Around 15mm for a young rat
Young brown rats can squeeze through openings of roughly 15mm. Fully grown adults need a little more, but a property proofed only against adults is a property open to the next generation. For proofing purposes, treat anything a thumb can enter as a rat-sized hole.
Gaps do not stay the same size
Rats gnaw constantly to keep their teeth worn down, and they gnaw at the edges of openings they want to use. Soft materials give way quickly: timber, plastic, uPVC trims, expanding foam and flexible sealant. A 10mm gap around a plastic waste pipe can be a 20mm hole within days. That is why what you fill a gap with matters as much as filling it — see proofing pipes and utility gaps.
Where rat-sized gaps occur
- Broken drain pipes and open connections underground — the subject of rats entering through drains
- Damaged air brick grilles opening into floor voids
- Gaps under garage and shed doors
- Holes left where old pipes were removed
- Cracks where extensions, porches or conservatories meet the house
- Gaps at the eaves and roof edge, reached by climbing
Rats climb, too
Brown rats climb rough brickwork, drainpipes, fences and trees well, so a rat-sized gap at first-floor level or in the roof counts. If the activity is overhead, rats in the loft covers that route.
Checking your own property
Look for gaps at ground level first, along the full base of the house and any outbuildings, then around every pipe that enters the walls. Our walk-round guide, the most common rodent entry points, takes you through it zone by zone. Signs of use — smears, gnawing and droppings — show which gaps matter most.
What proofing can and cannot fix
Small accessible gaps above ground can be sealed with our rodent proofing service at £180 for up to one hour. Broken drains need a drainage contractor, and large holes a builder. For the rat-specific approach, read rat proofing.
If rats are already inside
Treat first, proof after. Rat control in Brent is £190 for a flat and £240 for a house — book rat control and we will talk about the gaps at the same time.
