Correlating Hail-Strike Bruises With Hailstone Size, Roof Pitch, and Storm Direction

Hailstones from October 4, 2006, hailstorm in Gahanna, Ohio. ABSTRACT facing away from the incoming In 1969, while working for the National This paper is based on six years of hail- storm; and Bureau of Standards, Greenfeld3 published strike inspections (729 total) of residential 4. Roof elevations facing toward an a report that stated that at hail’s terminal properties gathered by EES Group, Inc. The incoming storm contained over two freefall velocity, damage to asphalt shingles data collected were correlated to determine times as many hail-strike bruises as occurred when hail measured from 1½ to 2 the following: roof elevations perpendicular to the inches in diameter. These data were found • Maximum likely size of hailstone incoming storm. by launching freezer-made ice stones at test compared to the number of hail- panels. Greenfeld3 also explained how ice is strike bruises, EES data demonstrate that there is a formed in alternating layers of milky ice, • Roof pitch versus number of hail- strong correlation between hailstone dam- which has a lower density than freezer- strike bruises, and age and the size of hail, the pitch of a roof, made ice. Therefore, the simulated ice • Bruise counts on roof elevations fac- and the direction a roof faces compared to stones are of higher density and weight ing toward, away, and 90 degrees the direction of an incoming hailstorm. than actual hailstones of the same size. from the incoming storm. Thus, he indicated that simulated hail may REVIEW OF BACKGROUND LITERATURE AS IT overestimate or conservatively estimate From this work, EES found: APPLIES TO THIS STUDY actual hail damage. 1. The estimated maximum size of hail Hail-impact damage has been the sub- In 1991, Koontz4 published a paper on must be between 1¾ inches and 2 ject of many studies over the past 55 years. ice-impact damage, which took into account inches in diameter to cause The first published ice impact studies were the effects of wind on the impact force of increased hail bruises per roof eleva- conducted by Rigby and Steyn6 in 1952. hail. He came to the conclusion that the tion (greater than ten hail-strike Since then, many test methods have been vertical free-fall velocity of hail, combined bruises per 100 square feet); created, implemented, and published to with the horizontal velocity of the wind, cre- 2. The bruise count vs. roof pitch sug- replicate hail impact damage to various ates a greater impact force than that of free- gests that shallow and steep roofs building surfaces, from asphalt shingles to falling hail. This was proven by Koontz using (defined as those measuring below a metal roof vents. Researchers have also trigonometry and the addition of vectors. He 4-in/12-in pitch and steeper than 9- tried varying the size of synthetic hail in also proved through mathematics that the in/12-in, respectively) had more these tests in an attempt to determine the impact force of hail increases exponentially hail-strike bruises than moderately hail damage threshold of asphalt shingles. as the mass and velocity of hail increase. sloped roofs (defined as roofs with 5- Other key variables include the velocity at In his 2001 publication, Noon5 devoted a in/12-in to 9-in/12-in roof pitches; which the hail travels, the angle of impact chapter to the effects of hail on a roof. Noon5 3. The shingles on roof elevations fac- between the hail and the roof elevation, and reported that the damage threshold for ing toward an incoming storm con- the effect that wind has on the velocity—all asphalt shingles is two inches in diameter tained almost 2½ times as many of which are factors on the impact force of a for roofs that are constructed of quality hail-strike bruises as roof elevations hail strike. materials and are properly installed. Noon 4 • INTERFACE M AY / J U N E 2 0 0 9 also found mathematically that hail-strike damage increases as the angle which the hail strikes the roof nears 90 degrees (i.e., perpendicular strike). He also reported that hail striking soft metal surfaces leaves dents up to half the size of the hail that struck the subject residence. Thus, one can estimate the maximum size of hail that struck a building’s finished surfaces by doubling the size of the largest hail-strike dents observed on soft metal surfaces such as box vents, gutters, and downspouts. Haag Engineering1,2 has published numerous reports concerning the effects of Figure 1– Direction of arriving hailstorm vs. roof elevation. replicated hail on composition (asphalt) roofing and other exterior fin- and West Virginia from June 2002 to hailstorm vs. count on opposite face, ishes. Haag’s first major hail publication August 2007. The database used for this and was in 1975 and contained the findings of paper included 729 hail damage inspections • Bruise count of roof slope facing the its study on red cedar shingles.2 Since then, on asphalt-shingled roof surfaces. This hailstorm vs. count on perpendicuHaag Engineering has become the industry study only included those roof elevations lar faces.