In the continuation of the series on Hail Damage Standards, we will now focus on the velocity and directionality characteristics of hailstones. As discussed before, our goal is to accurately and consistently determine hail damage to roofing and building components.
This series on hail damage first discussed the characteristics of hailstones and future blogs will discuss the difference between cosmetic and functional damage to shingles and the last blog will outline the appropriate inspection procedures.
Velocity of Hailstones
The velocity or speed at which a hailstone falls is limited by its aerodynamic shape, its size, and the quality of its surface.
The fastest speed at which an object of a specific shape can possibly fall is called its terminal velocity or free-fall speed. The terminal velocity of hailstones is important because the faster a hailstone falls, the more impact-energy it carries. A hailstone falling fast is more likely to cause damage when it hits than a similar hailstone falling more slowly.
The chart below reflects common hail damage scenarios based on size and velocity (speed).
Comparing the speed and impact-energy of a 1-inch hailstone to that of a 3-inch hailstone of equal density, it’s easy to see why larger hailstones do more damage. The 1-inch hailstone falls at about 50 miles per hour and carries less than 1 foot-pound of impact-energy. The 3-inch hailstone falls at almost 100 miles per hour and carries 120 foot-pounds of impact-energy. Impact-energy increases exponentially as hailstone size increases.
Hailstone shape is important because it affects the manner in which the impact energy from a falling hailstone is transferred to the material it strikes.
A conical hailstone hitting at the wide end will spread the energy of its impact over a larger area than a hailstone hitting at the narrow end. The smaller surface area of the narrow end will concentrate the force of the hailstone strike, which increases the chance of hail damage.
About 75% of hailstones are spherical, conical or ellipsoidal in shape. Spherical is by far the most common shape, especially with small and medium-sized hail. Occasionally, hailstones will form unusual geometrical shapes. Individual damage marks from stones like these will not fit the typical profile, although the overall pattern of damage across the roof and elsewhere around the building site will not be affected by hailstone shape.
Directionality of Hailstones
“Directionality” is a term used to describe the fact that hail usually blows in from a certain direction. Since hail is associated with storms, the roof slope and elevation facing the direction of the oncoming hailstorm will suffer the most severe damage.
The concentration of damage should vary according to which direction each building elevation faces. Hail can hit all exposed surfaces, so evidence of the direction from which hail came should be apparent on a number of different types of surfaces, and not just on the roof. This is called collateral damage.
Occasionally, hail will fall almost straight down, and, in these situations, damage on different slopes may be similar, and collateral damage may be limited.
Evidence of hail damage should be consistent in its directionality. If hail blew in from the southwest, the most severe hail damage should be on the surfaces facing that direction.
If hail damage of similar severity is observed on both the north and south or east and west sides, that’s not consistent with damage from a single hailstorm, but is more likely to be damage from two separate hailstorms. Compare the damage from each side, and look for signs that indicate damage of different ages.
The fall pattern should be random. Hail falls from thousands of feet in the air, so the pattern of damage across the roof should be random. If geometrical patterns of damage are observed, examine the damage closely for evidence of causes other than hail.
Angle of Impact of Hailstones
Hail falling in storms with higher wind speeds will impact at a steeper angle than hail falling through calmer air. The severity of damage from the angle of impact should be consistent with other evidence of directionality that you see on various surfaces around the property.
Blown or wind-driven hail will also produce more collateral damage than hail falling closer to straight down.
In the next blog in this series on hail damage, we will look at the difference between “cosmetic” and “functional” damage to shingles.
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