Contrary to popular opinion, the maintenance-free roof system is a misnomer. All types of roofs require a certain level of attention. In fact, from the moment of installation, the roofing system undergoes continuous deterioration. Extreme temperature fluctuations as well as snow, ice, hail and wind prevail upon the roofing surface. In short, the elements are the biggest deterrents to the roof system over its service life. Traffic on the roof and the installation of mechanical and other equipment can also cause physical damage that could lead to roofing failures.
Roofs are exposed to the elements 24 hours a day, every day of the year. In the summer, the roof surface temperature can exceed 200 degrees F. In the winter, the roof surface can be exposed to continual snow and ice for weeks at a time. In addition, the spring and fall winds exert more force at the roof level than at any other exterior building component.
The environment has a significant impact on a roof system. The environment's impact on roof systems is concentrated in three forms: radiation, water and direct chemical action.
RadiationUltraviolet radiation from the sun has a negative effect on all types of roof systems. In built-up roof systems, ultraviolet radiation degrades exposed bitumen through a chemical process known as photo-oxidation in asphalt-based systems, and the evaporation of volatiles from coal-tar-based systems. In photo-oxidation, the number of high-molecular-weight hydrocarbons and water-soluble products in bitumen are increased. This combination of heat and ultraviolet radiation manifests in a migration of oily constituents to the surface and hardens the bitumen. The bitumen, in its hardened form, cracks in a process known as "alligatoring." Mineral aggregate surfacing is applied on built-up roof systems to protect the bitumen flood coat from ultraviolet radiation.
Ultraviolet radiation has a devastating effect on unreinforced polyvinyl chloride (PVC) membranes. It has been widely reported throughout the industry that a defection of the plasticizer in the PVC compound migrates out through exposure to ultraviolet radiation. Since the plasticizer is applied in the PVC compound to provide flexibility to the membrane, the diffusion results in an embrittled, hardened membrane that is susceptible to shrinkage, cracking and shattering.
Ultraviolet radiation contributes to the loss of oils from EPDM membranes, causing shrinkage of the sheets. Most elastomeric membranes achieve ultraviolet resistance from a compound additive called carbon black. Carbon black screens the ultraviolet radiation from absorbing the chemicals included in the polymer mixture.
Modified bitumen membranes incur deterioration from the same photo-oxidation that attacks asphalt-based built-up roof systems. To protect against ultraviolet deterioration, modified bitumen membranes utilize a variety of shielding surfaces, such as factory-embedded mineral granules, metal foils and field-applied reflective coatings.
WaterWater in the form of vapor, rain, snow, ice and hail can lead to roof defects. The most common problem related to water in a roof system is in the form of leaks into the facility. In these instances, the water finds an opening or weak point in the roof system and travels into the facility. Obvious membrane defects such as holes, openings and membrane splits contribute to the moisture intrusion.
The most critical function of a roof system is to effectively shed water. In all cases, a roof that is designed to shed water will provide a longer service life than a roof that is not designed to resist water. This is primarily due to the fact that it is nearly impossible to apply a roof system with perfect precision. Some defects are bound to occur over time. It is possible to shed water even on a flat roof system. This can be accomplished with proper slope and the necessary roof drainage. Without these elements, ponding water can occur.
Ponding WaterPonding water can be detrimental to a roof system and, in some cases, the entire structure. Some of the defects attributable to ponding water include:
- The increased weight can cause structural roof collapse.
- Moisture intrusion into the system can occur at weak points and imperfections.
- Continual cycles of ponding and evaporation in concert with cycles of rainfall and sunshine accelerate the degradation of asphalt and polymeric materials.
- The freeze-thaw action in ponded water moves with changing temperatures. The thermal movement of ice can erode aggregate surfacing.
- Ponded water promotes the growth of vegetation, plants and fungi with roots that can penetrate the roof surface.
- Collected water at flashing joints can intrude into the system.
- Water that remains on the roof surface 48 to 72 hours after a rainfall may nullify the warranty with some manufacturers.
At the roof deck, moisture causes deterioration of the substrate and weakens structural integrity. Moisture weakens the structural integrity of wood and tectum, promotes rust in metals, and can damage concrete.
Once insulation becomes wet, it loses its structural and thermal integrity. The insulation is weaker due to loss of binders and rotting of organic fibers, and thermal resistance is dramatically reduced. It also causes a dimensional change in the insulation.
Water VaporWater vapor has the ability to flow wherever air can flow. In insulation, this can occur between fibers, through open cells that are interconnected, or in broken-down closed-cell structures. At all the points where the water vapor replaces the air, the thermal value of the insulation significantly drops. This is due to the fact that the thermal conductivity of water exceeds that of air by approximately 20 times.
Water vapor can migrate into the system from the interior of the structure. This predominantly occurs through condensation. Once water vapor finds its way into the system, even from the interior side, it has the same destructive effects.
Hail DamageWater in the form of hail could have negative effects on all roof systems. Depending on the type of membrane and the size of the hail, roof damage can be sustained in the form of punctures or holes. The roof area should be inspected after every hailstorm to ensure that no damage has occurred.
Direct Chemical ActionThe roof system is also subjected to direct chemical actions from pollution and ozone. These actions, often undetected by the human eye, have negative effects on membranes over their service life. Environmental chemical actions occur through the dumping of fuel from aircrafts and spills from within the building onto the roof surface.
Aging of the Roof SystemIn the quest for a more effective maintenance process, we should understand the aging of a roof.
There are a number of complex, systematic variables that cause distress in roofing systems. Rational planning for repair and replacement is necessary. It is important to note that distress of roof systems is not linear. The function of each system is defined in distinct terms, not on a continuum. If a roofing system's performance reaches a point of failure, its function and aging process change dramatically. From that point on, problems are not static, self-correlating, or reversible. A split in a roofing membrane amounts to an immediate failure to protect the building structure and its contents, as does a flashing failure, detachment due to wind forces, mechanical damage by man, or any change in the protective function of the roofing membrane.
The next important fact is that there exists a distinct synergism in the dysfunction of a roofing system; that is, one problem can lead to other problems, and the situation can get worse and worse. For example, if a perimeter flashing fails and the ingress of water subsequently takes place, the insulation undergoes a change in its thermal performance, and potentially its dimensional and/or chemical stability, compressibility, and its ability to restrain the membrane in movement. Improper stabilization of the surface flood coat may lead to migration of the surfacing bitumen, exposure of the bitumen to photo-oxidation, and subsequent change in its physical behavior. A change in the dynamics of the membrane behavior in thermal cycling will result, and the roofing system may begin to behave unpredictably. The possibility of splitting during thermal load will increase, stresses at penetrations increase, and the entire membrane may experience differential stress concentrations, causing it to become distressed at areas of weaker reinforcement.
Physical and Chemical AbuseRoof systems are exposed to a variety of physical and chemical distress, which range from dramatic physical actions, such as wind forces, thermal loading, or mechanical damage, to slow, insidious chemical processes like photo-oxidation. To perform satisfactorily against the more dramatic actions, the roof membrane has to resist the effects of three main physical forces:
- Upward force, which results in blow-offs or peel-offs.
- Downward force, which results in mechanical rupture of the membrane due to abuse, or weather factors such as hail.
- Lateral force, which results from dimensional change of the membrane, membrane fastening systems, or sheer transferred from contact surfaces.
- Rapid oxidation, or combustion, which causes total destruction of the roofing system.
- Environmental chemical action, such as the dumping of fuel from aircraft, or spills from within the building onto the roof.
The effects that progress more slowly and cannot be successfully eliminated, or even efficiently controlled in the original design, involve the slow aging process of the membrane. These effects can be divided into three categories:
- Photo-oxidation of asphalts/plasticizer loss in thermoplastics.
- Evaporation of volatiles from coal tar and oils from elastomeric sheets.
- Moisture ingress (chemical and physical) into the roof assembly materials.