How to Find Out Where Mice Are Getting In

Rodents are not random. They follow the same routes every night, hug walls rather than crossing open floor, and enter through a small number of predictable points. Which means the problem is findable — and once it's found, it's solvable.

Here is the walkthrough we recommend, in the order we would do it ourselves.

The rule that decides everything: 6 millimetres

An adult house mouse can pass through a gap of roughly 6 mm — about the diameter of a pencil. A young rat needs around 12 mm. If a gap admits a pencil, treat it as a door.

Step 1 — Read the droppings

Droppings are the most reliable evidence you have, because they tell you three things at once.

Where. Droppings accumulate along travel routes and near food. Find a concentration and you've found a highway, not just a stop.

What. Mouse droppings are 3–6 mm, dark, and pointed at both ends — roughly the size and shape of a rice grain. Rat droppings are 12–18 mm and blunter. Knowing which one you have changes the gap size you're looking for.

When. Fresh droppings are dark, soft and slightly glossy. Old ones are grey, dry and crumble under pressure. If everything you find crumbles, you may be looking at a historical problem rather than a live one.

Check: behind and under kitchen appliances, inside the plinth gap beneath cabinets, in the back corners of under-sink cupboards, along skirting boards, in the loft near the eaves, and in the corners of the garage.

Step 2 — Look for rub marks and runways

Rodents have poor eyesight and navigate by touch, running with one flank against a wall. Over weeks, the grease and dirt in their fur leaves a faint dark smear along that route — a rub mark.

Rub marks are easy to miss and enormously useful. Shine a torch at a low angle along skirting boards, pipework, joists and the edges of cupboards. A grey-brown smudge along a consistent line is a runway. Follow it in both directions and it will lead you to both the food source and the entry point.

In dusty areas — a loft, a cellar, behind a garage shelf — you may also find tail-drag marks and footprints. A thin dusting of flour left overnight across a suspected route is an old trick that still works perfectly.

Step 3 — Listen at night

Rodents are most active in the two hours after dusk and again before dawn. Sit somewhere quiet with the house lights off.

• Scratching or scurrying in the ceiling points to loft or first-floor void activity, usually entering at roof level via eaves, soffit gaps or where a cable enters.
• Scratching inside a wall means the cavity is being used as a vertical route between floors.
• Gnawing — a slow, rhythmic grinding — means they're enlarging something. Follow that sound; it's often at or near the entry point.

Note where you hear it and at what height. It narrows the exterior search enormously.

Step 4 — Walk the exterior perimeter

Go outside and walk the full footprint of the building slowly, twice: once looking at ground level, once looking up.

At ground level, check:

• Where pipes, waste outlets and cables enter the wall — the seal shrinks and cracks over time, and this is the single most common entry point
• The threshold gap under garage and shed doors, especially where the seal has worn at the corners
• Air bricks and ventilation grilles with missing or broken mesh
• Cracks in mortar at the base of the wall, and any gap where a driveway or patio has settled away from the building
• Drain covers that no longer seat flush

Looking up, check:

• Gaps where the soffit meets the wall, and any lifted or missing soffit panel
• Roof tiles displaced at the verge or ridge
• Where a satellite cable or aerial lead crosses into the roof space
• Overhanging branches and climbing plants touching the roofline — rodents are excellent climbers, and a branch is a bridge

Step 5 — Find the nest, or where it would be

Rodents nest close to food and water, in dark, undisturbed, insulated places. The shortlist is short:

• Behind or beneath a fridge, freezer, oven or washing machine — warm, dark, rarely moved
• Inside the void behind kitchen kickboards
• In loft insulation, especially near the water tank
• Inside stored cardboard, in a garage or under stairs
• In a garden shed, behind bagged compost or birdseed

Nests are shredded material — insulation, paper, fabric, cardboard — formed into a loose ball roughly the size of a grapefruit. Finding one confirms a resident population rather than an occasional visitor, and it changes your plan: residents need to be evicted, visitors just need the door closed.

Step 6 — Turn the findings into a plan

Once you know the entry points and the routes, three things happen, in this order.

Seal. Close every gap that admits a pencil. Steel wool packed into a gap and sealed with mortar or expanding foam works, because rodents won't chew steel. Foam alone will simply be chewed through. Fit bristle strips to door thresholds, replace broken air-brick mesh, and re-seal pipe entries.

Remove the reason. Decant dry food, pet food and birdseed into rigid sealed containers. Fix dripping taps and standing water. Move cardboard storage off the floor.

Deter the route. This is where electronic deterrence earns its place — not as a substitute for the first two steps, but as continuous pressure on the routes and voids you can't fully seal. Position devices in the rooms where you found droppings and rub marks, with clear line of sight along the runway rather than tucked behind furniture. Our guide to where to place an ultrasonic repeller covers this room by room.

An ultrasonic device works on the space the sound reaches. It doesn't reach inside a wall cavity or through a closed cupboard door — which is exactly why sealing comes first, and can't be skipped.

We explain the physics openly in do ultrasonic pest repellers actually work.