Rodent Proof Construction

Rex O. Baker. Professor and Research Project Leader. Horticulture/Plant & Soil Science. California State Polytechnic University. Pomona, California 91768

Gerald R. Bodman. Extension Agricultural Engineer — Livestock Systems Department of Biological
Systems Engineering. University of Nebraska. Lincoln, Nebraska 68583

Robert M. Timm. Superintendent and Extension. Wildlife Specialist Hopland Research and
Extension Center. University of California. Hopland, California 95449

Importance of Rodent-Proof Construction

Rats and mice cause serious damage to all kinds of structures if they are allowed access to them. Damage by rodents has been documented in homes, apartments, hotels, office complexes, retail businesses, manufacturing facilities, food processing and warehouse facilities, public utility operations (especially power and electronic media operations), farm and feed storage buildings, and other structures.

In urban settings, rodents most often cause damage to older, innercity buildings and utilities in poor repair. New housing developments may experience commensal rodent problems, but problems are more noticeable in neighborhoods 10 to 12 years of age or older. Ornamental plantings, accumulation of refuse, woodpiles, and other such sources of harborage and food are more quickly invaded and occupied by rodents when adjacent to an established rodent habitat.

Many types of land, air, and water transportation systems and their infrastructure also face serious rodent infestation problems. Infestations are of particular concern in the transportation of foodstuffs, feed, and other agricultural products. Commensal rodents consume and contaminate human and livestock feed. One rat can eat about 1/2 pound (227 g) of feed per week, and will contaminate and waste perhaps 10 times that amount.

Figure 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, was destroyed by house mice in less than 3 years.

Rodents destroy insulation, electrical wiring, plumbing, and other structural components of buildings (Fig. 1). Insulation damage alone may amount to a loss of several thousand dollars in only a few years. Energy loss from damaged buildings results in added annual costs. Rodent-induced fires from damaged electrical wiring or nest building in electrical panels cause loss of property and threaten human safety. Rodents also serve as vectors or reservoirs of a variety of diseases, such as salmonellosis, leptospirosis, and murine typhus, that are transmittable to humans. Additionally, they may be sources of swine dysentery, brucellosis, sarcoptic mange, and tuberculosis, all of which affect livestock or pets.

The most effective means of limiting rodent damage is rodent-proof construction. New buildings should be designed and built to prevent rodent entry. Rodent-proofing is a good investment. Designing and constructing a rodent-proof building is less expensive than adding rodent-proofing later. Nevertheless, poor maintenance or management practices, such as leaving entry doors and unscreened windows open, will make the best-constructed building susceptible to rodent entry. Techniques discussed here apply both to new construction and to the modification of existing structures.

Junctures where utilities (pipes, cables) enter structures require special consideration in preventing rodent entry. Some earthquake design criteria require open spaces in important joints and other support areas, to allow for limited movement of tall structures. These present a real challenge in the design of rodent-proof construction.

Physical Abilities of Rats and Mice

To prevent rodent entry, their capabilities 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 gain entry to a building (Fig.2);

-climb almost any rough vertical surface, such as wood, brick, concrete, weathered sheet metal, and many plastic products;

-crawl horizontally along or through pipes, augers, conveyors, conduit, and underground utility and communications lines;

-gnaw through a wide variety of materials, including lead and aluminum sheeting, window screens, wood, rubber, vinyl, fiberglass, plastic, and low-quality concrete or concrete block.

Figure 2. Rat traveling along an electric wire.

Rats can:
-crawl through or under any opening higher or wider than 1/2 inch (1.3 cm) (Fig 3);

-climb the outside of vertical pipes and conduits up to 3 inches (7.6 cm) in diameter; climb the outside of larger pipes attached to buildings by bracing themselves between the wall and the pipe; climb the inside of vertical pipes, wall voids, or earthquake safety seams and joints between 1 1/2 and 4 inches (3.8 and
10.2 cm) in diameter;

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

-drop 50 feet (15 m) without being seriously injured;

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

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

-swim as far as 1/2 mile (800 m) in open water, dive through water traps in plumbing, and travel in sewer lines against a substantial water current. In areas where high rat populations exist, it is common for both roof rats and Norway rats to enter buildings through toilets and uncovered drains.

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

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 surface;

-travel considerable distances crawling upside-down along screen wire;

-survive and reproduce at a temperature of 24^oF (-4^oC) if adequate food and nesting material are available.

Survey for Entry Points

When inspecting sites for potential rodent entry points, look for rub marks, droppings, tracks, gnawing, or other rodent signs. Special attention should be paid to areas discussed under Common Rodent Entry Points (below). Keep in mind the physical abilities and behavior of the particular rodents, especially their tendency to seek shelter behind, under, or in appliances, sinks cabinets, drawers, stored goods, wall voids, false ceilings, and other undisturbed areas.

To conduct a thorough survey, inspectors will need an inspection form and paper for noting and illustrating items needing attention; a good flashlight; a mirror (to see under and behind objects); and screwdrivers and other small hand tools to remove interior and exterior vent grills, appliance base plates, and service doors to attics, crawl spaces, and utility cabinets. A tape measure is usually necessary when preparing a plan and estimating materials needed for repair. A small dustpan, broom, and some lime, flour, or similar powdered material are useful in preparing an area for a follow-up observation of fresh tracks. A camera can be of great value, especially when trying to design a project after leaving the site, or when seeking assistance from someone unfamiliar with the site. A simple item to use when measuring gaps under doors or around pipes, screens, or vents is a common wooden pencil or ballpoint pen (usually 3/16 to 3/8 inch [0.5 to 1.0 cm] in diameter) — large enough for mouse entry.

Figure 4a. Low-profile wall vent with poorly attached hardware cloth, allowing for easy rodent entry.