Old and New Hail Damage to Roofs

By: Jeff Coyne, P.E., MSCE
In our October 2010 newsletter, Bob Whelan, P.E., discussed how to approach a roof hail study by using the scientific method. In summary, one should perform a collateral study of the property, or neighboring properties, to gather information about the particular hailstorm in question. Weather research and interviews are also important, to some degree. Knowing the general size of the hailstones, the cardinal direction from which they originated, the number of hailstones that fell per unit area, and whether the hailstorm is recent is important when studying any roof for hail damage. Knowing the size and direction of the hailstones before observing the roof will aid the observer in what slopes or areas to pay particular attention to, and whether the hailstones were even of a sufficient size to cause damage to the roofing material. The reason for the collateral study is simple: there are numerous reasons roofing materials fail.   To a seasoned professional, discerning between old and new hail is not as difficult as it sounds. A collateral study of oxidized or weathered metal, plastic, composite, and wooden surfaces should reveal indications of whether a hailstorm occurred within the last few years. When hail strikes an oxidized or algae-stained surface, it leaves a mark (clean spot) that will eventually fade back to its pre-storm color. In contrast, dents in metal surfaces from hailstones are permanent and will be present for the extent of their service life. Determining whether two or more hailstorm events have occurred at the property may also be important, as it may represent separate losses. [caption id="attachment_865" align="alignleft" width="717"]Hail Damage to Wooden Fence and Metal Door Figure 2: Examples of Clean Spots on a Wood Fence and Metal Door[/caption] Once the collateral study on and around the property is complete, the roof study should commence. If one suspects old damage to a roofing material, it is best to reaffirm their suspicions with weather research. Old hail damage to most roofing materials will typically have characteristics that are distinguishable from new hail damage. For example, old hail damage to asphalt shingles will appear weathered, with the asphalt layer having a grayish tint, whereas new hail damage will appear fresh, with the asphalt layer being a dark, black color. Old hail damage to asphalt shingles will have fiberglass particles exposed where the asphalt layer has deteriorated, while little, if any, fiberglass particles will be exposed for new hail damage. We’ve discussed the methods used to determine whether hail damage to a roofing material is recent or old. This involves a collateral study of the property to gather information about the particular hailstorm in question. It may also involve weather research and interviews if necessary. And lastly, knowing how the roofing material responds to hail impacts, and how the roofing material ages, are necessary steps.