How rodents and martens get into the house

A gap under a door, a loosely sealed pipe, or a shifted roof tile can be enough for an uninvited guest to enter the building. Mice need surprisingly little space, and martens can slip through openings much narrower than you might expect based on their fur. One of the most important steps in prevention is therefore identifying entry points and sealing them up permanently. We’ll show you how large the gaps can be, where it’s worth checking, and which materials can help improve the situation.

A mouse only needs a few millimeters.

The house mouse can get in through an opening of just about 6 millimeters. The CDC guide compares this size to the thickness of a pencil. A gap like this can easily seem insignificant under a garage door or next to a cable running through a wall, yet it is still worth addressing.

Rats generally need a bit more space, but even here we are talking about surprisingly small dimensions. According to the University of California’s summary, especially young rats can squeeze through a gap under a door of about 1.3 centimeters in height. So you should definitely not dismiss even a finger-width gap as insignificant. If you are also defending against mice, sealing the smaller openings should be the standard.

The tennis ball rule for martens

For martens, a useful rule of thumb is the tennis ball rule: where a tennis ball can pass through, you also have to count on a marten being able to get in. An opening this size next to the roof edge or under the eaves already deserves attention. A marten’s elongated body is narrower than its thick fur makes it appear, so it is easy to underestimate where it can squeeze through based on its outer appearance.

A tennis ball is not the smallest possible entry point, however. According to the Stadtwildtiere expert species profile, a marten can already get through gaps of about 5 centimeters. So if a ball does not fit through an opening, it is still worth taking a closer look. The comparison is a quick warning sign; when checking in practice, look for the smaller gaps as well.

Inspect the building from the basement to the roof.

Start with the area around doors, gates, and pipe penetrations. Check whether there is any gap at the threshold, whether the bottom door seal is damaged, and whether the openings around water pipes and cables have been properly sealed. The garage, basement, and outbuilding are just as worth checking as the living space.

Inside, also look for gaps behind the kitchen cabinets, around the pipes under the sink, and at the junction between the wall and the floor. This is especially useful after installation work or renovations, when finishing off a new opening made for a line or pipe can easily be overlooked. Take photos of the problem areas and measure the openings so you can choose the right material for the repair.

If you suspect martens, the area around the roof and eaves is also important. The Berlin environmental authority’s information sheet also mentions loose roof tiles, damaged roof windows, and ventilation openings as possible entry points. A tree standing close to the wall or vegetation reaching up to the roof can also make access easier. Leave hard-to-reach roof sections to a professional.

Expanding foam alone is not enough

A sealing solution only works if the animal cannot easily chew through or push it aside. According to the University of California’s guide to excluding mice, plastic mesh, rubber, wood, and conventional insulating foam alone are not suitable for sealing used entry points. Foam filling must be paired with a chew-resistant physical barrier.

For larger wall defects, a properly fitted metal sheet or a cement-based repair may be appropriate. Around pipes, you need a closure that matches the shape of the penetration, because any gap left at the edges can become an entry point again. In addition to replacing a loose door seal, also check how well the door fits.

Protect vents with a sturdy, properly secured metal grille or metal mesh. Against mice, the mesh openings should be smaller than the entry size mentioned above; chicken wire with large holes is not suitable for this. The necessary airflow must be maintained, so any modification of building-service vents should be coordinated with a professional.

Check the attic before sealing it up

If a marten is already visiting the attic, before sealing the entrance make sure there is no animal inside. A trapped marten may cause further damage while trying to get out. In spring and early summer, it is especially important to check whether there are any cubs: the mother leaving at night does not yet mean the entire hiding place is empty.

Observing movement, for example with a properly placed camera, can help identify the entry point in use. If there are cubs or hard-to-see roof cavities, ask a knowledgeable professional to help plan the exclusion. The goal is for the animals to get out safely, and for the repair to prevent them from getting back in.

After sealing, keep watching for signs as well

After the repair, check whether fresh chew marks, droppings, or new damage appear. These may indicate that another entry point remains, or that animals are still inside the building. Storing food and pet food in sealed containers, as well as reducing hiding places, remains part of prevention.

If you would also like to supplement your defense with an ultrasonic device, learn about the Vadalarm rodent repeller, and review our installation guide. Repairing defects in the building and checking for activity remain important even when using the device.

Frequently asked questions

How large a gap can a mouse fit through?

A house mouse may only need an opening of about 6 millimeters. Also check the tiny gaps around doors and pipe penetrations.

Can a marten really get in where a tennis ball can fit?

Yes, if an opening is large enough for a tennis ball, you should count on a marten being able to get in. It can even pass through a gap of around 5 centimeters, so the smaller openings must not be left out of the inspection either.

Is it enough to fill the hole with expanding foam?

Conventional expanding foam alone is not a durable barrier against rodents. You need a chew-resistant, securely fixed seal.

Can I seal off a marten entry point immediately?

Then seal it permanently only after you have made sure that neither an adult animal nor any cubs are left inside.