How small a hole can a rodent squeeze through?

A house mouse can squeeze through a gap as small as 6 mm (about the diameter of a pencil) — the rodent skull is flexible and can be compressed.

A rule of thumb: if you can fit a pencil into a gap, a house mouse can pass through it.

The Norway rat requires a gap of about 12 mm (the thickness of a pinky finger).

The roof rat falls in between, requiring about 8–10 mm.

These dimensions are the core parameters for rodent-proof sealing — all gaps larger than 6 mm must be sealed.

Stainless steel wool is the best physical rodent-proofing material — although rodent teeth are extremely hard, they cannot gnaw through stainless steel.

Stuff the gap with steel wool and then secure it with sealant.

Ordinary expanding foam can be easily chewed through by rodents and should not be used alone.