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
■ 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/
■ 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
THIS FOUNDRY ROOF, now 32 years old and counting, is a modified bitumen system using a high-tensile fiberglass-reinforced
modified bitumen cap sheet. It has lasted for nearly double the industry’s average roof lifespan of 17 years thanks to conservative
specifications and regular maintenance.