A bait station catches a rodent after it is inside your building. By then it has already run across a pallet of raw material, left droppings on a walkway and gnawed a bag in the store. Exclusion works earlier: it keeps the rodent outside the wall in the first place, where it belongs. This is why every experienced food safety auditor treats physical proofing as the primary rodent control and baiting as the backstop, and why a facility relying on bait alone is treating the symptom instead of the cause. It is why proofing sits ahead of baiting in our rodent control programmes.
The two species you are keeping out are the rats Rattus norvegicus (the brown or Norway rat) and Rattus rattus (the roof rat), and the house mouse Mus musculus. They are covered in detail in the pest species guide. Their behaviour dictates where you proof: R. norvegicus comes in low, through drains and under doors; R. rattus climbs, so it exploits roof and eaves gaps; M. musculus exploits everything because it is small enough to fit almost anywhere.
Key takeaways
- A mouse passes through a 6mm gap, a pencil's width; a young rat through about 12mm. Proof below these at every opening.
- Doors are the number-one failure: fit and maintain metal-backed sweeps and brush strips that meet the threshold with no light showing.
- Drains, dock levellers and service penetrations are the next tier of entry points and are often overlooked.
- Exclusion beats baiting because it stops entry before contamination, with no carcass risk inside food zones.
- A proofing survey walks the whole envelope, ground level and above, and every defect gets a corrective action with a close-out.
The 6mm pencil-width rule
Rodents are compressible. A house mouse can flatten its skull and body to squeeze through a gap of around 6mm, which is roughly the diameter of a standard HB pencil. That is the practical field test: if a pencil slides under a door or into a gap, a mouse can follow it. A young rat needs about 12mm. Rodents also gnaw, so a gap slightly under the limit in soft material will be enlarged; proofing materials must be gnaw-resistant, which means metal, concrete, or heavy-gauge mesh, not foam or plastic that a rodent chews through in a night.
Doors, sweeps and brush strips
Doors are where most rodents get in, because doors move, wear and get propped open. The controls:
- Door sweeps. Fit a metal-backed sweep or threshold seal to every external personnel door so the gap at the bottom closes to below 6mm across the full width. Rubber-only sweeps are gnawed through; a metal or bristle-and-metal sweep resists gnawing. When the door is shut, no daylight should show along the threshold.
- Brush strips. Roller shutters and larger doors rarely seal flat, so fit brush strips along the bottom edge and, where needed, the sides, to close the gap where the shutter meets an uneven floor. Keep the bristles dense and replace them when they wear thin or splay.
- Self-closers and discipline. A perfectly sealed door left standing open is useless. Fit self-closers to external doors and manage the site habit of chocking doors open for ventilation, especially at intake and dispatch.
- Fit and condition. Warped doors, dropped hinges and worn thresholds all reopen gaps that were once sealed. Proofing is not a one-off; it is inspected and maintained.
Drains, dock levellers and gulleys
Rattus norvegicus is a strong swimmer and a natural user of the drainage network, so drains are a primary route into food premises and one that gets forgotten because it is below the eyeline. Fit rodent barriers or one-way non-return flaps to drains and gulleys, keep gulley covers and gratings intact and correctly seated, and screen open drain ends with gnaw-resistant mesh. A cracked or missing drain cover is an open door.
Dock levellers are a classic weak point. The leveller pit and the gaps around the raised plate and side seals leave openings a rodent walks straight through when the dock is not in use. Fit dock shelters, side and rear seals, and where the design allows, brush or foam-backed metal seals around the leveller, and keep dock doors closed when there is no vehicle on the bay. Dock proofing is a standing item on our manufacturing pest control inspections for exactly this reason.
Service penetrations and the building envelope
Every pipe, cable, conduit and duct that passes through an external wall leaves a gap around it, and those gaps add up. Seal service penetrations with gnaw-resistant material: mortar, metal collars, or stainless wire wool packed and capped, not expanding foam alone, which rodents chew out. Check expansion joints, damaged brickwork, air bricks and vents, and screen any vent or opening larger than the gap limit with fine gnaw-resistant mesh.
Do not forget the vertical dimension. Rattus rattus climbs walls, pipes and cables and enters through eaves, roof gaps, damaged soffits and rooftop services. A proofing survey that only looks at ground level misses the roof rat entirely. Trees and shrubs touching the building give climbers a bridge, so keep vegetation cut back from the wall line.
| Entry point | Species most likely | Proofing measure |
|---|---|---|
| Under personnel doors | Mus musculus, R. norvegicus | Metal-backed door sweep, gap below 6mm, no daylight |
| Roller shutters | All | Brush strips, dense bristle, self-closing habit |
| Drains and gulleys | R. norvegicus | Non-return flaps, intact covers, meshed ends |
| Dock levellers | All | Dock seals, side and rear seals, doors closed when idle |
| Service penetrations | Mus musculus | Mortar, metal collars, wire wool packed and capped |
| Eaves and roof | Rattus rattus | Screen gaps, repair soffits, cut back vegetation |
Why exclusion beats baiting
Baiting is reactive. It waits for a rodent to be inside, then relies on that rodent finding and eating a bait before it does damage. In the meantime the rodent contaminates. Worse, a poisoned rodent that dies inside the building creates a carcass, odour and secondary insect problem in the very food zone you are protecting, which is exactly why internal stations use non-toxic monitoring only, as covered in the bait station mapping guide.
Exclusion is preventive. Seal the building and there is no rodent to bait, no contamination, no carcass. This is the logic auditors apply: physical proofing is the primary control, and the external bait line plus internal monitors are there to catch and warn about the rare rodent that gets past the proofing and to prove the exclusion is working. A facility whose only answer to rodents is more bait will be marked down under any credible food and beverage scheme.
Surveying and maintaining the proofing
Proofing decays. Sweeps wear, seals perish, mortar cracks, a forklift clips a dock seal, a contractor drills a new penetration and leaves it open. So a proofing survey is not a one-off installation but a recurring inspection of the whole external envelope, ground level and above, against the gap limits. Every defect found gets a corrective action: what, who, by when, and photographic close-out evidence, the same discipline described in the one-click audit pack. A proofing inspection checklist is on our free checklists and forms page.
Frequently asked questions
How small a gap can a mouse get through?
A house mouse (Mus musculus) fits through about 6mm, a pencil's width, and a young rat through about 12mm. If a pencil slides under a door, a mouse can follow. Proof every ground-level opening below 6mm with gnaw-resistant material.
Why is exclusion better than baiting?
Exclusion stops rodents entering, so there is no contamination inside food zones and no risk of a poisoned carcass decomposing internally. Baiting only responds to rodents already inside. Auditors treat proofing as the primary control and baiting as a backstop and monitor.
What are the main rodent entry points in a food factory?
Gaps under doors, worn door sweeps, roller shutters, drains and gulleys, dock levellers, service penetrations, damaged brickwork, and roof and eaves gaps for climbing roof rats. A survey walks the whole envelope, ground level and above, against the 6mm and 12mm limits.
