Rodent Exclusion Methods – Wildlife Damage Management

Exclusion Methods: Existing Structures and Equipment

Holes and Openings
By gnawing, rats can gain entry through any opening greater than 1/2 inch (1.3 cm) across, and mice through any opening larger than 1/4 inch (0.6 cm). The paired front (incisor) teeth of rats and mice curve slightly inward. This inward curve makes it difficult for them to gnaw into a flat, hard surface. When given a rough surface or an edge to bite into, however, they can quickly gnaw into most materials. To prevent rodent entry, seal all such holes with durable materials. Steel wool, copper gauze (Stuf-it^® brand) or screen wire packed tightly into openings is a good temporary plug. For long-term or permanent repair, mix a quick-drying patching plaster or anchoring such as Fixall^® into a wad of Stu-fit^® before pushing the material into the hole, and smooth over the outside (Fig. 9).

Figure 9. Patching small holes with copper gauze and a fast-drying patching compound (left), and 1/4-inch hardware cloth for larger holes (right).

If steel wool is used, rust stains are likely to result. Holes 3 inches (8 cm) or more in diameter should be covered or backed with 1/4-inch (0.6-cm) woven/welded hardware cloth prior to filling with a good patching compound (see recommendations under Foundations and Floors). Another backing material available is Strong Patch^TM (D. P. Wagner Mfg. Inc.), a 6 x 6-inch (15 x 15-cm) sheet metal patch to cover holes up to 5 x 5 inches (11 x 11 cm). It has a self-adhesive backing and a mesh on the surface for better adhesion of the patching compound or other texture.

To close larger openings or protect other areas subject to gnawing, use materials such as those listed in Table 1. Hardware cloth, if not woven, breaks easily. The woven/welded hardware cloth maintains its shape when cut to fit around pipes or other objects. Hardware cloth used to cover gaps and holes can be filled with foam caulk, Fix-all^®, Quick-Fix^®, or other fast-drying interior patching compounds. When used on the exterior, concrete mortar, plaster, or Concrete Patch^® can be used to provide longer-term rodent-proofing (Fig. 10). These are just a few of the many products available.

Figure 10.Frequently used patching materials on 1/4-inch woven hardware cloth backing.

Figure 11. Seal gaps or holes with rodent-proof materials where pipes, wires, or other similar objects enter buildings. Even a small unprotected opening can be an invitation to rodents. Figure 12. Metal siding may provide entry points for mice and rats where panel ends are left open (left). Properly installed metal siding rests on the concrete floor or has metal flashing or angle iron to block

entry (right).

Close openings around augers, pipes, and electric cables where they enter structures with Portland cement mortar, Concrete Patch^®, masonry, or metal collars (Fig. 11).

The ribs and corrugations in metal siding can be blocked with metal or mortar. Rubber or vinyl weather stops are quickly gnawed through. Design or modify buildings with metal siding by butting siding panels or sheets against solid materials (metal flashing or concrete) so the openings are not present

Caution: letting metal siding rest directly against concrete can lead to accelerated rusting and corrosion. The siding should be installed so that openings are no greater than 1/8 inch (0.3 cm) wide. If the siding is installed with the ribs horizontal, the ends must still be sealed or the bottom of the decorative corner trim flashed and closed.

Table 1. Recommended materials for rodent-proofing. Concrete: Minimum thickness of 2 inches (5.1 cm) if reinforced, or 3 3/4 inches (9.5 cm) if not reinforced. Galvanized sheet metal: 24 gauge or heavier for wall or pipe barriers; 22 gauge or heavier for kick plates or door edging. Perforated or expanded sheet metal grills should be 14 gauge. Brick: 3 3/4 inches (9.5 cm) thick with joints filled with mortar. Hardware cloth (wire mesh): Woven, 19-gauge, 1/2-x 1/2-inch (1.3- x 1.3-cm) mesh to exclude rats; 24-gauge, 1/4- x 1/4-inch (0.6- x 0.6-cm) mesh to exclude mice. Aluminum: 22 gauge for frames and flashing; 18 gauge for kick plates and guards.

Vents and Windows.
Use only metal window screening materials where windows or doors are accessible to rodents. Avoid unnecessary ledges outside windows. When necessary, screen ventilation openings and windows with woven/welded galvanized hardware cloth. Such screening is critical in commercial and farm buildings and where high rodent pressures in residential areas are found. For large openings or where the screen may be subject to abuse, add crossbars to support the hardware cloth. If the opening is an access route, install the screen on a hinged frame.

All vents and duct openings for heating and air conditioning should be screened or raised and/or guarded with an excluder device to prevent rodent entry. Residential cold air return grills can easily be mouse-proofed by placing 1/4-inch (0.6-cm) hardware cloth behind the grill where it is not unsightly. In some applications, power vents can be covered with hinged metal plates (louvered) that open with air flow and close when fans are off. These louvers are only effective if they fit tightly and the sides are recessed to prevent rodents from pushing through them. Caution: Hardware cloth less than 1/2 x 1/2 inch (1.3 x 1.3 cm) significantly reduces air flow. In buildings where ventilation is already marginally adequate or inadequate, such further restrictions may be unacceptable. In some locations, small mesh screens can become clogged with dust or freeze over. In such situations, the use of 1/2 x 1/2-inch (1.3 x 1.3-cm) hardware cloth is a reasonable compromise between ventilation requirements and rodent control.

Exterior Doors
Doors should fit tightly, the distance between the bottom of the door and the threshold not exceeding 1/4 inch (0.6 cm). In some instances, it is possible to build up the threshold rather than modify the door.

Metal thresholds can be fastened to floors. Steel pipes embedded in a concrete floor make good rodent-proof thresholds and allow doors to swing free when open. Pipe thresholds are especially useful where doorways are used by wheeled pallet jacks, heavy equipment, or livestock. Install flashing or a metal channel on the lower edge of doors, particularly softwood doors (Fig. 13)a plastic door boot has been successfully used where the door receives low use and the edges are not easily accessible to rodent gnawing.

Figure 13. Metal flashing or metal channel prevents rodent gnawing at bottom edges of a door.

Properly applied flashing extends to within 1/8 inch (0.3 cm) of the edge of the door at the sides and bottom. Close the gap at the top or sides of roll-up doors with conveyor belt material shaped to fit into the side channel frames and mounted on the top door jamb. Bent bottom rails on doors should be straightened. Concrete dam-age due to inadequate reinforcement or poor placement practices should be repaired or the concrete replaced. A metal or pipe threshold is sometimes preferable or required.

Mechanical door-closing devices save time and help overcome human negligence. Equip doorways used for ventilation with rodent-proof screen doors, or if the door surface is too slick for rodents to climb, modify the existing door so the upper half can be left open for ventilation. Always use a heavy kick plate and solid frame on screen doors in commercial and agricultural buildings. Light-framed screen doors easily get bent out of shape, allowing rodent entry.

Foundations and Floors.
Gaps or flaws along building exteriors where the wall framing or siding meets the foundation provide easy entry for rodents. Such openings can be pre-vented by well-formed and finished concrete work and installation of tight wall framing and siding, or installing metal screed-type flashing between the siding and the foundation. Use of rodent-proof exterior surface materials such as concrete, plaster, or metal sheeting is also effective if properly installed so that all ribs or corrugations are closed.

Rodents can gain entry into buildings with piers or shallow foundation walls by burrowing beneath the floor or foundation. To prevent rat entry by this route, extend foundation walls below ground at least 36 inches (91 cm). This also reduces damage from frost. A horizontal footing extension also may be added to deflect burrowing rodents away from the foundation (Fig. 14).

Figure 14. A 1-foot horizontal footing extension to deflect burrowing rodents.

Avoid the use of slab-on-grade construction techniques for agricultural buildings or bulk bin pads. The possible savings in initial construction may be quickly offset by the costs of rodent damage and control measures.

Interior floor surface
Rats exert more effort to enter buildings where feed is available. They frequently seek shelter under concrete floors and slabs, where they burrow to find protection. Ideally, install floors, slabs, and sidewalks with deep footings, or with curtain walls of concrete or 1/4-inch (0.6-cm) mesh wire (Fig. 15).

Figure 15. Left: A curtain wall made of concrete will prevent rats from burrowing under foundations to gain entry to buildings. Curtain walls can be installed on existing buildings. A horizontal footing extension often deflects burrowing rodents away from structures. Right: Feed sheds, corn cribs, and other existing wooden structures can be rodent-proofed by installing hardware cloth topped by a band of sheet metal. The hardware cloth can also serve as a curtain wall to prevent rodent burrowing.

The choice between concrete and wire mesh depends on the expected life of the structure. Though wire mesh costs considerably less than concrete, its usefulness generally lasts only 5 to 10 years.

Repair cracks in foundations and floors with concrete or masonry grout. There are numerous quick-setting types of products, such as Fix-all^® or Quick-Fix^®, which are for interior use, and Concrete Patch^®, Rockite^®, or Pour Stone^® for interior or exterior use (previously illustrated in figure 10). The four last-mentioned products are specifically designed for repairs and have quick setting, good adhesion, and nonshrinking properties which make them ideal for exclusion work. Each, however, is made for a specific application: Pour Stone^® and Rockite^® are designed to be easily poured into cracks in floors or into holes to anchor bolts or machinery, and set hard in 15 minutes. Concrete Patch^® is a mortar-type material for repairing masonry surfaces and has a vinyl polymer to increase adhesion. It sets in 2 hours and is hard after 12 hours. Quick-Fix^® is a durable patching plaster for inside use on plaster, drywall, or wood surfaces. Drying time may be within 30 minutes, depending on thickness. With all of these types of repairs, the use of reinforcement with hardware cloth is usually needed on vertical or overhead horizontal surfaces to add strength and provide the necessary backing. Rodents can claw and gnaw at concrete and Portland cement until it is fully cured, so the use of 1/2-inch (1.3-cm) hardware cloth laid in the top 1/4 inch (0.6 cm) of the repair area may be necessary if rats are currently using the repair area as an entry point. Otherwise, provide an effective temporary rodent-proof protective overlay until the concrete is fully cured. Caution: Metal products placed within 1 inch (2.5 cm) of a concrete surface will oxidize and corrode and may discolor the concrete.

If rats have gained access to crawl spaces under building floors, prevent them from getting into walls by using such modifications as illustrated below(fig. 16).

Figure 16. Method of excluding rats from double walls. In old buildings, galvanized sheet metal (a) may be cut to fit and nailed into place between studs, joists, floor, and sill; in buildings under construction, noncombustible stops of good quality concrete (b) or bricks (c) are recommended.

Maintaining a clean, 3-foot-wide (1-m) weed-free area around building foundations, concrete slabs, and footings often discourages rodents from burrowing as well as eliminates a food source and attractive harborage. Where erosion of bare soil is likely, this buffer can be maintained by regular, close mowing of vegetation or by installing heavy gravel. To discourage burrowing, install a strip of 1-inch-di-ameter (2.5-cm) or larger gravel laid in a band at least 2 feet (60 cm) wide and 1/2 foot (15 cm) deep. 1/4″ mesh

Interior Rodent-Proofing
When rats or mice are present in a building, attention must be given to interior as well as exterior rodent-proofing to remove all sources of shelter. A combination of actions is required in such instances, as no single effort is likely to yield the desired result.

Concrete floors are preferred to wooden floors. An attempt should be made to seal off rodents. Use traps to remove the rodents, or place poison bait packets through openings in the floor or wall and then seal the openings with galvanized metal or hard-ware cloth and patching plaster as previously discussed. Promptly treat new openings as they are found. In occupied buildings, always trap the rodents before sealing interior walls to avoid odors, stains, and an influx of insects that feed on decaying rodent carcasses.

Eliminate rodent hiding places beneath and behind equipment. Feeders in livestock facilities should have flat bottoms and be designed and installed so that rodents cannot find shelter beneath or behind them. Give special attention to storage rooms, closets, feed storage, or other areas where construction techniques may allow rodents access to walls, floor spaces, or attics. Stacks of wood and other stored items should be 18 inches (46 cm) above the floor and 18 to 24 inches (46 to 61 cm) away from walls to allow for proper cleaning and inspection. Ware-house stock should always be stacked off the floor on pallets and away from walls, and it should be rotated often to prevent development of infestations in undisturbed areas.

Rodents often gnaw into wall materials at corners or where joints in construction materials provide an edge. Poor construction techniques may allow rodents to gain access through materials that are otherwise considered rodent-proof.

Perimeter insulation is a necessary part of energy-efficient construction. Placing insulation on the exterior of foundation walls subjects it to mechanical damage as well as infestation and destruction by rodents. To prevent damage to perimeter insulation, use sandwich wall construction in which the insulation is placed within the concrete. Insulation placed on the outside of a foundation wall requires protective-covering material. Suitable materials include cement board, high density fiberglass-reinforced plastics, troweled-on coatings such as Block Bond^®, or Surewall^®. In such situations, metal flashing should be used to prevent the potential for entry routes for subterranean termites. Several companies now manufacture special coatings for exterior perimeter insulation. Examples include DuraWall^TM and Secure-wall^TM.

Extend protective cover materials at least 36 inches (91 cm) below finished grade. If the protective layer ends less than 36 inches (91 cm) below grade, add a horizontal ledge that extends outward at least 1 foot (30 cm). All top edges and corners must also be protected with a close-fitting heavy-gauge metal flashing (Fig. 17).

Figure 17. Perimeter insulation placed on the outside of a foundation, curtain, or stem wall must be protected to prevent rodent damage. Protection is required along the top and to a depth of at least 36 inches below the soil surface.

Drains and Pipes.
Both rats and mice use drainage pipes or sewage systems as routes to enter buildings. Equip floor drains with metal grates held firmly in place. Grate openings should not exceed 1/4 inch (0.6 cm). Maintain 1/2-inch (1.3-cm) hardware cloth over sewer roof vents in rat-infested areas. If the sewer system is known to be rat-infested, a “Rat Guard” one way flap valve may be placed in toilets (Fig. 18).

Figure 18.check openings that could allow rodent entry.

Sewer laterals should be checked for openings that could allow rodent entry. Smoke-producing leak detectors are often used by agencies checking sewer lines for leaks or openings. If openings are detected, replace the pipe or wrap the pipe break with 1/4-inch (0.6-cm) hardware cloth and use concrete patching material to seal the area. Rain gutter downspouts are often used by rats to gain access to roofs. It may be possible to screen over openings at the base of downspouts with 1/2-inch (0.6-cm) hardware cloth or a grate, but this will require continued maintenance to remove accumulated debris, particularly where leaves and small sticks are washed from roofs into the gutter system. Flap valves have been used here too—swinging shut except when water is flowing. Openings to floor or driveway drains should have covers. Gutter and other drain covers must be kept clean of debris to prevent water backup.

Manure management systems in livestock facilities may be of a type that periodically drains manure or water from the building to a lagoon or other storage area. In such cases, a “floating” metal cover or check valve-style closure at the open end of the discharge pipe, with a hinge at its upper edge, can be effective. The hinge must operate easily so that the cover will open when water or manure flows out but will fall back into place when the flow stops in a manner similar to a tide gate used on drains in coastal areas. The potential for such covers to freeze shut, however, can be a drawback. A better method is to extend discharge pipes far enough over the bank or into the lagoon to prevent rodents from jumping or crawling into the open end. Install rodent shields to prevent rodents from gaining access (Fig. 19). Always cap pump-out ports when under-building manure storages are not in use. Left open, they allow rodents easy entry.

Figure 19. Shield on manure discharge pipe to prevent access by rodents to open pipe.

Physical Barriers/Guards.
To pre-vent rodents from climbing or traveling along a particular route, install guards made of sheet metal or similar materials (Table 1). Guards must be wide enough and positioned to keep rodents from reaching their outer mar-gins by climbing or jumping. Dock areas may need guards to keep rodents from jumping or climbing from foundations, pipes, steps, or rough exterior wall surfaces, and from infesting trucks or rail cars transporting goods.

A sheet metal band attached to a wall will prevent rodents from climbing. Rodent guards should be at least 14 inches (36 cm) but preferably 18 inches (46 cm) wide (Fig. 20).

Figure 20.Guards prevent rats and mice from climbing at corners.

Inside buildings, such guards can prevent rats and mice from climbing at corners. Used in combination with hardware cloth or other suitable material, they can make a building essentially rodent-proof. These modifications are essential on pumping plants, water treatment facilities, power stations, and communications facilities. They have also been used to make corn cribs, barns, and other older buildings in current use rat- and mouse-proof.

Guards on walls should be at least 36 inches (91 cm) above ground or floor level. Flat guards have been used to prevent rodents from traveling along horizontal or vertical pipes or electric wires (Fig. 21).

Figure 21. Cones or discs act as rodent guards on suspended cables, rain gutter downspouts, conduit, ropes, augers, or pipes.

Figure 22. Guards can be used to prevent rats from climbing augers, pipes, or wires leading to buildings. Footings or foundations for grain bins should extend into the ground to prevent rat burrowing.

With some ingenuity, you can design rodent guards to fit any given situation. Free-hanging guards are easily damaged. Circular guards must extend out 18 inches (46 cm) around the line they guard. They are constructed of 24-gauge metal and anchored in place by one or more arms on the side opposite to that accessible to rats. Cone-shaped circular guards prevent rats from climbing vertical pipes, pilings, and trees. Shields or wire guards made of 1/4-inch (0.6-cm) wire mesh are useful in excluding rodents from the interior of conveyor belts, underground power and communications conduit, feed augers, fan housings, and similar openings.

Food Handling and Storage Areas.
Even when all of the holes are plugged, rodents seem to find a way into food storage and handling areas. Sometimes rodents come in with supplies, or they run in through open doors or windows. Often, one or more openings remain undetected. These hidden holes are often below sinks, behind equipment, in false or suspended ceilings, and behind or under cupboards. Once in an environment having all the basic needs, rodents quickly establish viable populations. The solution is to eliminate harborage and exclude rodents from food and water sources inside the building.

All equipment such as large refrigerators, freezers, counters, dishwashers, and sanitizers should be raised and easily movable, enabling cleaning underneath and behind them. Insulated walls and closed areas should be tightly closed off to avoid use as harborage. Openings are commonly seen in new stainless-steel work counters in supports under the work surface, or in areas provided for drains. Drains should be easy to clean but should have rodent-proof covers.

Store food products in rodent-proof enclosures or on shelving at least 18 inches (46 cm), but preferably 30 inches (76 cm) or more, above the floor. Tubular supports (legs) for shelving should be a minimum of 4 inches (10 cm) in diameter to prevent rats from shinnying up from the floor.

Keep all damaged goods and returns, as well as refuse, in a separate rodent-proof room. Loading docks should be very tightly rodent-proofed and only open during daylight hours. All out-side doors should be self-closing, have heavy kickplates, and be checked periodically for excessive gaps. In warehouses and storage situations, centralize highly susceptible foodstuffs into areas that can be securely rodent-proofed and closely monitored. These areas should also have rodent bait stations and multiple-capture mouse traps permanently installed around the perimeter.

Livestock Feed Bunks and Bins.
Rats typically burrow and nest under feed bunks that are placed directly on the ground or near ground level. Properly designed concrete bunks that sit tightly on a concrete base eliminate rodent habitat. Though cattle traffic may discourage burrowing under the concrete slab, a foundation may be needed to prevent burrowing around the sides of the slab that do not receive heavy cattle use. Concrete slabs on which feed bins are placed should have foundations extending 36 inches (91 cm) into the soil at the outer edge to prevent rats from burrowing under the slab. Installing heavy gravel and maintaining a clean, weed-free zone around the perimeter of the slab will also discourage rat burrowing and permit easier detection of rat activity.

Feed and Refuse Storage.
Livestock or pet feed and edible refuse attract rodents and are a common food source. Always store these materials in metal containers with tight-fitting lids. Food is often available to rodents around homes, kennels, and poultry and livestock feed storage areas because feed is kept in plastic or wood storage bins or hoppers. These storage containers are frequently open at the top, or may be gnawed through the sides. Check nonmetal hoppers frequently for holes and, when necessary, repair with sheet metal. Avoid the use of self-feeders for pets. Feed pets only as much as they will consume at one time and only during daylight hours.

Proper storage and disposal of house-hold garbage and dead animals is a very important part of rat control. Bulk dumpsters are often left with the tops open, or the tops are badly bent, allowing rodent entry. Constant vigilance and calls to the refuse company should correct these situations. Seal bulk trash compactors from rodents. Spilled refuse and juices from crushed contents often create rodent problems under and behind compactors and bulk dumpsters. Clean these areas of-ten and install rodent screening in container drains.