Air bricks are a common route into suspended timber floors, cellars and voids beneath older UK properties. This guide to proofing air bricks explains how to deny access to rats, mice and insects without making the costly mistake of stopping the ventilation your property needs.
An air brick is not an unwanted hole in the wall. It is part of the building’s ventilation system, allowing air to move beneath a suspended floor and helping to control moisture. The job is to fit pest-resistant protection over the opening while keeping enough free airflow. Blocking it completely with mortar, expanding foam, tape or a solid board can lead to damp timber, mould, musty smells and, in serious cases, decay.
Why air bricks attract pests
Air bricks sit at ground level, often beside planting, patios, wheelie bins or external storage. They offer darkness, shelter and a direct route into the subfloor void. Once underneath, rodents can travel around the property unseen, chew pipes and cables, disturb insulation, and find gaps around service penetrations or floor edges.
Mice are particularly good at exploiting damaged vents and loose grilles. A gap of around 6mm can be enough for a young mouse, while rats need a larger opening but can gnaw weak plastic and poorly fixed mesh. Wasps, spiders, beetles and other crawling insects may also use unprotected vents, although insect-proofing needs more careful consideration because very fine mesh restricts airflow and blocks with dust.
Do not assume an air brick is the only entry point. If you have signs of rodents indoors - droppings, gnawing, scratching, greasy rub marks or disturbed insulation - inspect the full external perimeter. Gaps around pipes, damaged drains, broken pointing, open weep holes and ill-fitting doors may need attention as well.
Inspect before proofing air bricks
Start outside in daylight and inspect every air brick, not only the one nearest the suspected activity. Look for cracked clay units, broken plastic grilles, missing slats, chew marks, loose mortar and soil or leaf build-up. An air brick that is partly buried by raised soil, a new patio or decking needs particular attention. The opening must not be covered or allowed to drain water towards the wall.
Then establish what type of ventilation you are dealing with. Traditional clay air bricks usually ventilate a suspended timber floor. Modern plastic telescopic vents may pass through a cavity wall to an underfloor void. Some grilles serve a cellar, utility area or extractor route instead. Do not fit mesh where it could obstruct a flue, combustion air supply or a purpose-designed mechanical ventilation outlet.
If practical, inspect the void using a torch through the vent or from an existing access hatch. You are looking for nesting material, droppings, standing water, damaged air ducts and evidence that a vent has become disconnected internally. If the void is tight, unsafe or contains suspect electrical wiring, do not climb in. A pest controller or suitable building professional can assess it safely.
Choose a cover that keeps air moving
For rodent proofing, purpose-made metal air brick covers or stainless-steel mesh are normally the right options. They should be corrosion-resistant, firmly fixed and sized to cover the whole accessible opening without crushing or sealing the vent face.
Mesh aperture matters. For mouse control, a welded stainless-steel mesh with openings no larger than about 6mm is commonly used. This is small enough to prevent most mice passing through while still permitting airflow. Larger aperture mesh may deter rats but can leave a mouse route. Avoid lightweight chicken wire: it is easy to bend, can rust and does not provide reliable mouse proofing.
A proprietary air brick cover can be neater and quicker to fit, particularly where there are several standard-sized vents. Choose a product designed for ventilation, rather than a general-purpose grille with unknown airflow performance. On listed buildings, period properties or properties with distinctive clay vents, a discreet mesh installation may be more appropriate, but it still needs to be secure.
Plastic grilles can work where they are specifically made for pest resistance, but assess the location. Low-level vents beside bins, gardens or known rat activity are better protected with metal. Rodents will test vulnerable materials repeatedly.
Do not use insect mesh as a default solution
Very fine insect mesh can look like a complete answer, but it may severely reduce the free area of an air brick. It also traps dust, fluff, spider webs and leaves, requiring regular cleaning. Where insects are the concern, identify the species and source first. A wasp nest in a wall cavity, for example, needs a different approach from occasional insects entering a subfloor vent.
If fine mesh is necessary, use it only where adequate ventilation has been confirmed and make inspection part of the maintenance plan. In many cases, rodent-grade mesh plus targeted insect treatment around the external area is the more sensible balance.
How to fit an air brick guard properly
Clean the face of the air brick first. Remove loose debris, old adhesive and vegetation, but do not force material deeper into the opening. Cut mesh with enough overlap to cover every slat and edge of the opening. Sharp mesh edges should be folded back or covered so they cannot cause injury during future maintenance.
Fix the guard to sound masonry around the vent, not just to brittle clay slats or a thin plastic grille. Stainless-steel screws and suitable masonry fixings are a reliable choice where the surrounding brickwork is sound. For uneven faces, a purpose-made frame or cover may give a more secure fit than trying to pull mesh tight with a few screws.
The finished guard should sit flat, with no lifted corners or gaps at the sides. Check it cannot be pushed inwards by hand. A rat does not need to remove the whole cover; it only needs enough movement to create a gap. Make sure the vent path remains open behind the guard and that no sealant has filled the louvres.
Where an air brick is damaged beyond a simple cover, replacement is usually the better repair. Match the size and ventilation function of the original unit, and make good surrounding mortar without obstructing the opening. If the vent is below external ground level or constantly fills with water, the underlying level, drainage or paving issue must be corrected. A cover alone will not solve it.
Common proofing mistakes that cause bigger problems
The most frequent error is sealing an air brick completely because pests have been seen nearby. Underfloor ventilation is there for a reason, especially in timber-floor properties. Reducing it can create a moisture problem that costs far more than the original pest issue.
Another mistake is proofing only one visible vent. Air moves through a network of vents around the building, and rodents will use the weakest point. Inspect the front, rear and side elevations, including vents hidden behind shrubs, stored materials and decking.
It is also unwise to fit mesh over an active rodent route without considering what is already inside the void. If rodents are established beneath the floor, blocking their preferred exit may encourage them to find a route upwards into the property. Control the population with appropriate traps, baiting by a competent person where suitable, or professional treatment, then proof the access points. In many cases, treatment and proofing should happen together rather than as separate jobs.
Finally, do not ignore nearby harbourage. Dense planting against walls, overflowing bins, bird food, pet food, compost and clutter all give rodents a reason to remain close to the building. Proofing works best when food, shelter and access are reduced at the same time.
Check your work and maintain it
After fitting guards, inspect them again after a few days and following heavy rain. Look for disturbed mesh, fresh gnawing, soil build-up and leaves caught against the cover. Repeat checks every few months, particularly in autumn when rodents start seeking warmer shelter.
Keep at least a clear strip around low-level vents where possible. Do not stack timber, bags of compost or garden furniture directly against the wall. If you notice persistent damp floors, a musty smell or condensation after proofing, review whether airflow has been reduced and seek advice before making further changes.
A properly fitted metal guard is a small repair, but it protects a part of the property pests rely on precisely because it is overlooked. Keep the air moving, remove the route in, and deal with any activity around the building before it becomes an indoor problem.