November 26, 2015
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:
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:
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.
Types of systems available
Eventually, the roof will reach the end of its warranted service life and show signs of aging, but not necessarily any of the aforementioned symptoms of severe conditions. In these instances, there are various products available that offer a wide level of performance. For simplicity, they can be classified into ‘restoration’ and ‘coating’ categories.
For severely aged mod-bit systems, employing a restoration system may provide the most sustainable option. Restoration systems typically include an asphaltic-based fluid or hot-applied system. The fluid-applied options can be water- or solvent-based; in high-performance systems, they can be 100 per cent solid chemistry. In all cases, the asphaltic system is applied to a clean and debris-free surface at the application rate recommended by the manufacturer.
In most instances, the product is heavier-bodied in nature and is applied with a slotted squeegee, roller, or spray unit. Still, it is normally self-levelling and works to fill existing voids and cracking. In the case of hot-applied systems, an asphalt kettle is utilized to preheat the asphalt or modified asphalt applied to the cleaned surface.2 In some cases, a primer is required with either liquid or hot applied systems. It is important to always consult the manufacturer’s recommendations before use.
The asphalt restoration is used to replace the lost waterproofing of the existing modified bitumen system. It serves not only as an additional layer of protection for the existing assembly, but also as a level platform for the subsequent application of additional system products.
In many applications of restoration systems, reinforcement is employed as part of the application. Polyester or glass mats are layered into the restoration product prior to cure. The reinforcement becomes part of the system and provides increased split- and puncture-resistance. Once cured, the restoration application is typically coated with a reflective coating. In some cases, while the applied system is still uncured, minerals or aggregate can be broadcast into the liquid restoration. The restoration ultimately sets and the aggregate becomes part of the wearing surface.
In situations where the mod-bit system is in good or better condition, a coating may instead be the best course of action. Coatings, like restoration systems, need to be applied to clean and debris-free surfaces. Available in wide varieties, they can be defined by their solution media (i.e. solvent- or water-based), their polymer type (i.e. urethane, acrylics, or thermoplastic), or their pigment (white coatings typically employ titanium dioxide as a pigment, while metallic coatings normally use aluminum flake). In many cases, primers are used on prepared surfaces first.
When selecting a coating it is important to consider the substrate over which the coating is applied—not all coatings can be used over all substrates. Further, the finished coat’s desired reflectivity must be taken into account. While white reflective coatings can be used to reduce energy used in cooling a facility, it may not provide annual energy savings because of the higher heating demand in northern climates like those found in Canada.
The environment in which the coating is being used must also be considered. For example, will there be chemical exposure or foot traffic? In some coating systems, a reinforcement can be used to create a more durable and long-term solution. As with restorations, the reinforcement is applied during the application process and, once cured, becomes an integral part of the system.
Working with a trained roofing professional or manufacturer is a great first step in deciding the best course of action. Selecting the right system for the roof top conditions is critical to creating the best sustainable solution.
Best practices in performing the restoration
As discussed in this article, the best practices for extending the life of a modified bitumen roof include:
Additionally, there is an extra ‘bullet point:’ stay in front of any issues. Performing frequent roof inspections is a key component to the long-term performance of the existing system and subsequently applied restorations or coatings. Keeping drains clear and repairing flashing splits and leaks is paramount in guaranteeing success. The best coating or restoration in the world cannot repair long-term neglect.
Charbel Boulos, P.Eng., is president of Garland Canada, and has the knowledge and technical expertise to solve a variety of complex roofing and building envelope concerns. A licensed professional engineer, he has 25 years of experience in all facets of roofing design and construction, including hot asphalt, modified bitumen, cold process, and sustainable roofing. Boulos studied engineering at the University of Toronto, specializing in chemical processes and petroleum-based products, and received his professional engineering designation and license in 1990. He can be contacted via e-mail at email@example.com.
Joe Mellott holds multiple patents for roof-related innovations. He received the 2006 Industry Statesman Award from the Roof Coatings Manufacturers Association (RCMA) for his work in advancing roof coatings industry technology. A graduate of Case Western Reserve University, Mellott holds a degree in polymer engineering, is a frequent contributor of technical articles to industrial publications, and has participated in innumerable roofing- and polymer-related organizations. He has served as the technical chair and president of the Roof Coatings Manufacturers Association (RCMA), as well as sat on the board of the Cool Roofs Rating Council (CRRC). Mellott is the vice-president/general manager of the Innovative Metal Company (IMETCO) for the Garland Industries of Cleveland. He can be reached at firstname.lastname@example.org.
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