One reason rodent problems catch people off guard is that the entry point is often much smaller than they expect. BPCA says rats only need a gap of about 15 mm to gain entry to a structure, while mice can squeeze through cracks as small as 5 mm.
That is why people can have an active problem without ever spotting an obvious “way in.” The building can look fine at a glance and still have enough weak points for rats or mice to use.
The usual routes are not especially glamorous.
They tend to be things like:
BPCA specifically advises checking gaps around pipes and windows, low-level gaps first, basements, and drains for rats, while its mouse guidance highlights small gaps, door brush strips, and exterior door gaps larger than 4 mm.
For both rats and mice, the lower part of a building often matters most.
BPCA says low-level gaps are the most likely areas for rats to enter, which is why it recommends focusing there first when checking for entry points. That usually means looking around the base of walls, pipe entries, garages, sheds, side returns, and utility areas before you start worrying about high roof spaces.
That does not mean rodents never come in higher up.
It just means the boring, ground-level gaps are often the real problem.
Mice are especially good at exploiting tiny gaps.
BPCA says exterior doors should have a door sweep or brush strip if the gap is larger than 4 mm, and it also says mice can squeeze through cracks as small as 5 mm. That is a useful reality check for anyone assuming the opening has to be visibly large before it becomes a problem.
This is why mice often turn up in kitchens, utility rooms, storage areas, and commercial premises that otherwise look fairly well sealed. A very small weakness can be enough.
Rodents do not need a purpose-built hole.
They often use the gaps left around pipes, cables, conduits, and service penetrations. BPCA’s public proofing advice for rats specifically says to check around pipes and windows, while its broader proofing guidance stresses restricting access to food, shelter, and warmth rather than only reacting to obvious holes.
That is why rodent entry points often show up around the practical edges of a building rather than in the middle of a clean wall.
This is one people often forget.
BPCA’s rat prevention advice says drain inspection covers should be kept in good repair and disused pipes should be sealed off. That is a strong clue that drains and pipe routes can be part of how rats gain access or move close to a structure.
So if the signs keep appearing indoors and the usual external gaps do not explain it, drains and below-ground routes are worth thinking about.
A rodent problem does not always begin inside the house.
BPCA says rats can burrow under sheds and into grassy banks, and it advises removing nesting sites by cutting back overgrown areas and clearing piles of wood or debris. For mice, BPCA also recommends keeping gardens tidy, reducing clutter, and covering waste and compost.
That matters because an outside harbourage problem often becomes an inside access problem later.
If rodents keep getting in, it is usually because the site still works for them.
CRRU UK’s code of best practice says long-term control depends on environmental management, hygiene, proofing, maintenance, and repair of buildings. In plain English, if the food, shelter, and access are still there, the building is still attractive.
That is why “how did they get in?” and “why do they keep coming back?” are usually linked questions.
If you are trying to work out how rodents are getting into your home or building, start by thinking smaller, not bigger.
Check the low-level gaps first. Look around pipes, doors, vents, windows, sheds, service entries, and anywhere the structure changes direction or material. If the signs keep pointing to active access and you are not sure where the weak point is, Pest Gone can help you work out the likely route and what makes the most sense to deal with first.
