Hail Damage to Built-up Roofing

Built-up roofs comprise a large percentage of the commercial roofs today. The hail resistance of these roofs has been known to be substantial especially when covered with gravel. However, much debate has surfaced over the size hail needed to cause damage to such roofs as well as determining the characteristics of such damage.

This paper will summarize the results of ice impact tests that have been conducted by the author's firm on built-up roofs. Ice stones of various sizes were propelled at various roof samples employing a special launcher. We found that the built-up roof samples resisted being damaged by icestones up to two inches in diameter. Damage appeared as a "donut-shaped" crater in the roof where the gravel and asphalt surfacing were removed and the roofing felts were ruptured. In no instance was the gravel driven downward into the roofing felts.