[Tech Talk] Flat Roofs Are Perfect For Pests

Columns - Technically Speaking

February 27, 2012

Typically, older commercial buildings have flat roofs comprised of some combination of wood (decking) and metal, covered with rubber, tar and gravel. Such roofs are penetrated by drains, stacks, ventilators, utility lines and access hatches.

Usually the roof perimeter is capped with metal crown flashing and the roof may be undercoated with a spray-on insulation containing cellulose. All of these elements work together effectively...for a while. Then the cumulative effect of sun-baking, rain, freezing (and thawing), ice and snow takes its toll on the integrity of flat roofs, making them vulnerable to a host of pests.

Invariably, cracks and tears occur and seams open up in the rubber surfacing of flat roofs. When this happens, moisture seeps down into underlying wood and insulation, where it is retained. Over time, mold and various fungi begin to spread throughout the decaying organic structural components. Such situations are ideal breeding sites for fungus gnats, fungus beetles (including the foreign grain beetle), springtails, psocids (a.k.a. barklice and booklice, and even termites and ants (e.g., odorous house ants, carpenter ants and acrobat ants). The winged reproductives (alates or swarmers) of termites and ants can be wind-blown up onto roofs by updrafts during breezy springtime days; from there, colonies of these social insects can develop from the invasion of just one fertilized female (queen).

Although the crown flashing may appear to be fastened tightly to the edge of the roof, gaps of only 1/5 inch width are sufficient to allow invasive insects into the sub-roof void. Mass invasions occur mainly in late summer/early autumn, when insects like lady beetles, cluster flies, bluebottle flies, boxelder bugs, leaf-footed bugs and female paper wasps seek out sheltered crevices in which to overwinter. Once indoors, these insects follow sources of light and warmth, which leads them through gaps in the false ceiling panels and into the work space of the top floor.

Oftentimes large AC condenser units are located on flat roofs and when the water that drips from these units is drained improperly, the resulting puddles of standing water can become breeding sites for chironomid midges (small flies with aquatic larvae) and mosquitoes. These flies continue to breed during the warm months and find their way indoors through air intakes, loading docks, delivery entrances and other outside doors.

The essential steps that building maintenance can take in order to avoid the above situations from occurring are: 1) twice annually, thoroughly inspect the roof — once in the spring, and again in autumn — to locate and repair rips and tears in the rubber surfacing; 2) dry out or replace wet areas of roofing, following the recommendations of structural engineers; 3) properly drain water from AC condenser units on roofs; 4) correct low areas on roofs to avoid standing water; and 5) use metal fasteners and silicone sealer to close the gap between the crown flashing and top edge around the roof perimeter.

If these steps are followed, indoor pest situations will be greatly reduced, resulting in a more pleasant work environment all year round.


The author is technical director of ProGuard Commercial Pest Solutions, Columbus, Ohio. He can be reached at gwegner@giemedia.com.

Copesan is an alliance of pest management companies with locations throughout North America. To learn more, visit www.copesan.com.