Common Modes of Failure in Roof Systems

by Derek Josephson

We are often asked by clients, "How can I maintain my roof?" Maintaining your roof, whether it is two months or 20 years old, is a critical step in achieving maximum service life. For persons charged with performing roof maintenance, identifying weaknesses or potential problems early on is paramount to extending service life.

Roofs can be subjected to extreme annual temperature differentials, sometimes as much as 200 degrees. In addition, UV rays beat down on the surface, wind threatens to blow the system off, and water, "the universal solvent" exerts its forces in the form of rain, ice, sleet and snow. These factors, combined with installation errors and material limitations, result in some of the frequent problems in roofing. This article will focus on the modes of failure for some of the more common roof systems, including built-up, modified bitumen and single-ply membranes.

Blisters are the result of a void created either between the felt plies or the membrane and substrate. This void will grow with the evaporation of moisture or the expansion of trapped water vapor inside the void. Small blisters should be monitored, but usually do not require immediate attention. Larger blisters, however, increase the chances of membrane damage from both physical and chemical forces. A large blister is susceptible to puncture from rooftop traffic, dropped tools, and moveable equipment. The sloping sides of a blister cause membrane surfacing to slide, exposing bitumen and felt plies. Bitumen will degrade 200 times faster in sunlight than when protected with proper surfacing. As this degradation process occurs, the membrane becomes brittle and cracks. Eventually, the waterproofing element of the bitumen will be compromised.

Modified Bitumen
With the advent of polymer modified asphalt products in the US market in the 1980s, a forward leap in low slope roofing was achieved. While modified bitumen roofing has advantages over built-up roofs, the old adage, "no free lunch" still applies.

The single largest mode of failure for a modified bitumen membrane is defective lap seams. As in all low slope roof applications, the membrane must be waterproof. An open end lap or field seam would certainly negate this quality. Open seams in modified bitumen membranes usually occur due to inadequately heated asphalt at the time of application. When asphalt temperatures fall below required parameters, a positive bond is rarely achieved. Conversely, overheating modified bitumen during torch application can "cook" the membrane, which also results in poor bonding.

Elastomeric membranes are chemically inert, which makes achieving a durable bond with adhesive difficult.  This critical step accounts for the reported high percentage of failures at field seams. Newer elastomeric membranes, however, use seam tape, which has helped to reduce the problem of adhesive bonds.

Membrane shrinkage is also a common deficiency on EPDM roof systems; particularly loose laid/ballasted applications. Over time, some of the processing oils found in the membrane sheet will be lost. Tensile stresses built into the membrane during manufacturing, combined with insufficient relaxation at the point of installation, can also result in membrane shrinkage. This shrinkage affects seam integrity and also causes base flashings to bridge at perimeters and penetrations. Though flashing deficiencies are independent of the membrane, reported data suggests that the aformentioned factors also contribute to flashing problems.

In contrast to elastomeric membranes, the first generation of thermoplastic membranes had more problems with the material itself rather than the flashings or seams. The major reported failure of these thermoplastics was embrittlement. This condition, in some cases, was severe enough to cause shattering or cracking of the membrane. Advancements in reinforcements and chemical composition have helped to virtually eliminate this occurrence in newer membranes.

Conclusion
Identification of every mode of failure is beyond the scope of this article. The important concept to remember is early recognition of these deficiencies, combined with timely repair will certainly reduce costs, minimize leakage, and prolong roof serviceability.