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THE ADDITION OF A FEW KEY WORDS TO THE SPECIFICATION, LIKE “COMPOSITE,” “WOVEN MAT,” OR “SCRIM REINFORCEMENT,” CAN IMPROVE THE LONGEVITY AND PERFORMANCE LEVEL OF THE ROOF SYSTEM BY 20-40% BEYOND THE ASTM STANDARDS.
Specifying types of polymer modified bitumen (MB) membrane reinforcement can play a large role in the performance and life of a roof system. There are multiple factors to consider before specifying a type of roof membrane, such as geographical location. In Texas, we have extreme heat and hurricane force winds which can shorten the lifespan of the roof. The addition of a few keywords to the specification, like “composite,” “woven mat,” or “scrim reinforcement,” can improve the longevity and performance level of the roof system by 20-40% beyond the ASTM standards. The American Society for Testing and Materials (ASTM) is the organization that sets the standards to define the specific types of reinforcement materials. ASTM’s standards should be viewed as having the minimal code requirements for construction. These standards create a foundation for the roof system but don’t provide the upgrades that can provide enhanced durability and or extended life cycle expectancy. ASTM relies on qualified members from around the world to develop the standards, but the organization does not have an in-house testing facility or technical research center for material testing, so it uses accredited facilities like the Z6 Commissioning laboratory to help develop them. These standards provide dimensional tolerances, physical properties, performance requirements and material appearance requirements.
The MB roof system standards are drafted and reviewed by industry professionals and manufacturer’s representatives for approval, then published if a two-thirds consensus is achieved. There are five typical ASTM standards used to define the structural performance characteristics of a styrene butadiene styrene (SBS) or atactic polypropylene (APP) MB membrane. There are three commonly used polymer modified roofing material standards for SBS by ASTM. These standards define reinforcement types using glass fiber (ASTM 6163), polyester (ASTM 6164), or combination (ASTM 6162). There are two standards which define reinforcements for APP polyester (ASTM 6222) or a combination of polyester and glass fiber (ASTM 6223). These standards are further defined with type classifications I, II, and III, differentiated by increasing weights and area of sheet per unit. The grade of the unit is distinguished with a granule-surface (G) or smooth surfaced (S) materials. An example of ASTM D6162, Type I or II, Grade S in the specification reads, “Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using a Combination of Polyester and Glass Fiber Reinforcements.” There are no additional distinctions defined in these standards to specify a type of fabric used for reinforcements.
The words scrim or non-woven (i.e. mat) are left out of the standards. SBS is a plastomer with elasticity characteristics that have consistent properties throughout a wide temperature range. The SBS will stay flexible even in cold temperatures below 32° F. SBS has a melting point of 210° F which allows a variety of application methods. SBS membranes can be cold applied, hot mopped, or propane torch applied. APP MBs tend to be stronger and stiffer and will provide greater resistance to high temperatures. APP MBs are typically only applied via propane torching with a melting point of 300°F or through cold applied polymer-modified adhesives. Specifying a scrim or composite in either APP or SBS will lead to a superior product line for just about any manufacturer.
The performance criteria to consider when choosing a membrane combination are watertight laps, blister resistance, resistance to splitting, delamination resistance, shrinkage resistance, and durability. Reinforcing materials of the MBs will serve as the carrier for the polymers and will work as a structural element to bridge substrate joints. The combinations of materials will then increase tensile strength, puncture resistance, and provide increased fire protection.
There are two main types of reinforcement fabrics – scrims and fabric mats. Scrims are fabrics woven together in both machine and cross machine directions and are used in high-performance membranes, providing a greater tensile and tear strength than minimum ASTM standards. Fabric mats are non-woven (unless specified as “woven mat”) comprised of randomly distributed fibers which are dependent on binders achieved through chemical adhesives, thermally, or mechanically. They typically have an overlapping arrangement and have less resistance to tensile and tear strength. Composites or laminates include both scrim and mats which are chemically or mechanically bonded. Composites will typically combine characteristics for superior tensile strength and puncture resistance. Glass fibers are more dimensionally stable and more heat resistant than polyester fibers. Glass fibers will not break down with ultraviolet (UV) exposure but polyester will. Polyester, however, has greater resistance to puncture, strain energy, and flexibility. Since polyester and fiberglass are complementary, the best approach is a two-ply membrane minimum incorporating both materials.
The placement and type of reinforcements can have a significant effect on the weathering characteristics of the finished roof membranes. Studies* have shown that the use of polyester mats, due to their dimensional instability, in cap membrane of both APP and SBS can accelerate the natural weathering causing cracking and crazing, which is a network of fine cracks on the surface of the material. The best-performing systems have an inner ply and cap membranes to incorporate both a scrim and a mat. Failures can occur with systems using only scrims due to excessive dimensional instability which can lead to failures with the membrane splitting. The use of only nonwoven mats can cause a less dimensionally stable reinforcement that can eventually result in stress to the seams due to shrinkage.
In the harsh environment of the Texas coastal region, there is little room to accept only the minimum criteria where every manufacturer meets the benchmark. Relying only on the standards can be shortsighted which tends to damage the health of the modified bitumen industry. This is evident due to the fact that so many roof systems are compromised by water infiltration, cutting their lifespans short before the warranty is over. Referencing reinforcement fabric type combinations in the specifications can provide a healthy boost in performance and durability for the roof system. *Source: Baxter, Richard and Tim Keamney, “Weathering