[VERTEBRATE PESTS] Don’t Forget About Those Roof Mice

In last month’s column, I discussed how mice harbor beneath large commercial slabs of food stores and distribution centers. I tried to emphasize that because slab mice are out of sight and out of mind, it is not uncommon for serious mouse infestations to occur, catching both the client and the servicing pest professional off-guard. This is also true for mice that can occur in the complete opposite direction of slab mice — namely roof mice. Roof areas from a general pest perspective were discussed in this column in July 2002; ceiling mouse infestations in April 2002. However, the goal of this month’s column is to focus specifically on understanding and managing mice that access and infest roofs and roof areas.

HOW DO MICE ACCESS ROOFTOPS? The roof rat isn’t the only domestic rodent that is a capable climber. The Norway rat, house mouse and deer mouse are all also excellent climbers. But mice are particularly adept at climbing quickly and can utilize more vertical avenues than can rats.

This is because the mouse is small, lightweight, relatively powerful and extremely agile. These characteristics help them to nimbly climb straight up surfaces of all types. But exterior walls constructed of rough materials such as brick, stone and rough concrete are especially easy for the mouse since there are virtually an unlimited number of small crevices, and nooks and crannies the mouse can anchor to with its small, narrow and sharp toes.

Many professionals can attest to witnessing mice climbing walls and many have seen a frightened mouse bolt from a station and quickly scale the wall behind the bait station in an effort to escape.

Even smooth vinyl and aluminum siding are not mouse-proof. Mice will climb up these coverings by holding on to various edges and corner folds. Jeff Maslan of Western Pest Services in Parsippany, N.J., shared with me photos taken of a mouse climbing up the side of a house by grabbing hold of such corners and other footholds.

And due to its small size, mice, unlike larger rats, can take immediate advantage of any existing small openings and gaps along aerial areas and on rooftops. But remember that rats are more capable than mice in gnawing their way through building materials to gain entry to roof spaces.

If the mouse doesn’t scale the walls directly, it will, like the roof rats, pack rats and tree squirrels access the roofs via any connecting tree and or shrub branches, structural lines such as pipes, downspouts and utility conduit lines (lighting, cabling, phone lines, etc.).

Depending on the type of building, mice may simply climb to the roof and forage about for an opening into the structure where food or warm air currents are escaping through the building.

But for some the roofs of commercial food-processing plants, mice (and rats) may not only climb up the walls following their noses to the food smells, but they may actually establish nests on the roofs. Rodents will establish nests directly beneath different roof coverings, such as rubber or vinyl sheathings, or within the protected spaces of roof air handling system penthouses, or any available junk piles or abandoned equipment left on top of the roofs. In the southern states and other warm areas, roof nests may be established any time of the year. But for the temperate regions, nests usually aren’t seen during the winter months for obvious reasons.

Hopefully, it should be obvious that rooftop inspections and periodic rodent control should be the rule rather than the exception for commercial food-production facilities. Here are some ways to prevent and manage roof-top rodent infestations.

SANITATION. As with all other aspects of pest management, the maxim "sanitation is pest control" applies to roof areas as well. Roofs — especially roofs of food-production accounts — must be kept clean at all times.

Remember, a significant amount of warm and food-laden air is vented via exhaust stacks through roofs. Process equipment components also can extend onto roof surfaces. Each of these conditions can either attract rodents (as well as insects and birds), or provide food for the rodents when product spills or "blow-outs" occur onto the roofs. Because roof areas are often out of sight of plant supervisors, it is common for spilled food to remain for several weeks and longer. Rodents living below rubberized roof sheathing or within or among any roof equipment clutter will be able to survive and reproduce if food is available on a nightly basis.

EXCLUSION TIPS. Similar to all the items and doors we inspect at ground floor levels to exclude rodents, aerial and rooftop areas must also be considered for exclusion efforts. For example:

• Fascia boards must be kept tight against foundation walls. Otherwise, rats and mice can squeeze beneath the fascia and gain entry.

• All outside downspouts should be rodent-proofed at the top and bottom to prevent rodents from reaching the roof by climbing.

• All pedestrian access or emergency exit doors leading onto roofs must be rodent-proofed in the same manner as is done for doors at ground-floor levels.

• Ventilator stacks projecting above the roof surface should be screened with hardware cloth baskets or boxes. Care must always be taken to ensure any rodent-proofing materials used over intake or exhaust ventilation components do not affect the necessary airflow.

• Screen vents and louvered in-wall vents should be sealed off using galvanized hardware cloth (¼-inch mesh).

• All tree limbs should be pruned back by 6 feet (this distance prevents roof rats and tree squirrels from jumping the gap). Similarly, any vegetation climbing up foundations to roof areas should be managed.

• All utility lines penetrating through walls or rooftops must be sealed at the point of entry.

• In areas of heavy rodent pressure on commercial food-production buildings, rodent guards may need to be installed around any type of vertical or horizontal lines leading to the buildings.

• For sealing and "patching" repairs of the holes and crevices that develop in roofs, foundation walls, fascias and around the various aerial penetrations, it is important that surface-specific high quality sealants are employed. Although easy to use, inexpensive caulks or expanding foam "sealants" are not appropriate materials for long-term repair of structural gaps. In fact, expanding foam sealants when "sprayed" beneath fascia boards often will cause fascia boards to warp away from the building’s foundation, creating even more access for not only rodents, but a wide range of other pests (e.g., cluster flies, carpenter ants, cockroaches, small birds, bats, etc.).

CONTROLLING INFESTATIONS. As mentioned previously, regardless of exclusion practice, mice will sometimes scale rough wall surfaces directly and gain access to the roof without any difficulty. This underscores the importance of inspecting commercial roofs on a regular basis. For those situations where mice have become established on the roof, the following is recommended:

• Inspections will dictate what areas the rodents are active on the roof (e.g., fecal pellets, burrow holes gnawed into the rubber sheathing etc.)

• For mouse infestations, the same multiple catch traps used for interior perimeter wall zones can also be used on rooftops to catch mice. The traps can be installed at spacings of 20 to 40 feet.

• Multiple catch traps should be installed to flank the interior of both sides of any roof access doors, or any temporary openings through the roof due to construction work on the roof.

• Multiple catch traps must be checked on a weekly basis until the mouse activity has been eliminated.

• For both mice and rats, exterior bait stations can also be installed onto the roof until the rodent activity has been eliminated. Similar to the practices used for ground-level exterior bait stations, roof-top bait stations must be anchored and meet all tamper resistance-baiting strategies. Only block baits should be used inside roof bait stations and the blocks should be installed onto rods or secured inside the station in some other manner.

• If the use of baits is undesirable for whatever reason, snap trap stations for either rats or mice can also be used in the same manner as is conventionally done at ground levels.

CONCLUSION. In closing, it must be stressed that roofs are an essential component of any commercial client’s service. (For more information regarding pest management on roofs, refer to the Vertebrate Pests column in the July 2002 issue of PCT.)

Consequently, sales professionals on the initial bids must not overlook the occasional inspection and servicing of roof areas.

The roof may be a bit more difficult and time consuming to service. Nevertheless, inspecting and servicing the roof illustrates to a client (who themselves may have not considered roof areas) that they are in good hands with a thorough and highly proactive pest management professional.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com or 765/939-2829.