Rodent Entry Points – Wildlife Damage Management

Common Rodent Entry Points

Many structures have inherently similar rodent entry areas due to similarity in design (particularly entry and service areas), utility sources, and building equipment. For the sake of simplicity, we have attempted to group similar structures and settings.

Commercial Office Buildings, Hotels, Hospitals, and Retail Stores. Commercial buildings are constructed from types of materials and design methods that vary greatly in the degree of susceptibility to rodent infestation (for example, metal and concrete versus wood). Most structures eventually become less rodent-proof due to deterioration, alteration, or repair. Heating, air conditioning, plumbing, electrical service, and fire sprinklers provide some of the most commonly encountered rodent entry points. Neoprene seals, spray-in-place foam, and similar products commonly used to close openings are not rodent-proof.

Figure 4b. Gap (3/4 x 26 inch) between wall covering and framing at roof joint, allowing rats and mice easy access.

Even in new buildings, utility pipes, electrical conduit (often at meters or circuit breaker panels), water and gas lines, and communication cables generally have large openings that permit entry of mice and rats. Once rodents have entered walls, they generally have ready access to much of a building via holes for utility pipes and wires in the framing, and via overhead suspended ceilings or other types of construction adjacent to utility enclosures. Specific problem areas include poorly sealed heating and air conditioning ducts; roof and wall vents installed without strong, well-attached hard-ware cloth screening (Fig. 4a); roof and wall joints and edges without properly installed metal flashing (Fig. 4b); and doors hung unevenly or too high, or lined with unprotected soft rubber weather stripping (Fig. 4c). Refuse and food handling areas are likely to have the greatest rodent pressure. In older buildings, cracks in concrete slabs, brick and concrete block walls, or worn or damaged drain covers allow rodent entry. Air and elevator shafts and laundry chutes also merit close inspection.

Figure 4c. Large hole gnawed by Norway rats in weather strip on the base of a warehouse door.

Food Handling Facilities and Warehouses.

Businesses in which food is stored or handled are especially prone to rodent invasion. Good sanitation practices are essential. Keeping food well-sealed is very important and more difficult than might initially appear. Scraps of food can often be found in floor drains, under food preparation equipment and stored products, and around refuse and entry areas. Outside doors are often left ajar or fit poorly due to heavy use, physical damage, or improper installation.

Space under equipment (mixers, stoves, counters, or refrigerators) should allow easy cleaning and inspection, or be closed off completely with rodent-proof materials (Fig. 5).

Figure 5. Mice and rats are sometimes found using freezer and refrigerator compressor areas for harborage and water (from condensation on cold coils). Mice are often found in the insulated walls of large coolers. Look closely at corners and edges of metal, or other material covering the insulation, for rodent openings. Drains should have adequate screens or grates to prevent rodent entry.

Food disposal, refuse, and damaged goods areas are often located close to food handling or storage areas and are not sealed from rodents. Areas near loading docks should be closely inspected for cracks, broken screens, damaged doors, and uneven floors near doorways. Interior loading docks served by rail cars are difficult to close due to the tracks, but rubber door guards made to fit the tracks are available and will deter rodent entry.

Rodent-infested goods in food warehouses commonly include cereals, flour, and baking mixes; waxed carton drinks; dry pet foods; dried fruits and nuts; fresh produce; paper goods; charcoal briquets, and damaged goods. Products in these categories should be kept in open, easily inspected areas, not in dark corners. Regular and routine removal of such nonsalable or nonusable products should be standard practice to enhance cleanliness and safety and to reduce harborage.

Apartments and Houses.

Utility entry points include underground electrical and communication trunk lines, and exhaust vents for clothes dryers. Power lines have always been a favorite route of travel for commensal rodents, especially roof rats. Check all roof joints for tightness and presence of flashing, if rats and mice have access to the roof via wire, pipes, plants, or rough-textured walls. Also check roof and sewer vents for adequate screening and sealing, including presence of tight roof jacks (Fig. 6).

Figure 6. Chimneys should be checked for properly installed flashing or for missing mortar.

Rats occasionally enter buildings through toilet traps in innercity areas with rat-infested sewer systems. In such cases, tracks and water may be found on the rim of toilet bowls. Both roof and Norway rats have been known to enter structures via the sewage system. This route usually occurs in older (20 years or more) established areas with poorly maintained sewer systems. Mice often enter under entry doors, through holes beside water pipes and electrical conduit, and through the cold air return ducts on forced air furnaces, especially those located in outside cabinets or garages, and underneath mobile homes. Mice and rats often find easy access to garage areas through open doors or under and beside poor-fitting garage doors. Once in the garage, they may gain entry into the main structure along electrical lines, pipes, poorly sealed fire wall sheathing, or around furnace ducts, hot water heaters, or laundry drains.

If rodents are able to reach the attic, they may travel from room to room or unit to unit through openings for pipes, ducts, and wiring. Attics provide excellent harborage in winter, spring, and fall, but are often too hot during summer. Common attics, basements, or raised foundations in condominiums and apartments are a frequent source of rodent infestation.

Another source of entry to residences, and a source of harborage for rats and mice, are fireplaces—especially the newer preconstructed zero clearance sheet metal units that eliminate the need for concrete mortar and brick. A hollow space is left in the siding and the fireplace support framing between the outside wall and the fireplace. Rats and mice can enter this area from the outside via the roof joint, between the siding and decorative wood corner trim, around gas pipes, or outside wood storage doors.

Once a rodent gets into the attic, inside entry to the fireplace void is often easy because of poorly fitted sheeting or metal collars. Entry to the inside of the fireplace is made from the damper area or cool air and warm air returns on units that provide for air circulation around the firebox. When the fireplace is in use, the heat will prevent rodent entry. If the outside cannot be sealed, glass doors that seal the burn area are recommended to prevent rodent entry throughout the year. Cracked and missing mortar, or poorly fitted siding or plaster, may allow entry through brick or rock fireplaces.

Tile or shake shingle roofs allow rodent entry if the roof is not solidly sheeted with plywood or similar material and the tile is not properly fitted and grouted. Vents without tightly fitted double roof jacks also facilitate access to rodents.

Gaps or flaws in foundations and slabs, or where the wall framing meets the foundation or slab floor, may provide large enough openings for rodent entry. Older buildings commonly have cracked foundations, cracked plaster or mortar, warped siding, or broken and torn vent screens. Wood or masonite siding is especially vulnerable to warping and cracking near corners and around the base of the building. Old, unused holes where utilities formerly entered the structure are also common, especially in raised foundation and basement homes. Window screens are often left off or fit poorly in older, low-cost apartments and homes, allowing rodent entry from exterior utility lines and pipes running along exterior walls. Runways going to window ledges are often observed on stucco and brick walls and in ornamental plantings next to buildings.

Manufacturing Plants and Farm Buildings.

Overhead or under-ground pipes, conveyor belts, and augers commonly found in farm buildings and factories are often used as entry points and routes into and between buildings. Such equipment, particularly if abandoned, may provide harborage as well as food. Rodent-proofing these areas is not easy if the equipment is still in use.

Figure 7. Rodent barrier or guard used between steps, foundation, or other sources of rodent entry and the loading dock doors when doors must be left open at night.

Utility entry points must be constantly monitored for excess openings caused by equipment repair, installation, or modification. Outside walls and doors must also be monitored for damage from equipment or livestock and for damage or wear from heavy use. If work patterns require doors to be open during hours of darkness, when rodent entry is most likely, rodent barriers may be needed, such as a solid fence or wall or a metal wing wall between the foundation and adjacent loading dock areas (Fig. 7).

Buildings constructed with ribbed or corrugated metal siding allow rodent entry if the bottoms of the siding panels do not rest flat on a solid surface or they are not otherwise closed off. Sections of prefabricated buildings should be assembled tightly, and gaps at joints should be covered with metal flashing. Often, however, they are left open, especially at corners and at the foundation/slab interface.

Figure 8. Roll-up or overhead doors often provide easy entry for rodents, birds, and bats. With the door closed, check for gaps along the sides, bottom, and top of the door.

A gap at the top is common. Rats and mice can easily climb up the space between the door and the inner wall or track to the top, where they gain entry and climb down the inside of the track. Gaps between the track and the wall are also common, especially if the track has been installed on brick walls. Door bottoms may be bent or damaged, leaving gaps along the floor. Uneven floors due to frost heaves may leave gaps when the door is closed. Screens on windows, crawl spaces, and vents are often damaged in farm and industrial buildings. Check these carefully for needed repair or replacement.

One of the greatest challenges in farm buildings is preventing feed and seed from being a food source for rodents. Good sanitation practices are very important. Clean up spilled feed, and store feed and seed in rodent-proof buildings and containers. Keep sacked materials off the floor when possible. This facilitates for inspection and reduces harborage.

Excluding rodents from livestock and poultry operations is another challenge due to livestock and manure management and various animal husbandry practices. Nevertheless, rodent-proofing is important and can be accomplished. Many of the entry points already identified for other types of structures apply to farm buildings. Additional problem areas include insulated walls used for harborage, feed bins, and portable feed bunks. These are but a few of the challenges discussed in more depth under Exclusion Methods (below).