Rodent Exclusion: How to Seal Entry Points for Good

Trapping and baiting eliminate the rodents currently inside a structure. Exclusion — sealing the gaps, holes, and openings they used to enter — is what prevents the next wave from moving in. Without exclusion, rodent control becomes an ongoing exercise in population management rather than a resolved problem. For homeowners who want a lasting solution rather than perpetual treatment, exclusion is the essential step.

How Little Space Rodents Need

The first thing to understand about exclusion is how small an opening is sufficient for rodent entry:

• House mouse: Can squeeze through a gap as small as 6mm — approximately the diameter of a pencil. If you can slide a pencil through a gap, a mouse can enter.
• Norway rat: Requires a larger opening — about 12mm (half an inch) or a gap large enough to fit two fingers side by side. Rats also gnaw to enlarge openings that are almost large enough.

This means gaps that look insignificant to a homeowner — a slight gap under a door, a small space around a pipe, an unsealed crack in the foundation — are functional entryways for mice.

Where to Look: Common Entry Points

Foundation and Exterior Walls

• Cracks and gaps in the foundation wall — particularly where the foundation meets the sill plate
• Gaps around utility penetrations: water pipes, gas lines, electrical conduit, HVAC lines where they enter the structure
• Gaps in brick mortar joints at ground level
• Weep holes in brick construction — these need to be screened with hardware cloth, not sealed completely (they serve a moisture management function)
• Gaps between the foundation and exterior siding, particularly where settling has created separation

Doors and Windows

• Gaps under exterior doors — particularly garage service doors and rarely-used exterior doors where weatherstripping has deteriorated
• Gaps around door frames where the caulk has shrunk or failed
• Damaged or missing window screens, particularly at basement level
• The gap between the bottom of the garage door and the floor when the door is closed — even a small gap allows mouse entry

Roof and Upper Structure

• Gaps where the roofline meets the wall (fascia gaps)
• Gaps at roof vents and soffit vents where screens have failed or are absent
• Gaps where utility lines enter through the roof or upper walls
• Gaps at pipe boots and chimney flashings
• Overhanging tree branches that provide access to the roof — cut back to at least 6 feet from the structure

Interior

• Gaps around plumbing under sinks — often a half-inch or more of open space around pipe penetrations through cabinet floors
• Gaps at the back of kitchen cabinets where they meet the wall
• Gaps around floor drains
• Open spaces inside wall voids accessible from the basement or crawlspace

Exclusion Materials That Work

• Steel wool or copper mesh: Stuffed into gaps as a temporary measure; rodents can gnaw through steel wool over time, making it suitable for smaller gaps combined with caulk but not as a standalone solution for larger openings
• Hardware cloth (1/4 inch galvanized wire mesh): The most durable solution for screening larger openings — vents, soffits, gaps in the foundation. Cut to size and secure with screws or staples.
• Sheet metal flashing: Used to cover gaps along foundations, around door frames, and at roof penetrations where a durable, gnaw-resistant cover is needed
• Concrete or mortar: For filling cracks and gaps in masonry foundations
• Expanding foam: For filling gaps around pipes and in wall voids — rodents can gnaw through standard expanding foam, so use specifically formulated rodent-resistant foam (with abrasive additives) or combine foam with hardware cloth in larger gaps
• Door sweeps: Install on all exterior and garage service doors; use heavy-gauge aluminum or stainless sweeps for garage doors

Exclusion as Part of a Complete Program

Exclusion is most effective when performed after active control (trapping or baiting) has reduced the interior population — sealing entry points with rodents still inside can trap them in wall voids where they die and create odor problems. The typical sequence: control interior population first, then systematically seal all identified entry points. The combination provides the lasting result that either approach alone cannot achieve.