When a mouse or rat turns up indoors, the first question is usually how it got in. In most homes, the answer is a small gap that has been there for years without anyone noticing. Finding and sealing these entry points is the most effective long-term way to keep rodents out, and much of it can be done with basic tools and materials. This guide covers the most common entry points in UK homes and how to block them properly.
How small a gap can a rodent use?
It is easy to underestimate how little space a rodent needs. A house mouse can get through a gap of around 6mm, about the width of a pencil. A young brown rat can manage a hole of roughly 15mm. Both are good climbers, and rats are strong swimmers that can use damaged drains and sewers to reach a property. If you can fit a pen or your little finger into a gap, assume a rodent could use it.
Gaps around pipes and cables
Wherever a pipe, cable or waste outlet passes through an outside wall, there is a chance of a gap around it. Check where the gas pipe enters, where the outside tap is fitted, around the boiler flue, and behind the washing machine and dishwasher where waste pipes leave the building. Look under the kitchen sink too, as the opening in the cupboard base is often much larger than the pipe going through it.
The best way to fill these gaps is with wire wool or a stainless steel mesh packed tightly into the space, finished with a sealant or exterior filler over the top. Rodents can chew through expanding foam and silicone on their own, so the metal layer is important.
Air bricks and vents
Air bricks are there to ventilate the space under suspended floors and must never be blocked. However, older clay air bricks often crack, and some have holes large enough for a mouse to fit through. Replace damaged air bricks, or fit a fine stainless steel mesh cover over the outside. This keeps the airflow while stopping rodents getting under the floor.
Doors, including the garage
Look at the bottom of every external door, including back doors, side doors and the door between the house and an integral garage. If you can see daylight underneath, a mouse can get in. A brush strip or rubber threshold seal is cheap and easy to fit. Garage doors are a particular weak point because the bottom seal often wears away, and the garage is frequently full of stored items that give rodents somewhere to settle before moving into the house. Keeping the floor clear makes any activity easier to spot. Our garage waste clearance page explains what's involved if the space needs a full clear-out.
The roofline and eaves
Mice and rats climb drainpipes, ivy and rough walls with surprising ease. Gaps where the roof meets the wall, missing mortar on the gable end, and broken soffits or fascias all give access to the loft. From the ground, look for damaged boards and missing pointing. Cutting back ivy and climbing plants that reach the roofline removes an easy route up. Roof repairs should be done by a qualified roofer, but spotting the problem early saves a lot of trouble later.
Drains and sewers
Rats often come up from the sewers through broken drains, loose inspection covers or old pipework that has been disconnected but not capped. If rats keep appearing in the same spot in the garden or near a drain, the drains may be the cause. Make sure inspection covers sit flat and are not cracked, and that any unused pipes are capped. Many councils can offer advice on drain-related rat activity. Salford City Council's guide to rats explains the signs to look for and what to do next.
Gaps inside the house
Once a rodent is inside the walls or under the floor, it will look for gaps into living spaces. Check behind kitchen kickboards, around the boxing that hides pipework, and where the skirting board meets the floor. Gaps around radiator pipes coming up through floorboards are a common route. Smaller internal gaps can be filled with wire wool and a decorator's filler.
Choosing the right materials
- Wire wool or stainless steel mesh: packed into gaps around pipes and cables as the main barrier
- Exterior sealant or filler: used over the top of the metal to hold it in place and keep out draughts
- Mortar: for repointing gaps in brickwork
- Brush strips and threshold seals: for the bottom of doors
- Mesh air brick covers: for damaged or oversized vents
Avoid relying on expanding foam alone. It is quick to use but easy for rodents to chew through.
Seal after treatment, not before
If you already have rodents inside, sealing every gap at once can trap them inside the walls or under the floor, which can lead to unpleasant smells and more damage. In this case, it's best to have the problem treated first. The British Pest Control Association's mouse advice covers the signs of an infestation and how a professional will approach it. Once the activity has stopped, seal the entry points to stop it happening again.
Don't forget what's inside
Sealing gaps works best when the inside of the home is also less inviting. Clutter in lofts, spare rooms and outbuildings gives rodents somewhere to nest if they do find a way in. If you're preparing an empty property for winter, such as a relative's home after a bereavement, it's worth clearing stored items so the house doesn't sit full and unheated over the colder months. Our pages on bereavement house clearances and flat and apartment clearances cover how these jobs are usually handled, and there's more local information on house clearances in Salford and waste removal in Salford.
A careful walk around your home with a torch, some wire wool and a tube of sealant is one of the best investments you can make before winter. Most of the gaps rodents use are small, cheap to fix and easy to miss, until something comes through them.