Membrane Maintenance for Modified Bitumen

Nov. 20, 2019

A roof that lasts as long as the building it protects is the Holy Grail of building construction. But is it a realistic goal? The answer is yes – this long-lasting roof is achievable, albeit elusive. Examine these factors that are conducive to meeting this goal and find out if a high-performance roofing solution can meet your needs. 

A roof that lasts as long as the building it protects is the Holy Grail of building construction. But is it a realistic goal? The answer is yes – this long-lasting roof is achievable, albeit elusive. Examine these factors that are conducive to meeting this goal and find out if a high-performance roofing solution can meet your needs.

Factors to Meet Your Roofing Goals 

Influences on Longevity

When it comes to the longest-lasting roof systems, certain factors come into play. These include:

  • Geographical location

  • Exposure to weather, especially in climates with severe seasonal temperature fluctuations

  • Substrate type and condition

  • Building design

  • Functional attributes (e.g. extensive rooftop equipment, solar arrays, chemical effluents, or rooftop foot traffic)

  • Roofing material used

  • Quality of installer workmanship

The complexities of these considerations make it difficult to generalize about one uniformly ideal roofing specification, so the commercial construction industry remains dependent upon ASTM’s minimum standards as a performance benchmark. However, relying on this benchmark is short-sighted and has been damaging to the health of our industry, as evidenced by the average 17-year lifespan of today’s commercial roofs when they should be much longer.

Aim for a roof system safety factor at least 20% higher than what ASTM requires; a threshold of 40% or more is not unreasonable and will help ensure the long-term success of the roof’s waterproofing.

The Proof Is in the Performance

One of the largest, most definitive studies of bituminous roofing membranes was conducted by the National Research Council of Canada in 1977. The objective assessments were made using a selection of more than 50 well-respected performance tests and revealed that performance of modified bitumen depends on:

  • The grade of bitumen used

  • The percentage of polymer used in the compound

  • Compatibility of the bitumen and polymer, as well as the dispersion consistency in mixing the two

  • The types of modifiers used (e.g. atactic propylene/APP, styrene butadiene styrene/SBS, styrene isoprene styrene/SIS, etc.)

  • The type and quality of fire retardants, fillers, and reinforcement scrim.

Given these findings, it’s no surprise that materials sharing identical generic descriptions and meeting the same ASTM and code standards achieve widely varying performance outcomes.

Because the study was conducted nearly four decades ago, however, we recently selected a handful of still-performing leak-free roofs installed 23-29 years ago to confirm and update the research findings with real-world applications.

All three are located in Columbus, IN, which sits in a region of the state notorious for frequent freeze-thaw cycles that create rooftop conditions more challenging than average. Columbus temperatures range between 15-32 degrees F. for 64% of the year and between 85-100 degrees F. about 15% of the year.

The roofs were installed on a variety of decks, including poured-in-place gypsum, wood, and metal, and were constructed with conservative specifications that called for mechanically attached insulation, a second course of mopped insulation, two plies of Type IV glass felts in hot asphalt and a modified bitumen cap sheet, dual-reinforced fiberglass scrim, and a flood coat of gravel in Type III asphalt.

In the case of one 25-year-old roof, I had the opportunity to review laboratory results on two core samples. The softening point of the asphalt used in the flood coat increased and penetration decreased compared to ASTM D312 Type III standard specifications, which is to be expected because asphalt ages through photo-oxidation and is accelerated by heat.

Nevertheless, the system was solidly adhered with no evidence of gaps or voids. It remained pliable and watertight even though the asphalt had hardened with age.

The modified cap sheet, an SBS modified membrane reinforced with fiberglass, was removed from the base layers to verify its stability, low-temperature flexibility, and tensile strength. It turned out that the quality compound and the high-yield fiberglass reinforcement helped the membrane retain a remarkable 80% of its initial tensile strength, which meant it still exceeded the requirements of ASTM D6163 after a quarter-century of service.

Some hardening of the compound and reductions of overall tensile strength are inevitable, but these results demonstrate that high-performance materials promote compliance with industry specifications not just when the roof is installed, but throughout its extended service life. The continued waterproofing integrity suggests an excellent return on initial investment, meriting the serious consideration of any owner expecting to hold onto a property for 25 years or more.

Three Must-Dos for Extending Service Life

If extended service life is an appropriate goal for your roof, there is much you can do to improve your odds of long-term success. The three roofs studied have effectively doubled the life span expected of today’s commercial roof offerings thanks to these three common factors.

1. Specifying quality materials.

All three roofs were on manufacturing buildings with higher-than-average foot traffic, which required systems with high puncture and tear resistance. Given the thermal variations in Indiana, a membrane with a generous safety margin in regards to elasticity and a reinforcing fabric with significant tensile strength were appropriate.

In all three cases, the owner chose a bitumen blend of SBS polymers with a mixing process that ensured uniform dispersion, plus a high-performance scrim that added tensile strength several times greater than the industry standard in the early 1980s when the roofs were installed.

However, the membrane only tells part of the story. The specifications also called for a comparably high level of quality in all other system components, including two courses of insulation, multi-layered construction, a high-performance waterproofing cap sheet, and a belt-and-suspenders flood coat of gravel surfacing. Some may call these specifications over-engineered, but in my mind, the sustained performance of such redundant systems is attributable in no small part to the quality of the materials used.

2. Vigilant monitoring during installation.

The highest quality materials can still result in a premature roof failure if they are improperly installed. Ensure every system component is added in full compliance with the specification and the manufacturer requirements.

This means vigilant job site monitoring during construction is recommended – in the case of these roofs, this task was performed by a representative of the membrane manufacturer responsible for warranting the systems.

3. Long-term preventive maintenance.

The final element indispensable to extending roof life is proactive maintenance. Roofing success is a responsibility shared by the building owner.

No homeowner would expect a roof to survive multiple freeze-thaw cycles without cleaning out roof gutters periodically, and the typical commercial roof experiences far more foot traffic, exposure to potential punctures and tears, and degrees of building movement. Commercial roofs also support far more roof equipment, making preventive maintenance vital to long-term performance.

The owner of these three roofs collaborated with his roofing partners across decades for ongoing inspections of flashings, gutters, drains, and other critical components while diligently performing any required minor repairs. As a result, all three exceeded their warranted performance.

There can never be one single roof system application that solves every problem, but this approach to roof system selection, installation, and maintenance has already proved successful and cost-effective. If owners are to ever achieve a roof service life that lasts as long as the building itself, they must move forward grounded in these principles.

Dr. Heshmat O. Laaly is an analytical research chemist, professional roofing consultant, and president of Roofing Materials Science and Technologies in Los Angeles, CA. He is also the author of Science and Technology of Traditional and Modern Roofing Systems. For more information, see

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