How Small a Hole Can a Mouse or Rat Fit Through?

Why they fit through such small gaps

A rodent's skull is the widest hard part of its body. If the head fits, the rest can usually squeeze through. Rodents also gnaw, so a gap slightly too small can be chewed wider, especially in wood, soft plastic, foam and old sealant. Treat any gap close to the size of a dime as a mouse door.

Where mouse-sized gaps hide on San Diego homes

• Under and beside the garage door, where the bottom seal has curled or the side seals are worn
• Under exterior doors without a sweep
• At the weep screed along the base of stucco walls
• Around AC line sets, gas lines, hose bibs and cable lines where they enter the wall
• Around dryer vents and water heater vents
• Behind the stove, dishwasher and laundry hookups, where pipes pass through the wall
• Gaps at the slab edge in older detached garages

Many of these come from later work, like a new AC unit or a cable install, where the hole was cut larger than the line.

Where rat-sized gaps hide

Roof rats come in high. Look for:

• The open ends of barrel tiles at the eaves on tile roofs without bird stops
• Gable and soffit vents with torn or rusted screens
• Gaps where a second-story wall meets a lower roof
• Dead valleys and joints between old and new rooflines
• Shrunken fascia boards on older ranch homes
• Openings around solar conduit and roof vents

Trees, palms and vines that touch the roof give rats easy access to all of these. The county advises trimming plants at least 4 feet from the roof. See rats in the attic.

How to find them

• From inside the garage with the door closed, look for daylight at the bottom and sides
• Check under every sink and behind appliances with a flashlight
• Walk the outside of the house slowly at ground level, looking at every pipe and line
• From the ground, use binoculars to check tile ends and vents along the roofline
• In the attic, look for daylight and droppings near the eaves

Rub marks, greasy smudges and gnaw marks around an opening mean it is in use.

What to close them with

The county's hantavirus brochure suggests caulk or expanding foam for small holes and 1/4 inch, 18 to 22 gauge wire mesh for repairs. For gaps rodents are actively using, use something they cannot chew as the barrier:

• Hardware cloth or wire mesh for vents and larger openings
• Copper or steel mesh packed tight into gaps around pipes, finished with sealant
• Metal flashing for gaps at roof joints and fascia
• Bird stops or mesh for open tile ends
• Door sweeps and rodent-resistant garage door seals

Foam alone is easy to chew through. Use it as a filler behind or over mesh, not by itself. See steel wool vs copper mesh.

Seal in the right order

If rodents are inside, do not close every gap at once. They will chew a new way out or die inside. Seal most gaps, trap until catches stop, then close the last route. See rodent exclusion.

When to call for help

Roof work, tile ends, second-story vents and attic gaps are hard and risky to reach. A technician can map every gap and give you a written list. Compare quotes by the number of entry points and the materials named for each.

A quick checklist for one afternoon

You can cover most mouse-sized gaps on a single-story San Diego house in an afternoon. Close the garage door and look for daylight. Check each exterior door for a gap at the bottom. Walk the outside wall and look at every pipe, line and vent. Look at the base of the stucco for gaps at the weep screed. Open every cabinet under a sink and look where the pipes enter the wall. Pull the stove and fridge out if you can. Write each gap down with its size and location, then decide which ones you can close yourself and which need a ladder or a pro.