Module 6     Exclusion

Learning Objectives

  1. Identify common ways that wildlife enter structures and surrounding areas.
  2. Explain how to determine whether an opening is being used by wildlife.
  3. Give examples of manufactured items used for excluding wildlife.
  4. Know techniques that should be avoided.

Terms to Know

Exclusion   Techniques and products that prevent wildlife from entering an area.

Low-impedance fence chargers   Electric fence chargers that deliver pulses of electricity that produce a painful, but not continuous, jolt of electricity. The gap in the pulse allows people and animals to move away from the fence.


Exclusion refers to the use of barriers to reduce access of wildlife to resources. It is a type of habitat modification. Wildlife enter structures through existing holes or cracks in walls and foundations, or by gnawing new holes. Chimneys and vents offer access opportunities as well. This module gives information on exclusion, a valuable method to prevent and control wildlife damage.

Exclusion as Prevention

Exclusion is a useful method for NWCOs. First, exclusion does not use chemicals that may harm non-target animals or people. Second, exclusion provides immediate, long-term, and, frequently, 100% protection. A disadvantage of exclusion is the cost; it may be very expensive, particularly when large areas need protection (Figure 1).

Protecting Structures

Knowledge of how to exclude rodents from structures provides a foundation for understanding how to use this technique on other animals. As valuable as exclusion techniques are, property owners should be made aware that excluding rodents from a structure will not guarantee rodent-free buildings forever. Rodents can gain access to a structure by gnawing a new hole or getting carried in with a box or appliance. Therefore, exclusion techniques should be part of an overall integrated pest management (IPM) program that includes sanitation, population control, and monitoring.

Figure 1. Fences are effective for excluding deer from areas, but can be expensive. Photo by Paul D Curtis.

Know the Animals

Effective exclusion requires awareness of an animal’s behavior and physical capabilities. Generally, an entire structure must be secured to prevent access by climbing animals, but only up to 4 feet to prevent access by those that cannot climb. Exclusion of birds requires consideration of the entire building, especially perching and nesting locations.

Holes and Openings

A thorough site inspection is necessary for any exclusion technique to be effective. All potential entry points must be located. Rats need slightly more than a ½-inch gap to enter; mice need slightly more than a ¼-inch gap. Do not ignore small crevices, as gnawing rodents can enlarge them quickly.

Screen, plug, or secure openings, but only after you are certain that animals are not using them. When uncertain about whether animals are using an opening, plug it with rolled up newspaper and monitor it for at least 3 consecutive days of fair, warm weather (Figures 2a to 2c). If the newspaper is untouched after 3 days, it is reasonable to assume that the opening is no longer in use. Do not use this method if bats may be present.

Figures 2a to 2c. Place paper on the end of a painter’s extension pole to plug holes without using a ladder. Do not use this technique near power lines or electrical wires. Photos by Stephen M. Vantassel.

Next, consider whether a potential opening may be filled or if it must be screened. If airflow is needed, use a screen. If the gap can be filled, several options are available.

Secure holes up to ½ inch in diameter with caulk or other sealant. Sealants have greater elasticity than caulks and are preferable where movement in the substrate is expected, such as joints. Gaps larger than ½ inch require a backer to help hold the sealant. Backer rod, Copper Stuf-it™, and Xcluder™ have the flexibility to be wedged in crevices. They also provide enough structure to support the sealant as it dries.

Caulks and sealants come in several formulations. When selecting a caulk or sealant, choose the one that is appropriate for the substrate, exposure to weather, and aesthetics. It is common for NWCOs to carry several formulations of sealant.

Fill large openings, such as holes made by squirrels, first with expanding foam or other insulation to prevent leakage of air. It is thought that by inhibiting air movement and potential heat loss, animals will be less attracted to the structure as a possible nest or den site. After sealing the hole(s), install a gnaw-resistant barrier over the gap to prevent entry (Table 1).

Table 1.
Materials recommended for rodent-proofing.

Concrete: reinforced – minimum thickness of 2 inches;
not reinforced – 3¾ inches
Galvanized sheet metal: 24-gauge or heavier. Perforated sheet metal grills should be 14-gauge
Brick: 3 ¾ inches thick with mortar-filled joints
Hardware cloth (wire mesh): 19-gauge, ½- x ½-inch mesh to exclude rats; 24-gauge, ¼ x ¼-inch mesh to exclude mice
Aluminum: 22-gauge for frames and flashing; 20-gauge for kick plates; 18-gauge for guards

For openings less than ¾ inches in diameter that cannot be secured by other means, wedge copper or stainless steel wool tightly into the gap. Coarse steel wool will work temporarily, but eventually will rust. Never rely on expanding foam alone to secure an opening.


Every flue in a masonry chimney should be protected with a professionally manufactured chimney cap that meets all requirements in your local fire code. Chimney caps are sold in single-flue (Figure 3a) and multi-flue designs (Figure 3b).

Figure 3a. Single-flue cap. Photo by Hy-C Co., Inc.


Figure 3b. Multi-flue cap. Photo by Hy-C Co., Inc.

Purchase and install only professionally manufactured chimney caps made with stainless steel or copper. Galvanized-steel caps do not provide sufficient savings to justify their use. Chimney caps have solid roofs that protect the chimney crown from water damage. Check existing chimney caps for damage. Ensure that animals are not residing in flues prior to capping. Chimney screens (also called raccoon screens) provide the same protection against animal entry as do chimney caps, but without a solid roof. They are less expensive and can be placed on flues closer than 6 inches apart.

Under no circumstances should hardware cloth be used to protect a chimney. Wire mesh eventually rusts and the screen may allow water from snow, rain, or exhaust fumes to freeze, thereby blocking the exhaust gases and forcing them into the home.


Consult state regulations before modifying vents. Vents require special care because they must allow air flow. As a rule, metal screening is best placed on the exterior of a vent so that mosquito netting may be protected from damage. Screen with a mesh size less than ½- x ½-inch may significantly reduce airflow.

Roof and attic vents are best secured from the outside to prevent animal entry. Professionally manufactured screens (stainless steel and galvanized; Figure 4) are available.

Figure 4. Screen for a mushroom vent.
Photo by Hy-C Co., Inc.

Stainless-steel screens are preferable for roof vents. Follow manufacturer recommendations for securing the screen to a roof to prevent leaking and ice damming. Check state regulations before securing vents. Although screens installed from inside an attic are effective and often easier to install, avoid using this option when protecting mushroom vents and other large vents. While your screen will prevent entry into the attic, squirrels and birds may build a nest on top of the screen under the protection of the mushroom vent.

Ideally, screens should have a mesh size of ¼ inch to prevent entry by bats and mice, although size should be balanced with air flow considerations. When installing vents, particularly on a roof, it is critical to seal screws to prevent leaks. Place a bead of roof sealant under and on top of the shingle where the screw is to be placed. After driving the screw through the sealant, cap the screw with a final dab of sealant. Consider whether snow or ice may be blocked by the screen. You may need to install barriers to shift snow and ice to one or both sides of the screen.

Weep vents are gaps that are purposely placed in brick walls to allow moisture to vent outward. With brick-veneer construction, a gap often is placed behind the brick where drywall does not come completely down to the bottom edge. Small animals can move freely behind the brick and find entry into the home through the gaps.

The act of securing these openings must balance the need for moisture to vent with the need for preventing insects and wildlife from entering. Use of ¼-inch wire mesh is discouraged as it will not prevent entry by insects or snakes. Devices such as Retrofit Weep Hole Covers™ (Figure 5) or a section of Xcluder™ material placed in the gap will limit access by insects, snakes, and mice.

Figure 5. Weep vent cover. Image courtesy of Wildlife Control Supplies, LLC.

Ridge vents frequently are missing end-covers that provide openings for wildlife (Figure 6a). Use Copper Stuf-it™ or Xcluder™ (Figure 6b) and sealant to secure the ends. Add hardware cloth to the spot to increase its resistance to animal entry.

Figure 6a. The end of this ridge vent is open to wildlife entry. Photo by Stephen M. Vantassel.
Figure 6b. X-cluder™ fill fabric.
Photo courtesy of Wildlife Control Supplies, LLC.

Dryer exhaust vents require special consideration, as improper screening hastens the build-up of lint and increases the likelihood of fires. Although some screens have air-activated flaps that seal when not in use, the flaps often fail as they become jammed with lint. Consider installing a dryer-vent cover with a floating ball that secures the vent when the dryer stops (Figure 7a).

Figure 7a. Floating-ball-type cover for dryer vents.
Photo by Stephen M. Vantassel.

Whatever method is chosen, be sure to follow all state and local codes and ask homeowners to check their screens monthly. Use professionally manufactured screens whenever possible (Figure 7b).

Figure 7b. Dryer Ventguard™.
Photo courtesy of Hy-C Co., Inc.

Homemade screens can be constructed in limited circumstances, such as when mouse exclusion is the priority, if legal. A large box (1 cubic foot or larger) constructed of ¼-inch mesh can be used to protect dryer vents. The large size is necessary due to reduced airflow, restricted by the mesh. Install a re-sealable flap to enable removal of lint. Inspect the screen at least monthly to ensure that lint does not obstruct airflow. Snow and ice buildup also are concerns. These screens may be used on shower vent exhausts, as well.

Sewer vents may attract squirrels that occasionally enter vents, thus raising the possibility of blockage. Crown-vent guards are designed to prevent animal entry. Ensure that exhaust pipes meet Consumer Product Safety Commission standards. Properly seal gaps between pipes and structures to prevent rodent entry.

Bathroom exhaust vent covers made from plastic are sufficient to protect bathroom exhaust vents from entry by birds (Figure 8).

Figure 8. Plastic vent cover suitable for bathroom exhaust vents. Photo by Stephen M. Vantassel.

Other vents on a wall may be secured with galvanized ¼- to ½-inch wire mesh. Cut the mesh at least 2 inches wider than the size of the vent to provide space to secure the screen to the structure. For triangular vents, measure the base and height of the triangle, add 2 inches or more to each measurement, and cut a rectangle or square. Place the mesh against the vent, outline the remaining sides of the triangle with a black marker, and then cut it (Figure 9).

Figure 9. Outline of a triangular vent screen.
Photo by Stephen M. Vantassel.

Screens may not be acceptable in livestock buildings or other structures where ventilation is needed. In some locations, ¼-inch mesh screens can become clogged with dust or ice. A screen with a larger mesh opening sometimes is preferable to no screen at all. For most homes, however, ¼-inch wire mesh is sufficient.

Secure a screen in several places with screws to prevent animals from pulling it off the structure. Screws with washers or expanded heads prevent the screen from popping over the screws. Use staples to secure the screen between the screwed areas. If aesthetics are a concern, paint the screen, screws, and washers to match the building. Though leaks are less of a concern with side screens, place a bead of water-proof sealant at the spot where the screw will pierce the structure. Add cross bars for support if the screen is very large (4 x 7 feet or more). If the opening is an access way, install the screen on a hinged frame.

Cover window wells 4 inches deep or greater to prevent wildlife entrapment (Figure 10). Window well covers are easy to install and effective.

Figure 10. Window well cover. Photo by S. Vantassel.

Crack and crevice sealers such as foam provide a convenient way to fill cracks, crevices, and voids. Although rodents can gnaw through foam, its insulating properties play an important role in assisting more permanent exclusion methods. Foam degrades when exposed to sunlight.

Caulk and sealants cost more than foams, but provide a more durable seal for cracks and crevices. The products are sold in a variety of formulations and colors. Have several available for the most common applications in your area. Caulk should only be used for openings ½ inch or less in size. Larger openings need filler, such as hardware cloth or a professional sealant, or should be secured by another means (Figure 11).

Figure 11. Seal around a pipe with Todol foam.
Photo by Erin Bauer.

Sealants allow limited movement and are used when closing gaps between two surfaces that you expect to move relative to each other, such as during seasonal freeze-thaw events.

Copper Stuf-it™ consists of woven copper mesh that is durable and highly flexible. It can be inserted around pipes and into small crevices with a flat-head screwdriver. Its woven nature allows caulk or foam to easily penetrate and lock the material in place. Cut the fabric to the desired length with tin snips or heavy-duty scissors. Be aware that rodents can gnaw through this material. X-cluder™ fill fabric consists of nylon fabric impregnated with strands of stainless steel wire (Figure 6b). Wear gloves when handling it to prevent injury. The material is flexible, but not as flexible as Copper Stuf-it™. The impregnated stainless steel fibers may provide greater resistance to gnawing by rodents.

Exterior Doors

Doors should fit tightly enough so that the distance between the bottom of the door and the threshold is less than ¼ inch. It may be easier to build up the threshold than to modify the door. Steel pipes make good rodent-proof thresholds and allow doors to swing freely. Install flashing or a metal channel on the lower edge of doors, particularly softwood doors. The flashing should extend to within 1/8 inch of the door edge at the sides and bottom.

Mechanical door-closing devices save time and reduce the likelihood of a door getting propped open. Doors that are left open for ventilation should have rodent-proof screen doors added, or modified so the upper half can be left open for ventilation.

Foundations and Floors

Gaps or flaws often exist along building exteriors where the wall framing or siding meets the foundation at the sill plate. These gaps provide easy access for rodents. Rats can burrow beneath the floor or foundation of a building that rests on piers or shallow foundation walls. Prevent their entry by extending foundation walls at least 36 inches belowground. The potential for damage from frost also will be reduced. Alternatively, place 18 inches of compacted sharp gravel under the slab to discourage rodent burrowing. Repair cracks in foundations with concrete or masonry grout. If rodents have access to the crawl space of a building, modify the floor to prevent them from getting into the walls.

Maintain a clean, 3-foot wide, weed-free area around building foundations, concrete slabs, and footings to discourage rodents from burrowing. This makes it easier to see holes, burrows, or other wildlife signs as well. Maintain the buffer by mowing vegetation regularly or by applying 1½ inches of crushed rock to a depth of 3 inches along a 24-inch-wide strip (or wider) around the structure (Figure 12).

Figure 12. Crushed stone can be an effective deterrent to burrowing animals. Photo by Stephen M. Vantassel

Climbing Animals

Prevent squirrels, mice, and rats from climbing a structure by installing a 12- to 18-inch wide aluminum sheet metal band at least 36 inches above the ground (Figure 13a). Seal openings in walls and floors with sheet metal.

Figure 13a. Sheet metal guard at the interior corners of rooms prevents rodents from climbing. Image by UNL.

Cut 2 or 3 lengths of Model S Nixalite® spikes (1 foot each, Figure 13b) and install them 4½ inches apart as measured from each base. The lowest row should be 8 feet above the ground to prevent accidental impalement. Never use spikes with an active raccoon infestation.

Figure 13b. Model S Nixalite® spikes installed to prevent raccoon from climbing. Photo by Stephen M. Vantassel.

Protecting Decks, Sheds, and Foundation Crawl Spaces

Structures that lack full foundations (e.g., trailers, sheds, and decks) are vulnerable to entry by skunks, woodchucks, raccoons, and other burrowing animals. Crushed gravel is not sufficient for these situations. Instead, use shallow trench-screen to prevent animal access (Figure 14).

Figure 14. Trench-screen will protect a crawl space. Image by Michael S. Heller.

Increase the depth and skirting of the screen in locations subject to frost heaves or with wildlife that have a tendency to dig aggressively. Use ½-inch mesh if more airflow is needed. Pay special attention to corners to ensure that they are properly protected. Overlap screens 4 to 6 inches to prevent any gaps that could be exploited by digging animals.

Special Situations

Some products are designed to address specific problems. Many homes with vinyl siding allow rodent access at the bottom of corners of the structure. Kritter Kaps™ provide an easy way to secure these openings (Figure 15).

Figure 15. Kritter Kap™ used to secure vinyl siding. Photo by Stouffert Technologies.

Protecting Structures from Birds

Nets are useful for preventing access to joists and trusses. Nets also can be installed vertically to prevent birds from accessing walls and ledges. Several ledge products such as spikes (Figure 16), coils, wires, electrical shock tape, and slants are available to exclude birds from ledges. Grid wire systems on flat roofs may prevent birds from nesting and roosting.

Figure 16. Cat Claw® (top) and Nixalite® spikes (bottom) are two of many models of bird spikes. Photo by UNL.

Other Exclusion Methods

Protecting Individual Trees and Plants

Use wire or plastic tree guards to protect trees from trunk girdling by wildlife. More expensive wire guards provide longer-term damage prevention. When using tubes to protect plants, place screen over the top to prevent entrapment of cavity-nesting birds. Use nets to protect trees and other plants from bird depredation. Ensure that the nets reach the ground, as birds may try to fly or walk underneath.

Protect young trees from beaver damage by installing a 1- x 2-inch wire mesh fence around the tree. Wire or plastic mesh will protect plants from deer (Figure 17).

Figure 17. Plastic mesh fence protects a shrub from deer. Photo by Paul D Curtis.

Anchor the fence securely to posts, as animals will bend it to reach branches or the trunk. The fence should be 4 inches away from the trunk for beavers and 4 inches away from the branches for deer. Extend the fence 4 feet high for beavers and 6 feet for deer. Bury the lower edge of the fence 6 inches into the soil to prevent beavers from digging under it.

Fences are the most reliable exclusion technique for preventing damage to nursery stock, gardens, and crops.

Protecting Large Areas and Crops
from Mammals

Non-electric barriers prevent access to many species and have the added benefit of low maintenance. Dimensions and the type of fence depend on the species to be excluded (Table 2).

Table 2. Types of fences to exclude wildlife

Species Fence Type Minimum fence dimensions
Deer Non-electric 8 ft high
Deer Electric 2 strands, 1 ft and 4 ft from surface.
Rabbit Non-electric 1-in mesh buried 4 in in the soil, extending 2 ft above the ground
Raccoon Electric 1-in mesh buried 2 in and extending underground 1 ft or more. Fence should extend 4 ft above ground with an electrified wire 6 to 8 in below the top. Or 2 strands of electric wire 5 & 10 in off the ground.
Woodchuck Non-electric 1-in mesh buried 2 in, extending underground 1 ft or more. Fence should extend 4 ft above ground and have 1-ft overhang to prevent climbing.
Woodchuck Electric Install non-electric fence without overhang. Place a strand of wire 6 to 8 in below the top. 1-in mesh buried 2 in and extending underground 1 ft or more. Fence should extend 4 ft above ground with an electrified wire 6 to 8 in below the top. Or a strand of electric wire 4 to 5 in off the ground.


Unfortunately, fences can be expensive if large areas need protection. While requiring lower maintenance than electric fences, non-electric fences typically cost much more to install due to the high cost of woven wire, posts, anchors, braces, fasteners, and labor.

Electric fences act as behavioral deterrents. They use a painful but harmless shock to interfere with wildlife movements. They often have lower initial construction costs but require higher continued maintenance. Electric fences fail to prevent wildlife damage when unsuitable designs are used, they are not installed according to manufacturer specifications, or maintenance is inadequate. Frequent monitoring and vegetation control are required to maintain sufficient shocking power (at least 3,000 volts) on the fence.

Electricity can be used exclusively with a fence, as with the poly-tape fence, or in conjunction with a non-electric fence. Electric fence technology has improved dramatically over the years. Fences can be powered using electrical outlets, disposable batteries, or rechargeable batteries connected to a solar electric panel (Figure 18).

Modern low-impedance chargers deliver pulses of electricity that produce a painful, but not continuous, jolt of electricity. The gap in the pulse allows people and animals to move away from the fence. While the shock generally is considered safe for adults and older children, always consider the presence of young children in the area. Some powerful electric fence chargers can cause serious harm to young children and people with heart pacemakers. Most chargers used in large agricultural settings can power over 200 miles of fence. Electric fences can be used to protect home gardens from deer and woodchuck damage during the growing season. Electric post-and-wire applications can be used to exclude birds from roof tops and ledges.

Figure 18. Solar and battery-powered fence chargers may power polytape fences. Photos by Jan Hygnstrom.


This module highlights only a sample of the products available to help property owners manage conflicts with wildlife. We encourage NWCOs to obtain product catalogs from wildlife control supply houses. By all means, search the internet for additional supplies and information. Membership in trade associations (e.g., National Wildlife Control Operators Association) will help keep you abreast of new products reaching the market.