RODENT-PROOFCONSTRUCTION ANDEXCLUSIONMETHODS

Rex O. BakerProfessor and Research Project LeaderHorticulture/Plant & Soil ScienceCalifornia State Polytechnic UniversityPomona, California 91768

Gerald R. BodmanExtension Agricultural Engineer —

Livestock SystemsDepartment of Biological

Systems EngineeringUniversity of NebraskaLincoln, Nebraska 68583

Robert M. TimmSuperintendent and Extension

Wildlife SpecialistHopland Research and

Extension CenterUniversity of CaliforniaHopland, California 95449

Fig. 1. (a) Electrical cord of a freezer in a retail market, severely damaged by house mice; (b)fiberglass batt insulation within walls of a hog finishing house near Lincoln, Nebraska, wasdestroyed by house mice in less than 3 years.

Importance of Rodent-Proof Construction

Rats and mice cause serious damage toall kinds of structures if they areallowed access to them. Damage byrodents has been documented inhomes, apartments, hotels, office com-plexes, retail businesses, manufactur-ing facilities, food processing andwarehouse facilities, public utilityoperations (especially power and elec-tronic media operations), farm andfeed storage buildings, and otherstructures.

In urban settings, rodents most oftencause damage to older, inner-citybuildings and utilities in poor repair.New housing developments may ex-perience commensal rodent problems,but problems are more noticeable inneighborhoods 10 to 12 years of age orolder. Ornamental plantings, accumu-lation of refuse, woodpiles, and othersuch sources of harborage and foodare more quickly invaded and occu-pied by rodents when adjacent to anestablished rodent habitat.

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Many types of land, air, and watertransportation systems and their infra-structure also face serious rodent infes-tation problems. Infestations are ofparticular concern in the transporta-tion of foodstuffs, feed, and other agri-cultural products. Commensal rodentsconsume and contaminate human and

EVENTION AND CONTROL OF WILDLI

operative Extension Divisionstitute of Agriculture and Natural Resourcesiversity of Nebraska - Lincoln

ited States Department of Agricultureimal and Plant Health Inspection Serviceimal Damage Control

eat Plains Agricultural Councilildlife Committee

livestock feed. One rat can eat about1/2 pound (227 g) of feed per week,and will contaminate and waste per-haps 10 times that amount.

Rodents destroy insulation, electricalwiring, plumbing, and other structuralcomponents of buildings (Fig. 1). Insu-

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1/2"1/4"

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Fig. 2. Rat traveling along an electric wire.

Fig. 3. Rats can gain entry through holes largerthan 1/2 inch (1.3 cm); mice can use holes largerthan 1/4 inch (0.6 cm).

lation damage alone may amount to aloss of several thousand dollars in onlya few years. Energy loss from dam-aged buildings results in added annualcosts. Rodent-induced fires from dam-aged electrical wiring or nest buildingin electrical panels cause loss of prop-erty and threaten human safety.Rodents also serve as vectors or reser-voirs of a variety of diseases, such assalmonellosis, leptospirosis, andmurine typhus, that are transmittableto humans. Additionally, they may besources of swine dysentery, brucel-losis, sarcoptic mange, and tuberculo-sis, all of which affect livestock or pets.

The most effective means of limitingrodent damage is rodent-proof con-struction. New buildings should bedesigned and built to prevent rodententry. Rodent-proofing is a goodinvestment. Designing and construct-ing a rodent-proof building is lessexpensive than adding rodent-proof-ing later. Nevertheless, poor mainte-nance or management practices, suchas leaving entry doors and unscreenedwindows open, will make the best-constructed building susceptible torodent entry. Techniques discussedhere apply both to new constructionand to the modification of existingstructures.

Junctures where utilities (pipes, cables)enter structures require special consid-eration in preventing rodent entry.Some earthquake design criteriarequire open spaces in important jointsand other support areas, to allow forlimited movement of tall structures.These present a real challenge in thedesign of rodent-proof construction.

Physical Abilities of Ratsand Mice

To prevent rodent entry, their capabili-ties must be understood. For example,both rats and mice can:- run along or climb electrical wires,

pipes, fences, poles, ropes, cables,vines, shrubs, and trees to gainentry to a building (Fig. 2);

- climb almost any rough vertical sur-face, such as wood, brick, concrete,weathered sheet metal, and manyplastic products;

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- crawl horizontally along or throughpipes, augers, conveyors, conduit,and underground utility and com-munications lines;

- gnaw through a wide variety ofmaterials, including lead and alumi-num sheeting, window screens,wood, rubber, vinyl, fiberglass,plastic, and low-quality concrete orconcrete block.

Rats can:- crawl through or under any opening

higher or wider than 1/2 inch (1.3cm) (Fig 3);

- climb the outside of vertical pipesand conduits up to 3 inches (7.6 cm)in diameter; climb the outside oflarger pipes attached to buildingsby bracing themselves between thewall and the pipe; climb the insideof vertical pipes, wall voids, orearthquake safety seams and jointsbetween 1 1/2 and 4 inches (3.8 and10.2 cm) in diameter;

- jump from a flat surface up to 36inches (91 cm) vertically and as faras 48 inches horizontally;

- drop 50 feet (15 m) without beingseriously injured;

- burrow straight down into theground for at least 36 inches (91cm);

- reach as high or wide as 13 inches (33cm);

- swim as far as 1/2 mile (800 m) inopen water, dive through watertraps in plumbing, and travel in

sewer lines against a substantialwater current. In areas where highrat populations exist, it is commonfor both roof rats and Norway ratsto enter buildings through toiletsand uncovered drains.

House mice can:- enter openings larger than 1/4 inch

(0.6 cm);- jump as high as 18 inches (46 cm)

from a floor onto an elevated sur-face;

- travel considerable distances crawl-ing upside-down along screen wire;

- survive and reproduce at a tempera-ture of 24oF (-4oC) if adequate foodand nesting material are available.

Survey for Entry Points

When inspecting sites for potentialrodent entry points, look for rubmarks, droppings, tracks, gnawing, orother rodent signs. Special attentionshould be paid to areas discussedunder Common Rodent Entry Points(below). Keep in mind the physicalabilities and behavior of the particularrodents, especially their tendency toseek shelter behind, under, or in appli-ances, sinks cabinets, drawers, storedgoods, wall voids, false ceilings, andother undisturbed areas.

To conduct a thorough survey, inspec-tors will need an inspection form andpaper for noting and illustrating itemsneeding attention; a good flashlight; amirror (to see under and behind ob-jects); and screwdrivers and othersmall hand tools to remove interiorand exterior vent grills, appliance baseplates, and service doors to attics,crawl spaces, and utility cabinets. Atape measure is usually necessarywhen preparing a plan and estimatingmaterials needed for repair. A smalldustpan, broom, and some lime, flour,or similar powdered material are use-ful in preparing an area for a follow-up

Fig. 4a. Low-profile wall vent with poorlyattached hardware cloth, allowing for easyrodent entry.

Fig. 4b. Gap (3/4 x 26 inch) between wallcovering and framing at roof joint, allowing ratsand mice easy access.

Fig. 4c. Large hole gnawed by Norway rats inweather strip on the base of a warehouse door.

observation of fresh tracks. A cameracan be of great value, especially whentrying to design a project after leavingthe site, or when seeking assistancefrom someone unfamiliar with the site.A simple item to use when measuringgaps under doors or around pipes,screens, or vents is a common woodenpencil or ball-point pen (usually 3/16to 3/8 inch [0.5 to 1.0 cm] in diameter)— large enough for mouse entry.

Common Rodent EntryPoints

Many structures have inherently simi-lar rodent entry areas due to similarityin design (particularly entry and ser-vice areas), utility sources, and build-ing equipment. For the sake ofsimplicity, we have attempted togroup similar structures and settings.

Commercial Office Buildings,Hotels, Hospitals, and RetailStores. Commercial buildings areconstructed from types of materialsand design methods that vary greatlyin the degree of susceptibility to rodentinfestation (for example, metal andconcrete versus wood). Most struc-tures eventually become less rodent-proof due to deterioration, alteration,or repair. Heating, air conditioning,plumbing, electrical service, and firesprinklers provide some of the most

commonly encountered rodent entrypoints. Neoprene seals, spray-in-placefoam, and similar products commonlyused to close openings are not rodent-proof.

Even in new buildings, utility pipes,electrical conduit (often at meters orcircuit breaker panels), water and gaslines, and communication cables gen-erally have large openings that permitentry of mice and rats. Once rodentshave entered walls, they generallyhave ready access to much of a build-ing via holes for utility pipes and wiresin the framing, and via overhead sus-pended ceilings or other types of con-struction adjacent to utility enclosures.Specific problem areas include poorlysealed heating and air conditioningducts; roof and wall vents installedwithout strong, well-attached hard-ware cloth screening (Fig. 4a); roof andwall joints and edges without properlyinstalled metal flashing (Fig. 4b); anddoors hung unevenly or too high, orlined with unprotected soft rubberweather stripping (Fig. 4c). Refuse andfood handling areas are likely to havethe greatest rodent pressure. In olderbuildings, cracks in concrete slabs,brick and concrete block walls, orworn or damaged drain covers allowrodent entry. Air and elevator shaftsand laundry chutes also merit closeinspection.

Food Handling Facilities andWarehouses. Businesses in whichfood is stored or handled are espe-cially prone to rodent invasion. Goodsanitation practices are essential. Keep-ing food well-sealed is very importantand more difficult than might initiallyappear. Scraps of food can often befound in floor drains, under foodpreparation equipment and storedproducts, and around refuse and entryareas. Outside doors are often left ajaror fit poorly due to heavy use, physicaldamage, or improper installation.

Space under equipment (mixers,stoves, counters, or refrigerators)should allow easy cleaning and inspec-tion, or be closed off completely withrodent-proof materials (Fig. 5). Miceand rats are sometimes found usingfreezer and refrigerator compressorareas for harborage and water (fromcondensation on cold coils). Mice areoften found in the insulated walls oflarge coolers. Look closely at cornersand edges of metal, or other materialcovering the insulation, for rodentopenings. Drains should have ade-quate screens or grates to preventrodent entry.

Food disposal, refuse, and damagedgoods areas are often located close tofood handling or storage areas and arenot sealed from rodents. Areas nearloading docks should be closely

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Fig. 5. Area under food preparation equipmentis raised above a smooth stainless steel floor,allowing for easy cleaning of food spills andopen to prevent harborage.

Fig. 6. Poorly installed light-gauge roof vent,allowing easy access of rodents between roofingand base of vent. Gaps were large enough toallow rats and pigeons to enter.

inspected for cracks, broken screens,damaged doors, and uneven floorsnear doorways. Interior loading docksserved by rail cars are difficult to closedue to the tracks, but rubber doorguards made to fit the tracks are avail-able and will deter rodent entry.

Rodent-infested goods in food ware-houses commonly include cereals,flour, and baking mixes; waxed cartondrinks; dry pet foods; dried fruits andnuts; fresh produce; paper goods;charcoal briquets, and damagedgoods. Products in these categoriesshould be kept in open, easily in-spected areas, not in dark corners.Regular and routine removal of suchnonsalable or nonusable productsshould be standard practice to enhancecleanliness and safety and to reduceharborage.

Apartments and Houses. Utilityentry points include undergroundelectrical and communication trunklines, and exhaust vents for clothesdryers. Power lines have always beena favorite route of travel for commen-sal rodents, especially roof rats. Checkall roof joints for tightness and pres-ence of flashing, if rats and mice haveaccess to the roof via wire, pipes,plants, or rough-textured walls. Alsocheck roof and sewer vents for ade-quate screening and sealing, including

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presence of tight roof jacks (Fig. 6).Chimneys should be checked for prop-erly installed flashing or for missingmortar.

Rats occasionally enter buildingsthrough toilet traps in inner-city areaswith rat-infested sewer systems. Insuch cases, tracks and water may befound on the rim of toilet bowls. Bothroof and Norway rats have beenknown to enter structures via the sew-age system. This route usually occursin older (20 years or more) establishedareas with poorly maintained sewersystems. Mice often enter under entrydoors, through holes beside waterpipes and electrical conduit, andthrough the cold air return ducts onforced air furnaces, especially thoselocated in outside cabinets or garages,and underneath mobile homes. Miceand rats often find easy access togarage areas through open doors orunder and beside poor-fitting garagedoors. Once in the garage, they maygain entry into the main structurealong electrical lines, pipes, poorlysealed fire wall sheathing, or aroundfurnace ducts, hot water heaters, orlaundry drains.

If rodents are able to reach the attic,they may travel from room to room orunit to unit through openings forpipes, ducts, and wiring. Attics pro-

vide excellent harborage in winter,spring, and fall, but are often too hotduring summer. Common attics, base-ments, or raised foundations in condo-miniums and apartments are afrequent source of rodent infestation.

Another source of entry to residences,and a source of harborage for rats andmice, are fireplaces—especially thenewer preconstructed zero clearancesheet metal units that eliminate theneed for concrete mortar and brick. Ahollow space is left in the siding andthe fireplace support framing betweenthe outside wall and the fireplace. Ratsand mice can enter this area from theoutside via the roof joint, between thesiding and decorative wood cornertrim, around gas pipes, or outsidewood storage doors.

Once a rodent gets into the attic, insideentry to the fireplace void is often easybecause of poorly fitted sheeting ormetal collars. Entry to the inside of thefireplace is made from the damperarea or cool air and warm air returnson units that provide for air circulationaround the firebox. When the fireplaceis in use, the heat will prevent rodententry. If the outside cannot be sealed,glass doors that seal the burn area arerecommended to prevent rodent entrythroughout the year. Cracked andmissing mortar, or poorly fitted sidingor plaster, may allow entry throughbrick or rock fireplaces.

Tile or shake shingle roofs allowrodent entry if the roof is not solidlysheeted with plywood or similarmaterial and the tile is not properly fit-ted and grouted. Vents without tightlyfitted double roof jacks also facilitateaccess to rodents.

Gaps or flaws in foundations andslabs, or where the wall framing meetsthe foundation or slab floor, may pro-vide large enough openings for rodententry. Older buildings commonly havecracked foundations, cracked plasteror mortar, warped siding, or brokenand torn vent screens. Wood or mas-onite siding is especially vulnerable towarping and cracking near cornersand around the base of the building.Old, unused holes where utilities for-merly entered the structure are also

Fig. 8. Large gap between roll-up warehousedoor frame and wall, allowing for easy rodentaccess.

common, especially in raised founda-tion and basement homes. Windowscreens are often left off or fit poorly inolder, low-cost apartments and homes,allowing rodent entry from exteriorutility lines and pipes running alongexterior walls. Runways going to win-dow ledges are often observed onstucco and brick walls and in orna-mental plantings next to buildings.

Manufacturing Plants and FarmBuildings. Overhead or under-ground pipes, conveyor belts, andaugers commonly found in farm build-ings and factories are often used asentry points and routes into andbetween buildings. Such equipment,particularly if abandoned, may pro-vide harborage as well as food.Rodent-proofing these areas is noteasy if the equipment is still in use.

Utility entry points must be constantlymonitored for excess openings causedby equipment repair, installation, ormodification. Outside walls and doorsmust also be monitored for damagefrom equipment or livestock and fordamage or wear from heavy use. Ifwork patterns require doors to beopen during hours of darkness, whenrodent entry is most likely, rodent bar-riers may be needed, such as a solidfence or wall or a metal wing wall be-tween the foundation and adjacentloading dock areas (Fig. 7).

Buildings constructed with ribbed orcorrugated metal siding allow rodent

Fig. 7. Rodent barrier or guard used betweensteps, foundation, or other sources of rodententry and the loading dock doors when doorsmust be left open at night.

door frame

loadingdock

steps

30"

8"

metal w. well

Door frame

Metal wall Steps

30"

8"

Loadingdock

entry if the bottoms of the sidingpanels do not rest flat on a solid sur-face or they are not otherwise closedoff. Sections of prefabricated buildingsshould be assembled tightly, and gapsat joints should be covered with metalflashing. Often, however, they are leftopen, especially at corners and at thefoundation/slab interface.

Roll-up or overhead doors often pro-vide easy entry for rodents, birds, andbats. With the door closed, check forgaps along the sides, bottom, and topof the door (Fig. 8). A gap at the top iscommon. Rats and mice can easilyclimb up the space between the doorand the inner wall or track to the top,where they gain entry and climb downthe inside of the track. Gaps betweenthe track and the wall are also com-mon, especially if the track has beeninstalled on brick walls. Door bottomsmay be bent or damaged, leaving gapsalong the floor. Uneven floors due tofrost heaves may leave gaps when thedoor is closed.

Screens on windows, crawl spaces,and vents are often damaged in farmand industrial buildings. Check thesecarefully for needed repair or replace-ment.

One of the greatest challenges in farmbuildings is preventing feed and seedfrom being a food source for rodents.Good sanitation practices are very

important. Clean up spilled feed, andstore feed and seed in rodent-proofbuildings and containers. Keep sackedmaterials off the floor when possible.This facilitates for inspection andreduces harborage.

Excluding rodents from livestock andpoultry operations is another challengedue to livestock and manure manage-ment and various animal husbandrypractices. Nevertheless, rodent-proof-ing is important and can be accom-plished. Many of the entry pointsalready identified for other types ofstructures apply to farm buildings.Additional problem areas includeinsulated walls used for harborage,feed bins, and portable feed bunks.These are but a few of the challengesdiscussed in more depth under Exclu-sion Methods (below).

Exclusion Methods:Existing Structures andEquipment

Holes and Openings. By gnawing,rats can gain entry through any open-ing greater than 1/2 inch (1.3 cm)across, and mice through any openinglarger than 1/4 inch (0.6 cm). Thepaired front (incisor) teeth of rats andmice curve slightly inward. Thisinward curve makes it difficult forthem to gnaw into a flat, hard surface.When given a rough surface or anedge to bite into, however, they canquickly gnaw into most materials. Toprevent rodent entry, seal all suchholes with durable materials. Steelwool, copper gauze (Stuf-it® brand) orscreen wire packed tightly into open-ings is a good temporary plug. Forlong-term or permanent repair, mix aquick-drying patching plaster or an-choring such as Fixall® into a wad ofStuf-it® before pushing the materialinto the hole, and smooth over the out-side (Fig. 9). If steel wool is used, ruststains are likely to result. Holes 3inches (8 cm) or more in diametershould be covered or backed with1/4-inch (0.6-cm) woven/weldedhardware cloth prior to filling with agood patching compound (see recom-mendations under Foundations and

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Table 1. Recommended materials forrodent-proofing.

Concrete: Minimum thickness of 2 inches (5.1cm) if reinforced, or 3 3/4 inches (9.5 cm) ifnot reinforced.

Galvanized sheet metal: 24 gauge or heavier forwall or pipe barriers; 22 gauge or heavier forkick plates or door edging. Perforated orexpanded sheet metal grills should be 14gauge.

Brick: 3 3/4 inches (9.5 cm) thick with jointsfilled with mortar.

Hardware cloth (wire mesh): Woven, 19-gauge,1/2- x 1/2-inch (1.3- x 1.3-cm) mesh toexclude 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; 18gauge for kick plates and guards.

Fig. 9. Patching small holes with copper gauzeand a fast-drying patching compound (left), and1/4-inch hardware cloth for larger holes (right).

Fig. 10. Frequently used patching materials on1/4-inch woven hardware cloth backing.Fomofill®, Pour Stone®, Fix-all®, and CustomPlug® are shown as examples of the manymaterials available.

Fig. 11. Seal gaps or holes with rodent-proof materials where pipes, wires, or other similar objectsenter buildings.

Floors). Another backing materialavailable is Strong PatchTM (D. P.Wagner Mfg. Inc.), a 6 x 6-inch (15 x15-cm) sheet metal patch to coverholes up to 5 x 5 inches (11 x 11 cm).It has a self-adhesive backing and amesh on the surface for better adhe-sion of the patching compound orother texture.

To close larger openings or protectother areas subject to gnawing, usematerials such as those listed in Table1. Hardware cloth, if not woven,breaks easily. The woven/weldedhardware cloth maintains its shapewhen cut to fit around pipes or otherobjects. Hardware cloth used to covergaps and holes can be filled with foamcaulk, Fix-all®, Quick-Fix®, or otherfast-drying interior patching com-pounds. When used on the exterior,concrete mortar, plaster, or ConcretePatch® can be used to provide longer-term rodent-proofing (Fig. 10). Theseare just a few of the many productsavailable.

Close openings around augers, pipes,and electric cables where they enterstructures with Portland cement mor-tar, Concrete Patch®, masonry, ormetal collars (Fig. 11). Even a smallunprotected opening can be an invita-tion to rodents.

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The ribs and corrugations in metal sid-ing can be blocked with metal or mor-tar. Rubber or vinyl weather stops arequickly gnawed through. Design ormodify buildings with metal siding bybutting siding panels or sheets againstsolid materials (metal flashing or con-

crete) so the openings are not present(Fig. 12). Caution: letting metal sidingrest directly against concrete can leadto accelerated rusting and corrosion.The siding should be installed so thatopenings are no greater than 1/8 inch(0.3 cm) wide. If the siding is installedwith the ribs horizontal, the ends muststill be sealed or the bottom of thedecorative corner trim flashed andclosed.

Fig. 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 blockentry (right).

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

Vents and Windows. Use only metalwindow screening materials wherewindows or doors are accessible torodents. Avoid unnecessary ledgesoutside windows. When necessary,screen ventilation openings and win-dows with woven/welded galvanizedhardware cloth. Such screening is criti-cal in commercial and farm buildingsand where high rodent pressures inresidential areas are found. For largeopenings or where the screen may besubject to abuse, add crossbars to sup-port the hardware cloth. If the openingis an access route, install the screen ona hinged frame.

All vents and duct openings for heat-ing and air conditioning should bescreened or raised and/or guardedwith an excluder device to preventrodent entry. Residential cold airreturn grills can easily be mouse-proofed by placing 1/4-inch (0.6-cm)hardware cloth behind the grill whereit is not unsightly. In some applica-tions, power vents can be coveredwith hinged metal plates (louvered)that open with air flow and closewhen fans are off. These louvers areonly effective if they fit tightly and thesides are recessed to prevent rodentsfrom pushing through them. Caution:Hardware cloth less than 1/2 x 1/2inch (1.3 x 1.3 cm) significantly reducesair flow. In buildings where ventilation

is already marginally adequate orinadequate, such further restrictionsmay be unacceptable. In some loca-tions, small mesh screens can becomeclogged with dust or freeze over. Insuch situations, the use of 1/2 x 1/2-inch (1.3 x 1.3-cm) hardware cloth is areasonable compromise betweenventilation requirements and rodentcontrol.

Exterior Doors. Doors should fittightly, the distance between the bot-tom of the door and the threshold notexceeding 1/4 inch (0.6 cm). In someinstances, it is possible to build up thethreshold rather than modify the door.

Metal thresholds can be fastened tofloors. Steel pipes embedded in a con-crete floor make good rodent-proofthresholds and allow doors to swingfree when open. Pipe thresholds are es-pecially useful where doorways areused by wheeled pallet jacks, heavyequipment, or livestock. Install flash-ing or a metal channel on the loweredge of doors, particularly softwooddoors (Fig. 13); a plastic door boot hasbeen successfully used where the doorreceives low use and the edges are noteasily accessible to rodent gnawing.Properly applied flashing extends towithin 1/8 inch (0.3 cm) of the edge ofthe door at the sides and bottom. Closethe gap at the top or sides of roll-updoors with conveyor belt materialshaped to fit into the side channelframes and mounted on the top doorjamb. Bent bottom rails on doorsshould be straightened. Concrete dam-age due to inadequate reinforcementor poor placement practices should berepaired or the concrete replaced. Ametal or pipe threshold is sometimespreferable or required.

Mechanical door-closing devices savetime and help overcome human negli-gence. Equip doorways used for venti-lation with rodent-proof screen doors,or if the door surface is too slick forrodents to climb, modify the existingdoor so the upper half can be left openfor ventilation. Always use a heavykick plate and solid frame on screendoors in commercial and agricultural

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Fig. 15. Left: A curtain wall made of concrete will prevent rats from burrowing under foundations to gainentry to buildings. Curtain walls can be installed on existing buildings. A horizontal footing extensionoften deflects burrowing rodents away from structures. Right: Feed sheds, corn cribs, and other existingwooden 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.

Interiorfloorsurface

36"

12"

18-24"

1/4" mesh

4'

Ground level

buildings. Light-framed screen doorseasily get bent out of shape, allowingrodent entry.

Foundations and Floors. Gaps orflaws along building exteriors wherethe wall framing or siding meets thefoundation provide easy entry forrodents. Such openings can be pre-vented by well-formed and finishedconcrete work and installation of tightwall framing and siding, or installingmetal screed-type flashing between thesiding and the foundation. Use ofrodent-proof exterior surface materialssuch as concrete, plaster, or metalsheeting is also effective if properly in-stalled so that all ribs or corrugationsare closed.

Rodents can gain entry into buildingswith piers or shallow foundation wallsby burrowing beneath the floor orfoundation. To prevent rat entry bythis route, extend foundation wallsbelow ground at least 36 inches (91cm). This also reduces damage fromfrost. A horizontal footing extensionalso may be added to deflect burrow-ing rodents away from the foundation(Fig. 14). Avoid the use of slab-on-grade construction techniques for agri-cultural buildings or bulk bin pads.The possible savings in initial con-struction may be quickly offset by thecosts of rodent damage and controlmeasures.

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Fig. 14. A 1-foot horizontal footing extension todeflect burrowing rodents.

Ground level

2' min.

1' min.

4" min.

Rats exert more effort to enter build-ings where feed is available. They fre-quently seek shelter under concretefloors and slabs, where they burrow tofind protection. Ideally, install floors,slabs, and sidewalks with deep foot-ings, or with curtain walls of concreteor 1/4-inch (0.6-cm) mesh wire (Fig.15). The choice between concrete andwire mesh depends on the expectedlife of the structure. Though wire meshcosts considerably less than concrete,its usefulness generally lasts only 5 to10 years.

Repair cracks in foundations andfloors with concrete or masonry grout.There are numerous quick-settingtypes of products, such as Fix-all® orQuick-Fix®, which are for interior use,and Concrete Patch®, Rockite®, orPour Stone® for interior or exterior use(previously illustrated in figure 10).The four last-mentioned products arespecifically designed for repairs andhave quick setting, good adhesion, andnonshrinking properties which makethem ideal for exclusion work. Each,however, is made for a specific appli-cation: Pour Stone® and Rockite® aredesigned to be easily poured intocracks in floors or into holes to anchorbolts or machinery, and set hard in 15minutes. Concrete Patch® is a mortar-type material for repairing masonrysurfaces and has a vinyl polymer to

increase adhesion. It sets in 2 hoursand is hard after 12 hours. Quick-Fix®is a durable patching plaster for insideuse on plaster, drywall, or wood sur-faces. Drying time may be within 30minutes, depending on thickness. Withall of these types of repairs, the use ofreinforcement with hardware cloth isusually needed on vertical or overheadhorizontal surfaces to add strengthand provide the necessary backing.Rodents can claw and gnaw at con-crete and Portland cement until it isfully cured, so the use of 1/2-inch (1.3-cm) hardware cloth laid in the top 1/4inch (0.6 cm) of the repair area may benecessary if rats are currently using therepair area as an entry point. Other-wise, provide an effective temporaryrodent-proof protective overlay untilthe concrete is fully cured. Caution:Metal products placed within 1 inch(2.5 cm) of a concrete surface will oxi-dize and corrode and may discolor theconcrete.

If rats have gained access to crawlspaces under building floors, preventthem from getting into walls by usingsuch modifications as illustrated infigure 16.

Maintaining a clean, 3-foot-wide (1-m)weed-free area around building foun-dations, concrete slabs, and footingsoften discourages rodents from bur-rowing as well as eliminates a food

a

bc

1/4" mesh

Fig. 16. Method of excluding rats from double walls. In oldbuildings, galvanized sheet metal (a) may be cut to fit andnailed into place between studs, joists, floor, and sill; inbuildings under construction, noncombustible stops of goodquality concrete (b) or bricks (c) are recommended.

Fig. 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 36inches below the soil surface.

source and attractive harborage.Where erosion of bare soil is likely, thisbuffer can be maintained by regular,close mowing of vegetation or by in-stalling heavy gravel. To discourageburrowing, install a strip of 1-inch-di-ameter (2.5-cm) or larger gravel laid ina band at least 2 feet (60 cm) wide and1/2 foot (15 cm) deep.

Interior Rodent-Proofing. Whenrats or mice are present in a building,attention must be given to interior aswell as exterior rodent-proofing toremove all sources of shelter. A combi-nation of actions is required in suchinstances, as no single effort is likely toyield the desired result.

Concrete floors are preferred towooden floors. An attempt should bemade to seal off rodents. Use traps toremove the rodents, or place poisonbait packets through openings in thefloor or wall and then seal the open-ings with galvanized metal or hard-ware cloth and patching plaster aspreviously discussed. Promptly treatnew openings as they are found. Inoccupied buildings, always trap therodents before sealing interior walls toavoid odors, stains, and an influx ofinsects that feed on decaying rodentcarcasses.

Eliminate rodent hiding places beneathand behind equipment. Feeders in live-

stock facilities should have flat bot-toms and be designed and installed sothat rodents cannot find shelterbeneath or behind them. Give specialattention to storage rooms, closets,feed storage, or other areas where con-struction techniques may allowrodents access to walls, floor spaces, orattics. Stacks of wood and other storeditems should be 18 inches (46 cm)above the floor and 18 to 24 inches (46to 61 cm) away from walls to allow forproper cleaning and inspection. Ware-house stock should always be stackedoff the floor on pallets and away fromwalls, and it should be rotated often toprevent development of infestations inundisturbed areas.

Rodents often gnaw into wall materi-als at corners or where joints in con-struction materials provide an edge.Poor construction techniques mayallow rodents to gain access throughmaterials that are otherwise consid-ered rodent-proof.

Perimeter insulation is a necessary partof energy-efficient construction. Plac-ing insulation on the exterior of foun-dation walls subjects it to mechanicaldamage as well as infestation anddestruction by rodents. To preventdamage to perimeter insulation, usesandwich wall construction in whichthe insulation is placed within the con-

crete. Insulation placed on the outsideof a foundation wall requires protec-tive-covering material. Suitable materi-als include cement board, high densityfiberglass-reinforced plastics, trow-eled-on coatings such as Block Bond®,or Surewall®. In such situations, metalflashing should be used to prevent thepotential for entry routes for subter-ranean termites. Several companiesnow manufacture special coatings forexterior perimeter insulation. Exam-ples include DuraWallTM and Secure-wallTM.

Extend protective cover materials atleast 36 inches (91 cm) below finishedgrade. If the protective layer ends lessthan 36 inches (91 cm) below grade,add a horizontal ledge that extendsoutward at least 1 foot (30 cm). All topedges and corners must also be pro-tected with a close-fitting heavy-gaugemetal flashing (Fig. 17).

Drains and Pipes. Both rats andmice use drainage pipes or sewagesystems as routes to enter buildings.Equip floor drains with metal gratesheld firmly in place. Grate openingsshould not exceed 1/4 inch (0.6 cm).Maintain 1/2-inch (1.3-cm) hardwarecloth over sewer roof vents in rat-in-fested areas. If the sewer system isknown to be rat-infested, a “RatGuard” one way flap valve may be

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Fig. 18. “Rat Guard” one-way flap valve isavailable for placement in toilets where thesewer system is known to be rat-infested.

18"

Fig. 20. Sheet metal band attached to outsidewalls to prevent rodent climbing. Band shouldbe 14 to 18 inches wide and 30 inches abovewhere a rodent can jump from.

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

placed in toilets (Fig. 18). Sewer later-als should be checked for openingsthat could allow rodent entry. Smoke-producing leak detectors are oftenused by agencies checking sewer linesfor leaks or openings. If openings aredetected, replace the pipe or wrap thepipe break with 1/4-inch (0.6-cm)hardware cloth and use concretepatching material to seal the area. Raingutter downspouts are often used byrats to gain access to roofs. It may bepossible to screen over openings at thebase of downspouts with 1/2-inch(0.6-cm) hardware cloth or a grate, butthis will require continued mainte-nance to remove accumulated debris,particularly where leaves and smallsticks are washed from roofs into thegutter system. Flap valves have beenused here too—swinging shut exceptwhen water is flowing. Openings tofloor or driveway drains should havecovers. Gutter and other drain coversmust be kept clean of debris to preventwater backup.

Manure management systems in live-stock facilities may be of a type thatperiodically drains manure or waterfrom the building to a lagoon or otherstorage area. In such cases, a “floating”metal cover or check valve-style clo-sure at the open end of the dischargepipe, with a hinge at its upper edge,can be effective. The hinge must oper-ate easily so that the cover will open

46

when water or manure flows out butwill fall back into place when the flowstops in a manner similar to a tide gateused on drains in coastal areas. Thepotential for such covers to freezeshut, however, can be a drawback. Abetter method is to extend dischargepipes far enough over the bank or intothe lagoon to prevent rodents fromjumping or crawling into the open end.Install rodent shields to preventrodents from gaining access (Fig. 19).

Always cap pump-out ports whenunder-building manure storages arenot in use. Left open, they allowrodents easy entry.

Physical Barriers/Guards. To pre-vent rodents from climbing or travel-ing along a particular route, installguards made of sheet metal or similarmaterials (Table 1). Guards must bewide enough and positioned to keeprodents from reaching their outer mar-gins by climbing or jumping. Dock ar-eas may need guards to keep rodentsfrom jumping or climbing from foun-dations, pipes, steps, or rough exteriorwall surfaces, and from infestingtrucks or rail cars transporting goods.

A sheet metal band attached to a wallwill prevent rodents from climbing.Rodent guards should be at least 14inches (36 cm) but preferably 18 inches(46 cm) wide (Fig. 20). Inside build-ings, such guards can prevent rats andmice from climbing at corners. Used incombination with hardware cloth orother suitable material, they can make

a building essentially rodent-proof.These modifications are essential onpumping plants, water treatment fa-cilities, power stations, and communi-cations facilities. They have also beenused to make corn cribs, barns, andother older buildings in current userat- and mouse-proof.

Guards on walls should be at least 36inches (91 cm) above ground or floorlevel. Flat guards have been used toprevent rodents from traveling alonghorizontal or vertical pipes or electricwires (Fig. 21). Cones or discs act asrodent guards on suspended cables,rain gutter downspouts, conduit,ropes, augers, or pipes (Fig. 22). With

36" min.

12" min.

Fig. 21. Guards of various designs can prevent rodents from climbing along wires or pipes.

Fig. 22. Guards can be used to prevent rats from climbing augers, pipes, or wires leading tobuildings. Footings or foundations for grain bins should extend into the ground to prevent ratburrowing.

12"

12"

36"

18"

some ingenuity, you can design rodentguards to fit any given situation. Free-hanging guards are easily damaged.Circular guards must extend out 18inches (46 cm) around the line theyguard. They are constructed of 24-gauge metal and anchored in place byone or more arms on the side oppositeto that accessible to rats. Cone-shapedcircular guards prevent rats fromclimbing vertical pipes, pilings, andtrees. Shields or wire guards made of1/4-inch (0.6-cm) wire mesh are usefulin excluding rodents from the interiorof conveyor belts, underground powerand communications conduit, feed au-gers, fan housings, and similar open-ings.

Food Handling and StorageAreas. Even when all of the holes areplugged, rodents seem to find a wayinto food storage and handling areas.Sometimes rodents come in with sup-plies, or they run in through opendoors or windows. Often, one or moreopenings remain undetected. Thesehidden holes are often below sinks,behind equipment, in false or sus-pended ceilings, and behind or undercupboards. Once in an environmenthaving all the basic needs, rodentsquickly establish viable populations.The solution is to eliminate harborageand exclude rodents from food andwater sources inside the building.

All equipment such as large refrigera-tors, freezers, counters, dishwashers,and sanitizers should be raised andeasily movable, enabling cleaningunderneath and behind them. Insu-lated walls and closed areas should betightly closed off to avoid use as har-borage. Openings are commonly seenin new stainless-steel work counters insupports under the work surface, or inareas provided for drains. Drainsshould be easy to clean but shouldhave rodent-proof covers.

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

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Keep all damaged goods and returns,as well as refuse, in a separate rodent-proof room. Loading docks should bevery tightly rodent-proofed and onlyopen during daylight hours. All out-side doors should be self-closing, haveheavy kickplates, and be checkedperiodically for excessive gaps. Inwarehouses and storage situations,centralize highly susceptible foodstuffsinto areas that can be securely rodent-proofed and closely monitored. Theseareas should also have rodent bait sta-tions and multiple-capture mousetraps permanently installed around theperimeter.

Livestock Feed Bunks and Bins.Rats typically burrow and nest underfeed bunks that are placed directly onthe ground or near ground level. Prop-erly designed concrete bunks that sittightly on a concrete base eliminaterodent habitat. Though cattle trafficmay discourage burrowing under theconcrete slab, a foundation may beneeded to prevent burrowing aroundthe sides of the slab that do not receiveheavy cattle use. Concrete slabs onwhich feed bins are placed shouldhave foundations extending 36 inches(91 cm) into the soil at the outer edgeto prevent rats from burrowing underthe slab. Installing heavy gravel andmaintaining a clean, weed-free zonearound the perimeter of the slab willalso discourage rat burrowing andpermit easier detection of rat activity.

Feed and Refuse Storage. Live-stock or pet feed and edible refuseattract rodents and are a common foodsource. Always store these materials inmetal containers with tight-fitting lids.Food is often available to rodentsaround homes, kennels, and poultryand livestock feed storage areasbecause feed is kept in plastic or woodstorage bins or hoppers. These storagecontainers are frequently open at thetop, or may be gnawed through thesides. Check nonmetal hoppers fre-quently for holes and, when necessary,repair with sheet metal. Avoid the useof self-feeders for pets. Feed pets onlyas much as they will consume at onetime and only during daylight hours.

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Proper storage and disposal of house-hold garbage and dead animals is avery important part of rat control.Bulk dumpsters are often left with thetops open, or the tops are badly bent,allowing rodent entry. Constant vigi-lance and calls to the refuse companyshould correct these situations. Sealbulk trash compactors from rodents.Spilled refuse and juices from crushedcontents often create rodent problemsunder and behind compactors andbulk dumpsters. Clean these areas of-ten and install rodent screening in con-tainer drains.

Rodent-Proof BuildingDesign Considerations

The degree of a structure’s susceptibil-ity to rodent infestation depends onseveral considerations that the archi-tect, engineer, builder, and end-usershould keep foremost in mind prior toand during construction. All struc-tures, but particularly those intendedfor human occupancy, should be builtto be as rodent-proof as possible.

One consideration is the current andhistorical level of rodent problems inthe general area of the constructionsite. A project that is in an inner-cityredevelopment area with a known his-tory of rodent problems warrantsmore attention to rodent exclusionthan a project in a new urban fringearea with little or no history of prob-lems. Rodent control should be consid-ered during the preconstruction andconstruction phases. Infestations atthese times are common and some-times even include larger animals suchas skunks and opossums. Contractsshould require that contractors main-tain a clean and debris-free site.

If the area is known to have heavy ratand/or mouse problems, use exteriorsurface materials that have a hard,smooth surface on at least the lowerlevels that will be subject to contactwith the ground, ornamental plant-ings, fences, and other potential areasof rodent harborage. A viable alterna-tive is to utilize a smooth-surfaced

decorative band 14 to 18 inches (35 to46 cm) wide around the structureabove the rodent access height, in or-der to prevent rodents from climbingup exterior walls. Tightly rodent-proof sewer, electrical, communica-tion, water, and natural gas services,as well as exterior doors and windows.Loading docks should have exclusiondevices, automatic door-closingdevices, and good construction materi-als that preclude rodent climbing andentry. Stairs to the dock area shouldhave a tight-fitting personnel door farenough from the dock that rodentscannot jump from stairs to the dock.Dock bumper pads should be highenough (30 to 36 inches; [76 to 92 cm])to avoid being used as steps byrodents.

Another consideration is the building’sinterior design and intended use.Office buildings with tight-fitting inte-rior doors, tightly sealed wall voidsand utility access areas, and no food orwater sources are less attractive torodents than food handling facilities,apartments, hospitals, and ware-houses. Pallets commonly used inwarehouses provide good harboragefor rodents and should be rotatedregularly. Coolers and other food stor-age areas should be centrally locatedand, if possible, in sealed rooms, sepa-rate from other warehouse goods. Thisconfines rodent exclusion to a smallarea and provides for better observa-tion if problems arise. Even the clutterof machinery, parts, and nonfoodstored goods may attract rodents asharborage if food and water arenearby.

Containers for refuse disposal, includ-ing bulk dumpsters, should have tightlids. They should be kept clean andtightly sealed.

Earthquake safety joints can provide aroute from the basement to the topfloor of a building. Use 1/4-inch (0.6-cm) woven/welded hardware clothbetween floors in these joints to effec-tively stop rodent travel. Urethanefoam caulk or sheet metal screws havebeen used to hold the hardware clothin place without interfering with the

Fig. 23. Chimney with metal stack, heavy screen(animal excluder and spark arrestor), and tightroof to wall and Z-bar flashing joints.

Fig. 25. “Bird stop” shaped metal for tile roofsto prevent rodent and bird entry.

Fig. 26. A side wall exhaust vent with 1/4-inchhardware cloth.Fig. 24. Z-bar and roof-to-wall pan flashing,

when properly installed, prevent rodent entry.

Roof

Z-bar

Felt

Stucco

Roof to wall pan

Stucco

Felt

Z-bar

Roof to wall pan

Roof

purpose of the safety joints. Installadjustable metal collars around utilitypipes or other types of openings. Theyare available through building supplystores or can be easily cut from 24-gauge (or heavier) galvanized sheetmetal. Double walls can best be pro-tected near the foundation or floorarea with galvanized sheet metal,hardware cloth, concrete, or othermethods, as previously discussed. Useheavy (24-gauge or better) galvanizedflashing, called “screed,” between thesiding and foundation to close anyopenings created by warped wood or

Masonite, board and batt construction,stucco, or corrugated metal siding.Installation of fireplaces, especiallyzero clearance sheet metal types, posesa challenge when sealing the wall voidcreated around and above the fireboxto the roof. This area must be solidlysheeted with sheet rock or dry wall,and a metal collar called a draft stopmust be used at the ceiling and rooflines. When tightly fitted, it serves toprevent rodent movement. It is alsoimperative that good, tight Z-bar androof-to-wall pan flashing is properlyinstalled around the outside of thechimney. A metal cap and secureheavy mesh screen should be installedto prevent bird or small mammal entry(Fig. 23). There are many types ofmetal flashing. The Z-bar flashing is in-stalled so that it is under the felt paperand then bends outward to overlap theroof-to-wall flashing that rests againstthe roof and wall in an L shape (Fig.24).

Roofs and eaves often have openingslarge enough for raccoons andpigeons, as well as for small rodents toenter. Many of these problems insingle-family residences and apart-ments seem to occur where roof lineschange angles or elevations, leaving anarea under the eaves that is hard to fitwith siding. Problems are especiallycommon where the proper roof-to-wall and Z-bar flashing are not tightlycut and installed during initial con-struction. Shingle or tile roofs aresometimes a problem when they buttup against a wall or around roof ventsand at roof edges. Tile and shingles areless of a problem if solid sheathingunderlies the roofing material andadjacent walls have properly installedmetal flashing. Tile on roofs must bewell placed to avoid gaps and the endsshould all be sealed with preshapedmetal (commonly called “bird stop,”Fig. 25) or tile fillers. All other gapsshould be filled with concrete grout.

Vents must have openings of 1/4 x 1/4inch (0.6 x 0.6 cm) or less to preventhouse mouse entry (Fig. 26). There aremany different types of vents for spe-cialized placement. They include gablevents, exhaust vents, roof vents, foun-dation vents, and many others. The

building industry has had a problemgetting good-quality vents with strong1/4 inch galvanized hardware clothproperly fastened in place. Manymanufacturers are using nongalvan-ized hardware cloth or light screenwire, and others are simply shapingthe wire into place with no fasteningmethod used or using light spot-weld-ing spaced too far apart, allowing thescreen to be pushed away from thehousing of the vent.

Openings larger than 1/4 inch x 1/4inch (0.6 x 0.6 cm) can be used if airflow is inhibited, but the vent shouldbe located to prevent exposure to

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Fig. 27. Soft (upper) roof jack under tile.

rodents. This can easily be accom-plished on roofs with smooth sheetmetal raised 40 inches (1 m) or higher,constructed in a manner to avoidrough edge ribs or other surfaces thatrodents can climb. Place vents on thesides of buildings at least 40 inches(1 m) above the ground level, fences,or shrubs. Use a smooth surfacearound the vent if the wall materialhas a rough texture.

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Poorly installed roof jacks are a com-mon entry point for rodents. Tile andshake shingle roofs require the use ofdouble jacks, one rigid and the otheroften soft or flexible. The solid jack isinstalled over one layer of roofing feltand the sheeting or other substrate,and an overlapping piece of felt is thenlayered over the jack. The second jackis placed between the layers of roofing(tile or shingles, Fig. 27). When rigidmetal is used for the upper (second)jack, gaps left between the tile and themetal should be grouted to preventrodent or bird entry or nesting.

Permanent Bait/Census Stations.Another method that can be used toprevent the build-up of rodent popula-tions is the installation of permanentrodent “service access areas” aroundthe exterior of buildings. These cup-board-type areas can be built atground level into walls or foundationsnear loading docks, trash areas, nearutility service panels, corners, or rearalleys. The stations should be largeenough to contain an automaticmultiple-capture mouse trap (Ketch-AllTM or Tin CatTM) and a bait stationcontaining toxic baits. Placebo baitscan be used to monitor rodent

population pressure. The panel doorshould have tamper-resistant screws,bolts, or locks and have two 2 1/2- x 21/2-inch (6.4- x 6.4-cm) openings atground level to allow entry by rats andmice.

For additional information on the con-trol of commensal rodents, see thechapters House Mice, Norway Rats,and Roof Rats.

Acknowledgments

Figures 1a, 2, 4a, b, and c, 5, 6, 8, 9, 10, 12, 18, 20,23, 25, 26, and 27 by Rex O. Baker.

Figure 1b by Robert M. Timm.

Figures 3, 7, 14, 15, 17, 19, 22, and 24 byFrances I. Gould, David Thornhill andClint Chapman.

Figures 11, 13, 16, and 21 adapted by Frances I.Guld and David Thornhill from Pratt, H.D.,B. F. Bjornson, and K. S. Littig. 1977. Controlof domestic rats and mice. Publ. Health Serv.US Dep. Health, Educ. Welfare, Pub. No.(CDC) 77-841. 47 pp.

EditorsScott E. HygnstromRobert M. TimmGary E. Larson