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UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary

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Architectural

Library

TH

2

U58

no. 23

Contents

1. Introduction
2. Apparatus

2.1. Test apparatus
2.2. Hailstone carriers
2.3. Hailstone molds -

2.4. Specimen construction 3. Procedures

3.1. Shooting hailstones at roofing

3.2. Evaluating failure 4. Results

4.1. Asphalt shingles 4.2. Built-up roofs

4.3. Nonbituminous roofing 5. Conclusions 6. References

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Hail Resistance of Roofing Products

Sidney H. Greenfeld

A test was developed for evaluating the hail resistance of roofings, in which synthetic hail. stones (ice spheres) of various sizes were shot at roof assemblies at their free-fall terminal velocities. Indentations, granule loss and roofing fracture were observed. The following conclusions have been made from these results:

(a) All roofing materials have some resistance to hail damage, but as the size of the hail increases, a level of impact energy is reached at which damage occurs. This level lies in the range of 142 to 2 inch (3.8–5.1 cm) hailstones for most prepared roofings.

(b) Because of the ways in which prepared roofings are applied, most products have areas of different vulnerability.

(c) The solidly supported areas of roofing tend to be the most resistant to hail damage. (d) Heavier shingles tend to be more hail-resistant than Type 235 shingles. (e) Weathering tends to lower the hail resistance of asphalt shingles. (f) Built-up roofs on dense substrates tend to resist hail better than those on soft substrates.

(g) Built-up roofs made with inorganic felts tend to be more hail resistant than those made with organic felts.

(h) Coarse aggregate surfacing tends to increase the hail resistance of roofing. Key words: Asphalt shingles; built-up roofing; hail; roofing; shingles; storm damage.

1. Introduction

Hail, as a destructive force of nature, has plagued man, his crops and his property since the very beginnings of civilization. By far the vast majority of hailstorms contain hailstones that are relatively small. These small stones can damage crops, but not roofings. However, every year there are a number of storms in which hailstones occur in the range of 11/2 to 3 in. (3.8 to 7.6 cm), or more, in diameter.

In the United States, except on rare occasions, storms containing large hailstones are encountered in the States between the Appalachian and Rocky Mounains. While there is no evidence that the number of such storms has been increasing in recent years, the population has grown in this part of the country, more buildings have been constructed and, consequently, the incidence of building damage has increased.

It has been extremely difficult, for a number of reasons, to determine precisely the damage attributable to hail. The same storm fronts that spawn large hailstones contain high winds, not too infrequently of tornadic velocities. The short hail period is usually followed by torrential rains. Consequently, in the “post-mortem” analysis of building damage caused by a storm, the allocation of the causes cannot always be made. Therefore, the Weather Bureau Reports (1) usually lump these three causes of damage together, but where possible, have separated them.

The hailstones in a storm are rarely of uni

form size and, consequently, some damage remains hidden and does not appear until months or years later, in another storm, which might not be damaging on its own, or in cold weather, when ice penetration increases the destruction sufficiently to be observable. Even when only the damage unequivocally attributable to hail is considered, hail produces a greater annual loss through building damage than the morespectacular tornado.

It is beyond the scope of this paper to discuss the theories of hail formation and growth, or storm development; this information may be found in references (2–6].

Two types of damaging hailstorms are encountered in the United States [5]. The most prevalent type is known as the frontal storm. It involves the encounter of a cold, high air mass with a low, moist, warm air mass. The cold air tends to fall and the warm, moist air tends to rise, carrying its moisture with it. The moisture cools through heat exchange with the cold air and evaporation as the air expands upward. Eventually it becomes cooled significantly below the freezing point and remains subcooled until it encounters a nucleus upon which to freeze. As more water hits any particular ice particle, the particle grows. Because everything in these upper regions is at a temveloped. It was conceded, following Laurie, that (1) ordinary impact tests were not satis

1 Figures in brackets indicate the literature references at the end of this paper.

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