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Major roof rat infestations do not start overnight, but an ineffective inspection can make it feel like they do. Roof rat infestations can go unnoticed for several months, sometimes ranging upwards of six to nine months, before it becomes apparent that a situation has developed that requires immediate attention.

Figure 1: Pads and digits.
Photos courtesy of Don Foster, A.C.E., except where noted.

This seemingly quick infestation often happens when roof rat populations gain traction and accelerate quickly. Sometimes your service specialist focuses more on the number of visits vs. the quality of the visits to satisfy client protocols. Do not be the technician whose wide eyes are like that of a stranded motorist. Often a stuck driver exits their vehicle to open the hood, looks, then returns to the driver’s seat expecting some miraculous event to have occurred that will cause the car to fire right up. When you arrive at the account, be prepared to deliver exceptional service. You must “commit to the chase” if your intention is to deliver on your client’s zero-tolerance rodent expectation.

ALL ABOUT ROOF RATS. Roof rats, as the name suggests, should provoke some thought about their habitat. Roof rats are arboreal in nature, meaning that they naturally reside outside in a variety of trees and prefer to travel through a structure from an elevated position. Their hind digits are longer, and additional pads on the paw make them exceptional climbers (see Figure 1 below). Their tails are used for balance and temperature regulation. The tail is not prehensile; but the rat can wrap its tail around objects for added stability.

Figure 2: Urine stains on an insulated refrigeration line.

Rafters, conduit and refrigeration lines all have a common theme. They mimic the limbs of the arboreal world that roof rats travel on in their natural habitat. Following the “veins” of the structure will lead you to harborage and feeding areas. Refrigeration lines provide more than just travel routes to and from the harborage or food source (see Figure 2). In this photo, a blacklight was used to identify a runway. Refrigeration lines are insulated with foam rubber. A rodent’s inherent nature to gnaw and grind its teeth often exposes the copper tubing beneath this insulation. Differences in temperature allow the outside of the line to condensate, providing a water source during travels.

Sometimes, roof rats choose to take the stairs (see Figure 3). Of course, they need snacks along the way! Peridomestic cockroaches are an inviting delicacy in pipe chases and wall voids. Evidence of cockroach wings in wall cavities or scattered along ceiling tiles should be an indication that roof rats are in the area. Even a dragonfly (a rare delicacy) occasionally has been found in drop ceilings.

Figure 3: Sebum trail down the left side of the treads next to a refrigeration unit.

Roof rat harborages are just as debris-laden as those of a Norway rat. Most harborages are disguised very well; in commercial establishments, most are found in voids that are not easily detectable. Being armed with the knowledge that roof rats will exploit void areas and elevated positions means that you must elevate your position and aim high if you intend to win the battle. You will not win this war at ground level, or even at 6 feet. The focus of this war is more than 8 feet above the floor in practically all situations. We will take a look at some areas that were destined for notoriety in the roof rat playbooks. Grab a ladder (a tall one!), and let’s look at the following situations that all take place above the ceiling grid.

GROCERY STORE. A retail grocery establishment was having a major issue with roof rats. This was not the account’s first encounter with rodents.

Multiple rodent captures were taking place on a daily basis with no indication that the activity was slowing down. Inspections had occurred above the drop ceilings and on top of the coolers. Runways were found on the refrigeration lines in the ceiling as well as on top of the coolers. Peering down from the top of a meat cooler that was 70 feet in length, a void between the exterior wall and the meat cooler was filled with debris (see Figures 4 and 5).

Some parts appeared 3 feet deep, whereas other sections were as tall as the cooler wall in this void. Debris removal was scheduled, and over the course of three days, the wall void was cleared. During the process, it was discovered that items in the debris had been present for at least 15 to 20 years based on conversations with the client. Multiple nesting sites were found throughout the debris pile. More than 50 live juvenile roof rats were extracted with several litters throughout the debris. Lifting the material out was a chore (see Figures 6 and 7), and the knowledge gained from removing the debris became priceless. A total of six holes were discovered within the vertical expansion joints and around plumbing that penetrated the floor. This permitted the roof rats to emerge from the expansion joints in the floor into other areas of the stockroom. The debris removed from the wall void was placed in a roll-off dumpster (see Figure 8).

This was just one area of the store that had significant activity. Further inspections above the ceiling led to an area above a produce cooler. It is crucial to remember one important aspect of performing inspections: confirm you have exhausted all capabilities to inspect an area up close. If you cannot physically get into an area, find someone who can, or use technology to explore the area. Areas above a drop ceiling can be very misleading in large commercial accounts. Peering across a drop ceiling over a retail floor does not meet the criteria. Partition walls may be present from previous renovations that can obscure the view and prevent you from identifying a major nesting site.

A produce cooler was located on the opposite side of the store, directly next to the deli/bakery, and was supporting several litters of roof rats. A partition wall extended from the floor well into the ceiling space of the store. The partition wall was insulated with fiberglass insulation that extended into the roof trusses of the building. Not only does the insulation create a prime nesting area, it also provides an avenue for roof rats to travel from the floor to the rafters.

The tops of coolers in any type of food establishment should always be a part of your routine inspections. Some may exist in the back of the facility. Sometimes packaged food is thrown on top of coolers, creating a food source for rodents. In some situations, the top of the coolers may be used for seasonal storage and the materials forgotten about as new personnel cycle through. When accessing these areas, pay attention to the tops of the ceiling tiles. Evidence of foraging can be seen in some cases if the populations frequent these areas in search of food as seen in Figure 9.

After placing traps and physically removing rodents from the area, the stored items that blanketed the top of the cooler were removed. The amount of debris consisted of several 55-gallon contractor bags full of shredded material and two full pallets of debris towering between 6 and 7 feet tall. You can clearly see where one sebum trail was left from a runway in Figure 10. These were located in several areas across the top of the cooler. The fiberglass insulation that extended upward towards the overhead trusses was visible. Runways were followed through the fiberglass insulation to overhead refrigeration lines.

FINAL THOUGHTS. Roof rat populations can rapidly increase in size if your inspection neglects the hidden voids and spaces concealed within the four walls of a facility. These areas must be explored during routine inspections and have devices placed out as “feelers” to monitor these areas even when no visible evidence is present. Getting into these areas can be challenging at times. Voids can exist behind refrigeration units and separated by dividing walls that can serve as a primary harborage for rodents. Insulated panels can be hollowed out to conceal the nesting area. In cases such as these, openings must be made when the refrigeration units cannot be moved.

In some instances, being aggressive in your approach will be the only solution to resolve an ongoing infestation that could otherwise place a “stop sale” order and force the business to close until the source is mitigated. When you stop making progress and the situation appears to be escalating, reach out for help. There is no shame in doing this, because you cannot find the needed skills from reading books.

Figure 10: Runway on top of a produce cooler. 

Don Foster, A.C.E., is a 27-year veteran in the pest management industry and serves as a technical manager for Terminix International in the Southeast.