How Small a Gap Can a Mouse or Rat Use?.

A house mouse can pass through a gap about a quarter inch wide, roughly the width of a pencil. A young rat can use a gap around half an inch, about the size of a quarter-dollar coin laid on its edge, and adults can squeeze through a gap a little larger. These are working figures, and animals will often enlarge a gap by chewing.

The key point is that rodents do not need a hole. They need an edge to work on. A small gap along a seam becomes bigger after a few nights of gnawing, especially in soft materials such as wood, foam, plastic and drywall.

Gaps hide in places that no one looks at. Most of them come from construction, from movement over time or from changes made after the house was built. The list below covers the spots we find on most inspections.

• Around pipes, wires and cables where they pass through exterior walls.
• Beneath and around garage doors and side doors.
• At crawl space vents with torn screens.
• At the roof edge, soffits and fascia.
• Around dryer, bath fan and range hood vents.
• Where siding meets the foundation.
• Around chimneys and where additions meet the original house.

Utility penetrations are a favorite. When a cable or a pipe was installed, the hole made was usually a little larger than the pipe, and the gap was never filled. Over time, caulk dries and falls out.

The right material depends on the opening. Stainless mesh or hardware cloth works for vents and larger openings. Copper mesh packed into a pipe gap and sealed over is excellent for small spaces. Sheet metal covers damaged edges and trim. Masonry-grade patches fill cracks in concrete.

Some popular materials fail. Steel wool rusts and can crumble in wet air. Expanding foam is chewed through quickly. Plain caulk shrinks and cracks. They can serve as one layer in a good repair, but they are not enough alone. See rodent exclusion for how we choose materials.

Walk the outside slowly with a flashlight, even in daylight. Look at every place something enters the wall, and at the base of every door. Run a pencil along suspect gaps. If it goes in, a mouse can. Inside, check under sinks, behind appliances and around the pipes in a utility room.

A useful trick is to look for light. On a dark garage floor, light coming under the door in daytime is a visible gap. Another is to listen for drafts around a pipe or a vent.

Do not seal a gap if you think an animal is inside. Sealing a hole with a rodent in the wall or attic traps it, and it may chew a new hole or die inside, which leads to odor. The order should be trapping, then sealing once catches stop. See rodent trapping and removal for how that works.

Some gaps hide inside the structure. A hollow wall around a pipe, a gap behind a bath tub apron or a void under a kitchen cabinet connects the crawl space to the living space. You cannot see these from the outside, which is why an interior check matters as much as an exterior one.

Remodels are another source. Where an old wall was opened and the framing was rebuilt, the new work may not meet the old exactly, leaving a channel. If your house has had work done, mention it, and we will look at those joints first.

When a patch fails, the cause is nearly always the material or an unnoticed second gap. Foam is chewed. Caulk shrinks. Steel wool rusts. A single patch beside an untouched opening just moves the animal along. A full inspection lists every opening so that none are left.

Seals need a follow-up. Wet weather and temperature changes move materials, and a patch that looked fine in September may have cracked by spring. A short walk around the outside each season catches those changes while they are small.

If you find fresh gnaw marks at an old repair, tell us. It usually means the material was wrong for the job, or the animals found a second gap nearby. Both are easy to correct once you know.