By Charbel Boulos, P.Eng., and Joe Mellott
Over the last 40 years, the Canadian roofing market has experienced growth in product diversity, expectation, and demand. In short, there are many different acceptable systems and solutions to offer long-term rooftop service. These assemblies have been challenged to provide performance criteria beyond just keeping water out of the building.
With many years of use and design, the modified bitumen (mod-bit) roof assembly has become well-recognized for its performance and durability. Sadly, even the strongest system ages and eventually may need replacement or restoration. Understanding the choices available and the best practices in selecting and installing a restoration solution, coupled with expert knowledge of whether a system is suitable for restoration, is critical in the continued success of the waterproofing system.
What are modified bitumen roofs and how do they age?
Mod-bit roofing is a family of products developed in Europe in the early 1970s, and brought to North America shortly thereafter. The use of thermoplastic elastomers (specifically styrene-butadiene-styrene thermoplastic elastomer [SBS TPE]) in combination with suitable asphalt allowed formulators to develop roofing compounds exhibiting improved flexibility and a broader functional thermal window. This means a higher softening point and a lower glass transition temperature.
The polyester, glass, or composite scrims (reinforcement) is web-coated with the modified asphalt to form a sheet, which is then surfaced with pulverized coal slag, sand, or a variety of mineral aggregates. This sheet is installed in a multi-ply configuration with hot bitumen, cold-process adhesives, or, in some cases, torch welding.
An ideal roofing system would offer exceptional durability and a long-term waterproofing solution. All things age and degrade over time, and the abuse of rooftop exposure takes its toll on mod-bit assemblies. The compound in the modified bitumen sheet becomes exposed through mineral loss or, in cases where mineral was not part of the original design, the degradation of the installed coating. The key culprits in aging are heat, ultraviolet (UV) exposure, freeze-thaw cycling, and time.
Once exposed to the forces of nature, the mod-bit compound begins to age. This is a result of compatible fractions of the original asphalt ‘baking off’ reducing the SBS-asphalt compatibility. Further, the butadiene fraction of the SBS (i.e. rubber) has a tendency to crosslink, resulting in ‘stiffening’ of the polymer structure. This aging results in compound cracking and, if left unattended, compound failure and potential systems leaks. While this can happen anywhere in the system, it is much more critical to address issues of aging near flashing details. A majority of leaks happen at the edge details or at flashings around mechanical units, drains, and roof penetrations.
The good news is this type of aging is not rapid. Many SBS roofing systems, when expertly formulated, professionally installed, and properly maintained, can last 20 to 40 years. Catching the signs of aging early is critical in maximizing system life expectancy.1 Some easily observed indicators include:
- exposed compound or bitumen;
- areas of mineral loss; and
- small splits and tears in flashing or the field of the roof.
A roof inspection by a qualified facility manager or roofing professional can reveal areas needing attention and help sustain the system. It is important to review any existing warranties for the roof system before taking the next step. Many manufacturer warranties cover extraordinary aging of the system. However, many warranties may be voided if unauthorized repairs or restorations are made. A quick check can save time, money, and aggravation.
Knowing whether the roof system is suitable for restoration
A great starting point is a quick rooftop walk with a trained facility manager or roofing professional. Taking photos of the existing conditions of the roof and compiling notes on observations are necessary for documenting the current condition. This process is very helpful in reviewing and establishing any progressive change in the conditions of the roof.
A great habit is to inspect the roof once or twice annually. Cleaning drains and removing debris is a simple way to keep a facility tight and leak-free. Addressing any splits or tears in flashing or penetrations helps keep leaks from entering the building or soaking the insulation and destroying the system and the roof deck from within.
There are several key issues to consider when thinking about restoring the roof. They are best discussed in the ‘negative’—in other words; if one were to see these conditions, it may be better to remove and replace the existing system or strategically re-roof specific areas. Negative observations would be:
- severe ‘alligatoring’ or cracking of the roof compound and compound loss;
- large areas of exposed scrim or reinforcement;
- large splits or cracks penetrating the membrane;
- large or frequent blisters on the roof surface;
- delaminated plies (i.e. separation of the original roof construction);
- wet insulation (observed visually or through a thermal scan);
- large areas of ponded water; and
- under-deck inspections revealing severe rusting of a metal roof deck, deterioration of wood decking, and indications of moisture in gypsum or tectum decks.
It is important to remember restoration can repair an aging assembly, but it cannot correct a failed roof or deck. While applying a coating or a restoration system may hide the problem for a time; it cannot correct existing conditions that ultimately result in system failure. Depending on the extent of negative observations, it may be appropriate to install a new membrane over the existing membrane. Modified bitumen membranes and base sheets can be an intermediate level, or method, to extend the life of an existing roof.
While most of these observations can be achieved during a simple noninvasive inspection, there are techniques such infrared (IR) scans, core analysis, and moisture metering that can reveal even more about the existing system. In any case, collecting this data each year helps with the scheduling of repairs and extends the roof’s life.