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  GAF Materials Corporation   GAF Composite Roof  

The Strength of BUR - Page 1

   

GAF Composite Roof

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A century of commercial roofing experience has enabled GAF Materials Corporation to combine the exceptional strength of premium GAFGLAS® BUR with the superior elongation of RUBEROID® modified bitumen membranes. The result is a roof that is strong yet flexible, with unparalleled strain energy for long-lasting, reliable performance . . . The Next Generation in Roofing.
The Strength of BUR
   Since their introduction over 100 years ago, BUR systems have relied on tensile strength to withstand normal rooftop abuse. Contraction and expansion due to ambient temperature changes, HVAC-induced vibration, the uplift due to strong winds, and rooftop traffic are only some of the stresses to which a roofing system is subjected. As a result, these systems must either accommodate these stresses (through elongation) or withstand them (through strength).
    Modern BUR felts are manufactured with strong fiberglass-reinforcements for superior tensile strength. In many of today's demanding applications, traditional ASTM Type IV felts have been replaced by the newer Type VI felts, which have a minimum of 36% higher tensile strength.* The chart below compares the tensile strength of roofing systems composed of ASTM Type IV and Type VI BUR felts versus typical fiberglass- and polyester-reinforced modified bitumen systems. Note that the BUR systems exhibit high tensile strength, while the modified bitumen systems—both the fiberglass- and polyester-reinforced—have relatively lower tensile strengths. Higher tensile strength increases the system's ability to withstand normal roof movement and stresses, and is one on the key factors in long-term roof performance.

Typical Tensile Strengths of
BUR and Modified Bitumen Roofing Systems
(lbf/inch @ 0ºF)

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Note the superior strength of BUR ply felts, particularly the Type VI felts. Higher tensile strength allows a roofing system to withstand the stresses that can lead to premature roof failure.

The Flexibility of Modified Bitumen
    Modified bitumen membranes utilize high performance polymers to enhance the properties of asphaltic roofing systems, particularly at low temperatures. Some of these properties include improved flexibility, elongation, weatherability, impact resistance and ease of installation. Flexibility and elongation, in particular, are critical to the ability of a modified bitumen system to accommodate normal roof movement and resulting membrane stresses. As illustrated below, modified bitumen systems, particularly those made with polyester reinforcements, exhibit moderate to high elongation. In comparison, BUR systems (and MB systems with fiberglass reinforcements) have low elongation and must instead rely on their strength to withstand the stresses that might otherwise pull the roof apart.

Typical Elongation of Modified Bitumen and BUR Roofing Systems
(% @ 0ºF)

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The polyester-reinforced modified bitumen systems exhibit moderate to high elongation. The fiberglass MB system has only slightly greater elongation than the BUR systems. Greater elongation combined with inherent flexibility give the modified bitumen systems the ability to better accommodate normal roof stresses.

*ASTM D-2178 requires Type IV felts to exhibit at least 44 lbf/inch tensile strength and Type VI felts to exhibit at least 60 lbf/inch tensile strength.



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