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    Developing a Test Method for a Very Severe Hail Rating for Low Slope Roofing Assemblies

    Published: 04 December 2015

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    Increased property damage from hail impact has become more common in recent years, with a majority of the damage occurring to roofs. To help address this concern, a very severe hail (VSH) rating for low slope roofing is being developed. This work is being done using a modified version of the existing test method described in ANSI/FM 4473, American National Standard for Impact Resistance Testing of Rigid Roofing Materials by Impacting with Freezer Ice Balls. During this first phase of development, samples of single-ply, built-up, and modified bitumen low slope roof assemblies were subjected to impacts from 2.0 in. (51 mm) diameter freezer ice balls with impact energies between 23.75 and 26.13 ft·lbf (32.2 and 35.5 J). These impact energies are nearly double the 14.95 ft·lbf (19 J) kinetic energy delivered by the 2.0 in. (51 mm) diameter steel balls currently used in the severe hail rating test in FM Approvals Standard Number 4470. Of utmost importance to this study was evaluating the appropriateness of using freezer ice balls with the less rigid materials more commonly found in low slope roofing. Following the procedures in ANSI/FM 4473, each impact location was subjected to two ice ball impacts. Samples were impacted at seam locations, over metal plates (where applicable), and in the field of the roof sample in order to determine if any currently FM approved severe hail rated roofing assemblies could meet the pass criteria. The samples were evaluated for surface cracking and damage as well as for damage to the substrate (insulation or coverboard). Although the validity of using ice balls for impacting roofing samples has previously been demonstrated, the testing conducted in this study has shown the ability of ice balls to be used in granting a more severe hail rating than is currently available for low slope roofing.


    roof, hail, membrane, impact, ice, low slope

    Author Information:

    Boardman, Daniel A.
    FM Approvals LLC, Norwood, MA

    Brown, Daniel E.
    FM Approvals LLC, Norwood, MA

    Committee/Subcommittee: D08.20

    DOI: 10.1520/STP159020150022