs.Where a soil pipe or a heating pipe passes through a floor or a wall there is nearly always a gap. Under the bath, in the airing cupboard and where a boiler pipe drops into the floor are the classics.
- Check the meter cupboards and the consumer unit.Gas and electricity meter boxes are usually let into the external wall with a large hole behind them, and the seal is frequently missing entirely.
- Read the evidence rather than guessing.Smear marks, droppings concentrated at one point, a scatter of insulation, or a dusting of flour showing footprints. Evidence at a gap converts a maybe into a definite.
- Seal from the outside inwards, and leave one route until last.Closing everything at once with mice inside leaves them trapped, which means damage and a smell. Seal the exterior, trap indoors until activity stops, then close the last internal gaps.
- Use materials they cannot chew.Stainless steel or copper wool packed tightly into the gap and held with filler or mortar. Fine stainless mesh over anything that must stay ventilated. Expanding foam and silicone alone are chewed through in days.
- Re-survey after a month.New activity means a route you missed. Mice will find the one gap you did not, so the second pass is part of the job rather than a sign of failure.
Tips
- Carry the pencil test in your head: if a pencil fits, a mouse fits. It settles almost every 'surely not that one' argument.
- Take the kitchen plinths off and photograph everything behind them before you start. It is the single highest-yield twenty minutes of the whole survey.
- Do the survey in autumn. Mice come in when it gets cold, and sealing in September prevents a winter you would otherwise spend trapping.
Common mistakes
- Filling gaps with expanding foam alone — Foam is soft and mice gnaw straight through it, often using it as nesting material. It is only useful as a backing behind packed wire wool, never as the barrier itself.
- Blocking an airbrick to keep mice out — It causes damp and, where there is a fuel-burning appliance, can be dangerous. Airbricks must stay open — cover them with fine stainless mesh instead.
If it doesn't work
Mice still appearing after sealing everywhere obvious
Cause: A missed route, usually a service pipe or the void behind kitchen units — Fix: Dust flour along skirtings overnight and read the tracks in the morning. Follow them back — they lead to the entry.
Scratching inside a wall after sealing outside
Cause: Mice sealed inside the cavity — Fix: Open the last exterior route temporarily, trap indoors, and only close it once activity has stopped for a fortnight.
Mice in an upstairs room with no ground-floor activity
Cause: Cavity or pipe boxing acting as a vertical route, or entry at the roofline — Fix: Check the eaves, the soffit and the point where a lower roof meets the wall, and seal the pipe boxing at floor level in the room itself.
Questions people ask
Can mice really get through a hole that small?
Yes. An adult house mouse can pass a gap of around 6mm because the skull is the limiting dimension and the body follows. Young mice get through less. That is why the survey has to be done at the level of small gaps rather than looking for holes.
Do I need to seal inside as well as outside?
Sealing the exterior stops new arrivals; sealing inside stops movement between rooms and into cupboards. Do the exterior first and thoroughly, and treat the interior as containment while you trap.
Is steel wool or copper wool better?
Either works. Ordinary steel wool rusts and can stain masonry, so stainless steel or copper wool is the better choice outdoors. Pack it tight — loosely stuffed wool is pulled out.