National I ^ards

Li^irary, E-01 A^min. BIdg.

'^01 1 1969 BUILDING SCIENCE SERIES

Hail Resistance of

Roofing Products

Co U.S. DEPARTMENT OF COMMERCE

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

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Hail Resistance of Roofing Products

Sidney H. Greenfeld

Research Associate

Asphalt Roofing Manufacturers Association*

* (Formerly Asphalt Roofing Industry Bureau)

Building Research Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 23, 11 pages (August 1969)

CODEN: BSSNB

Issued August 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402—Price 25 cents

Contents

Page

1. Introduction 1

2. Apparatus 3

2.1. Test apparatus 3

2.2. Hailstone carriers 3

2.3. Hailstone molds 4

2.4. Specimen construction 4

3. Procedures 5

3.1. Shooting hailstones at roofing 5

3.2. evaluating failure 5

4. Results 5

4.1. Asphalt shingles 6

4.2. Built-up roofs 7

4.3. Nonbituminous roofing 8

5. Conclusions 9

6. References 9

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 1% 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 I14 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 uniform size and, consequently, some damage re- mains 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 en- countered 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 satis1 Figures in brackets indicate the literature references at the end

of this paper.

1
perature much below the freezing point of

water, the ice that forms does so rapidly and

traps air in the process. This forms a milky

layer of low-density ice. When an ice particle

becomes too heavy to be raised farther by the

updraft, it starts to fall. More condensation is collected during the fall, and once it reaches an

area with temperatures above the freezing

Each pin represents a storm in which at least $5,000 worth of building damage was done by hail. Figure 1. Hail storm distribution map.
point, the condensation on it is liquid water,

and air can escape. This forms a layer of clear,

high-density ice. Sooner or later, the particle encounters an- other strong updraft, starts back up and

freezes, subcools and goes through its tumbling

cycle over and over again. Thus, the hailstone

is found to consist of alternate layers of milky

(low density) ice and clear (high density) ice. When the hailstone encounters no updraft sufficient to lift it, it falls to earth, usually at a

velocity approximating the free-fall terminal

velocity [9].

The second type of storm occurs on the east- em slopes of the Rocky Mountains; thus, it is called an orographic storm. A front of warm,

moist air hits the base of the mountains, ex- pands upward until the nucleation, freezing

and tumbling processes occur and then the

hailstones drop out as in the frontal storm.

This type of storm tends to drop its hailstones

at about 6000 ft. Figure 1 is a map of the central United States

showing the distribution of storms during the

years 1960-1966 in which at least $5000 worth

of building damage was done by hail in each

storm. The orographic storms form an imperfect line at the left of the figure; the frontal

storms account for the rest of the points. Only

infrequently do building-damaging storms oc- cur outside of this area. Hailstorms occur all over the world in open

regions where rapidly moving air masses can

develop. However, only meteorological reports on storms and studies on the physics of hail

formation can be found in the literature. Occasionally reports appear in the trade literature

[7, 8] on hail damage to buildings, but only one paper has appeared in which a serious

effort has been made to evaluate the effects objectively. In this paper [9], J.A.P. Laurie

reported that he used 2V2-in (6.4 cm) artificial hailstones, made by cutting cylindrical cores from blocks of ice, cutting them to heights

equal to their diameter and molding them to roughly spherical shape. He fired these missiles at various velocities at building materials with

a grenade launcher and determined the thresnold energy of damage. The velocities were controlled by the size of the charge in the blank

cartridges used in the launcher.

Because of the difficulties in controlling the

velocities of the hailstones, an air-operated

piston was developed and used as the launcher

in the latter part of Laurie's study.

Laurie's paper, being the only one in its field, was the base from which this work was defactory, (2) the use of ice spheres was ex- tremely desirable, if not absolutely necessary,

(3) hail usually struck at its approximate freefall termin'^.l velocity (corroborated by others),

and (4) a criterion for failure was damage that

would permit the penetration of liquid water to an appreciable extent. However, it was decided

to use a less complicated launcher, use "hailstones" of various sizes, cast the "hailstones" to approximate spheres more closely and explore areas of diflferent vulnerability on various roofing systems. The work was primarily directed

at bituminous roofing materials, but a sampling

of other roofings was made.

2. Apparatus

2.1. Test Apparatus

The apparatus consisted of a compressed air

gun, for launching the hailstones, a timer, for

determining their velocity, and a target area, for positioning the specimen to be tested. The

physical layout of the apparatus is shown in

figure 2. The apparatus consisted of a specimen (target) area (1), timing range (2), gas gun (3),

gas cylinder (4), timer (5), hailstone carrier

(6), hailstone molds (7), and a triggering

mechanism (8). The roofing specimen to be

tested is mounted on a roof deck, just as in

service, and clamped in place against the backstop in position (1). The timing range consists

of a metal frame of %-in (1.9 cm) angle iron on which are mounted two microswitches 2.0 ft (61 cm) apart. The actuating levers on the

microswitches contain metal hooks, which are

used to hold one end of 1-in (2.5 cm) paper

computer tapes, the other ends of which are fastened to the top members of the frames with

masking tape, also 2.0 ft (61 cm) apart. The

tapes are kept under tension such that any

impact on them will close the microswitches and

actuate the triggering mechanism to start and

stop the counter (5)

.

The compressed air gun (4) is a commercially

available device manufactured by Diamond

King, Inc.2 (El Segundo, Calif.). It is their

Mark 14 model, with a 3l^-in (8.3 cm) inside

diameter barrel and a maximum muzzle velocity of 300 ft/s (9144 cm/s) . The counter is a Hewlett-Packard Model No. 523B microsecond

counter, with both starting and stopping gates

and a direct readout.

2.2. Hailstone Carriers

The hail carriers were made from 3-in (7.6

cm) diameter foamed polyethylene cylinders

(Ethafoam—Dow Chemical Co., Midland,

Mich.). This material was obtained as cylinders

9 ft (274 cm) long, sliced into short cylinders

6-in (15.2 cm) long and split in half longitudi3
The hail resistance of building materials was determined by shooting progressively larger hailstones at different parts of these materials until failure occurred. 5. Timer.

1. Test specimen. 6. Hailstone carriers—one in gun and one in open to

2. Timing section. show cavity.

3. Compressed gas gun. 7. Hailstone mold.

4. Gas cylinder, 8. Triggering mechanism. Figure 2. Hail resistance apparatus.

nally (Item 6 fig. 2). Each hemicylinder was

truncated at one end at 45 deg to its long axis

from the central cut to its outer wall and milled

with one of a series of sizes of hemispheres

centered in (5.7 cm) from its other end.

Thus, when the two hemicylinders were reas- sembled, they formed carriers for the several

sizes of hailstones and permitted one size barrel to be used for all of the hailstones. Carriers with recesses for II/2 (3.8 cm), 2 (5.1 cm),

21/2 (6.4 cm) and 2% in (7.0 cm) hailstones were made. The IV^-in (3.2 cm) hailstones were

carried in the li/^-in (3.8 cm) carrier and the

1%-in (4.5 cm) hailstones, in the 2-in (5.1 cm)

carrier.

2.3. Hailstone Molds

The hailstones were cast in molds made from

a silicone casting resin (RTV-60—General Electric Co.). The models for the hailstones were

plastic fishing floats, which are produced in increments of l^-in (0.6 cm) diameter from

1-in (2.5 cm) to 3-in (7.6 cm). Each float was

suspended on the end of a rod, which fit the

indentation in the float, in the center of a cylindrical polyethylene container of suitable size. The casting resin was deaerated, poured into

the mold and cured. The following day the cast- ing was removed from the polyethylene container and sliced through with a razor blade at a great circle of the float. The float and rod were removed and the cut interface covered

with a thin layer of silicone grease.

The hailstones were cast in these molds in

two stages, in order to permit expansion of

water during freezing to occur without shattering the hailstones. Water was poured into

the mold through the opening (called the gate)

left by the removal of the suspending rod

until the cavity (left by the float) was about

one-half full and frozen in the freezing com- partment of a conventional refrigerator. Four

hours later water was added to fill the mold

just to the bottom of the gate and the mold was returned to the freezer. Only by this twostage process was it possible to freeze ice 2.4. Specimen Construction

The shingle specimens were applied with four

staples (per strip) to 1 ft 6 in x 3 ft 0 in (46 x

spheres without shattering. While the struc- ture of these synthetic hailstones is different

from that of naturally formed hailstones, it was felt that the differences in structure did

not affect their performance.

^ References to specific articles in the description of apparatus

used in these experiments are for the purpose of definition of the experimental details, and should not be construed as preferential endorsements of these articles.

4
The ice spheres were stored in a chest-type

freezer at about 10°F (—12°C) until ready for

use. 91 cm) decks, representative of those used in construction (% in (1 cm) and 1/2 in (1-3 cm)

plywood, 1 in X 6 in (nominal) T & G boards)

.

The decks were supported on 2-2 in x 4 in (nominal) "rafters," to which they were fastened 6 in (15.2 cm) from each of the short

sides by 8d common nails. Thus, each deck

represented a 1 ft 6 in x 3 ft 0 in (46 x 91 cm)

section out of a conventional roof.

Wood, slate, asbestos cement, tile, and sheet

metal roofing were applied as directed by their

suppliers to decks supported on 2 in x 4 in rafters, 2 ft (61 cm) on centers. The built-up roofing specimens, 1 ft (30.5 cm)

square, were solidly mopped to 1/2 in (1-3 cm)

plywood or 1 in (2.5 cm) asbestos cement board

(to simulate a concrete deck) or to various

types of insulation mopped solidly to these

decks. Also, where metal decking was used,

the insulation was mopped solidly to the

decking.

3. Procedures

3.1. Shooting Hailstones at Roofing

The specimen on its deck was held against

i the backstop in figure 2 with large C clamps.

The 1-in (2.5 cm) paper computer tapes were

I hooked to the microswitches and fastened to

i the top of the timing frame with masking tape,

j They were held under tension, just insufficient

I to close the switches. A hailstone of the desired

size was taken from the freezer, cleaned of any

'. burrs or projecting pieces of ice (from the gate

in the mold) , weighed, and placed in its carrier,

• which was slid into the barrel of the gun as

far as possible. Air, or nitrogen, was permitted

I to enter the gun until the desired pressure was

reached. The valves between the gun and the

tank were closed (to protect the pressure reguj lator), the pressure gage was removed from

the gun, and the gun was fired by opening the

, solenoid valve, which relieved the pressure

I behind the floating cylinder in the gun and

permitted the remainder of the pressurized

! gas to escape into the barrel and expel the

hailstone carrier.

I The carrier was propelled out of the gun,

ij where the air resistance opened the two halves

and permitted the hailstone to travel alone

i toward the target. As the hailstone hit the first

I tape it started the counter, and as it hit the

1 second tape, it stopped the counter. Then it hit

the specimen.

The indentation on the specimen was measured and the condition of the specimen noted

after each firing. A minimum of two hits in each area of vulnerability was observed and

average values of damage used. Granule losses,

coating and felt fractures and deck damage

were recorded. The average velocities and energies of the hailstones in the 2 ft (61 cm) of

travel immediately in front of the test specimen

also were calculated and recorded.

3.2. Evaluating Failure

Damage to roofing by hail falls into two general categories: (1) Severe damage, which leads

to penetration of the structure by the elements

and (2) Superficial damage, which affects appearance but does not materially interfere

with the performance of the roofing. While the

latter is distracting and leads to insurance

claims, the former is the type of damage that

should be of most concern, because the possible

loss can exceed the replacement cost of the

roofing many fold. Thus, while the dents will be reported, only the fractures of the coating,

felt or other shingle material will be called failure in this report. For each material and

roofing system, the thresholds of failure, or the

smallest hail size producing these failures, are

reported.

4. Results

Although hailstones vary in size, shape, density, and velocity, those that do damage t^^

buildings tend to fall within the narrow limits

of ice spheres falling at about their free-fall terminal velocity [9]

.

The density of large hailstones has been

shown to approximate that of solid ice [10]

and sepms to range between 0.89 and 0.91

g/cm^. Hailstones, while rarely smooth spheres, can be treated aerodynamically as smooth

spheres and conclusions reached are close to observed results [11]. The terminal velocities

and energies of ice spheres have been calculated

and reported graphically by Laurie [9] ; they

are tabulated below as taken from these graphs

for the hailstone sizes used

:

Table 1. Terminal velocities and energies of hailstones"-

Diameter Terminal velocity Approximate

impact

energy

inches cm ft/s mi/hr (m/sec) /( lbs 1 (2.5) 73 50 (22.3) <1 IM (3.2) 82 56 (25.0) 4 (3.8) 90 61 (27.4) 8 m (4.5) 97 66 (29.6) 14

2 (5.1) 105 72 (32.0) 22

(6.4) 117 80 (35.7) 53 2% (7.0) 124 85 (37.8) 81

3 (7.6) 130 88 (39.6) 120

Joules «1.36)

(5.42) (10.85)

(18.96) (29.80)

(71.9) (109.8) (162.7)

• Read from graphs in reference [9].
All of the results reported are based on hail- stones of a given size traveling at velocities

within ± 10 percent of the terminal velocities

reported in table 1 for hailstones of that size. The results are reported under the types of

roofing studied.

4.1. Asphalt Shingles

When applied according to the recommendations of their manufacturers, Type 235 squaretab shingles have three regions of different

vulnerability: (1) The tab edges, (2) The sur- face over the unsupported areas between the

top of one strip and the "line" where the strip

above it contacts the deck or underlayment,

and (3) The triple coverage area solidly supported from the deck up [12].

The resistances of these areas to hail damage

are different; therefore, results are reported

for each area. The results for the Type 235

square-tab shingles are shown in table 2. These specimens were also exposed to ll^-in

dentations were made in the shingles by the

(3.2 cm) hailstones. Only small, superficial in- 114-in (3.2 cm) hailstones. The larger size hail- stones produced progressively larger dents. In

general, the smaller hailstones produced cir- cular indentations approximating one half their

diameter and the larger hailstones, those above

the felt-damage threshold, produced dents

greater in diameter than one-half the hailstone

diameter. Hailstones 2% in (7.0 cm) in diameter produced damage to the decks on which the

shingles were mounted.

Table 2. Hail resistance of Type 235 square tab shingles exposed 5 in {12.7 cm)

Deck

Smallest hail size cracking shingle felt

No underlayment 15# Felt imderlayment

Edges Unsupported Triple portion coverage Edges Unsupported Triple portion coverage

Yi-ln Plywood (1 cm)

J^in Plywood (1.3 cm) 1x6 in nominal TiG

in cm in cm in cm m (4.5) 1^4 (4.5) 1% (4.5) IH (4.5) IM (4.5) IH (4.5)

2 (5.1) 1?4 (4.5) (6.4)

in em in cm in cm

IVi (3.8) IVt (3.8) m (4.5)

(3.8) IH (3.8) IVi (3.8) IH (3.8 ) 114 (3.8) 2 (5.1)

Shingles on %-in (1 cm) and V2-in (1-3 cm)

plywood performed equally well; those on 1 x

6-in T & G roof boards were more resistant to

hail damage than those on plywood.

The shingles without an underlayment consistently had a higher threshold of hail damage

than did those with the conventional 15 lb saturated felt underlayment on all three decks.

Apparently, the soft layer of felt makes the

shingle slightly more vulnerable. The improved

performance usually involved only l^-in (0.6

cm) larger hailstones, but this represented re- sistance to 6.3 or 9.5 more foot pounds (8.5 or

12.8 joules) of kinetic energy. From these

results, it would seem that the solidly supported roofings performed better than those

with some soft underlying layer in their construction. This observation is consistent with

the fact that shingle materials are stronger in compression than in tension and the best performance can be expected when the impact

forces can be kept as pure compression forces. Any soft layer within the system permits the

back of the layer above it to be in tension and

fail more easily.

As shingles age during exposure they tend to undergo a number of physical changes, which

may affect their resistance to hail. A number

of shingles that had been exposed on (1-3

cm) plywood to the weather in Washington,

D. C. for 91/2 years became available and were

tested. These shingles had been exposed at a

4-in pitch (10 cm in 30 cm) facing due south.

Three different Type 210 shingles showed fail- ures (felt cracking) on all three areas of dif- ferent vulnerability with li4-in (3.2 cm) hail- stones. One Type 255 and one Type 290 shingle

experienced spalling of the coating with 114-in (3.2 cm) hailstones, but felt damage did not

occur until li/4-in (3.8 cm) hailstones were

used. Two Type 250 shingles showed felt damage in all three areas of vulnerability with I14-

in (3.2 cm) hailstones; however, one Type 250

and one Type 275 shingle showed no damage

below 1%-in (4.5 cm) hailstones on the tab

centers, but both developed felt damage in the

other two areas with 114,-in (3.2 cm) hailstones.

No direct comparison can be made between

these aged shingles and unexposed ones be- cause of changes in design and production. However, the aged shingles tended to be less re- sistant to hail damage than the new ones. A number of heavy weight and premium

shingles were also investigated. Some of these

resisted hail no better than the regular* Type

235 square-tab shingles. However, a few performed significantly better, as discussed below.

A Class B shingle based on a glass fiber mat,

instead of the conventional organic felt, did not

show felt-type failure on its tab edges and un- supported areas with hailstones smaller than

2 in (5.1 cm). It failed with 21/2-in (6.4 cm)

6
I hailstones on the solidly supported areas. Simij larly, three other shingles, all Class A, based

I on glass mat felts showed no felt damage on

' their obverse sides with hailstones below 2 in

' (5.1 cm) in diameter; one of these had a damage threshold at the 21/2-in (6.4 cm) hailstone on all three portions of its surface. Some of the

conventionally made heavy shingles, usually

with Number 9 granules or with high concenj trations of mineral additives, performed equally

' well. One Type 290 Class C shingle actually had

a damage threshold at the 2%-in (7.0 cm) hail- stone. While it is outside the province of this

report to identify these heavy Class C and

Class A shingles more specifically, the manufacturers have peen informed of how their indi- vidual products performed and the basic prin-

, ciples required to make more hail-resistant

! products. All of these shingles were mounted

j on i/^-in (1.3 cm) plywood.

I Because the vast majority of hailstorms oc- cur in warm weather, the roofs are above ambient temperatures when the hailstorm starts,

j Hailstorms that produce large hailstones are

I always of short duration and are preceded by

j a cloud cover, which drops the roof tempera-

[ tures below their daily highs. Therefore, the

hail resistance evaluation was conducted at 75

to 80°F (24 to 27°C). However, one Type 235

and one Type 315 Class C shingle and one Type

240 Class A shingle were tested at 120 °F

(49°C).on a 1/2-in (1.3 cm) plywood deck with

I a 15 lb saturated felt underlayment. The hail

resistance of the Type 235 shingle was in- creased to the 214-in (6.4 cm) hailstones from

the li/^-in (3.8 cm) hailstones on all three surfaces. That of the Type 315 shingle was improved only on the unsupported areas and that

of the Type 240 was not changed. Thus, the

results on these three shingles indicate that

shingles tend to be more resistant to hail damage at higher temperatures. It is fortunate that

hailstorms occur in warm weather.

4.2. Built-Up Roofs

Occasionally in residential construction and

much more frequently in commercial and in- dustrial construction relatively flat roofs are

used. These roofs are not "factory manufactured," but "built up" on the site from alternate

layers of bitumen and reinforcing membranes.

Some of these roofs are surfaced with a smooth

layer of bitumen and others are surfaced with

a layer of pebbles, crushed stone or light weight

aggregate particles in addition to a layer of

bitumen. There are many variations of this type

of roof system ; only a few representative ones were tested. The construction of these roofs and

the results of the hail-resistance tests are summarized in table 3.

The conventional smooth-surface built-up roof

[la and le in table 3] on a dense deck showed

visible signs of damage; i.e., cracking of the

surface, when 2-in (5.1 cm) hailstones were

used. Smaller hailstones usually indented the

flood coat, but did not crack it. When fiberboard

(lb) or glass fiber (Ig) insulation was installed

between the deck and the roof membrane the

indentations were larger and coating cracks

appeared with 1%-in (4.5 cm) hailstones. The

roofing on Foamboard A insulation (Ic) performed better than on the dense decks when

2-in (5.1 cm) hailstones were used, but 214-in

(6.4 cm) hailstones penetrated through "the roofing into the insulation. Foamboard B (Id)

delaminated; i.e., the insulation broke away

from its protective asphalt coated felts, when

impacted with 2-in (5.1 cm) hailstones. The

roofing on glass fiber insulation (Ig) on steel decking was penetrated by 2i/2-in (6.4 cm)

hailstones.

The flood coat of the built-up roof made with

asbestos felts on a plywood deck (2a) did not

crack or become indented by 2i/2-in (6.4 cm)

hailstones ; however, the flood coat was indented

and cracked by 2-in (5.1 cm) hailstones when

fiberboard insulation (2c) was used between the

membrane and the deck. The asbestos-felt roofs

had better hail resistance than the rag felt built-up roofs on comparable decks.

The built-up roofs made with coal tar pitch

[3] , referred to as tar in table 3, did not indent,

but developed concentric cracks with all sizes of hailstones. The 21/2-in (6.4 cm) hailstones

caused some of the flood coat to spall from the

top felts [3a]. Coal tar pitch generally tends

to be more brittle than asphalt and would be

expected to respond to the hail impact as a

brittle material.

The roofs built up with glass fiber felts on

the dense decks C4a and 4b) (plywood and

asbestos cement) did not experience flood coat

cracking with hailstones 2i/2-in (6.4 cm) in diameter and smaller, but when insulation was

present (4c. 4d, 4e, and 4f), cracks were produced with 21/2-in (6.4 cm) hailstones. The glass

felt roofs fell in between the organic felt built- up roofs and asbestos felt built-up roofs in hail resistance.

The roofs constructed of two base sheets (5)

performed much better on the asbestos cement

deck (5b) than on plywood (5a) ; their performance on plywood or insulation were about

the same as conventional asphalt-organic-felt

built-up roofs on the same substrates. Where

these roofs were covered with 300 Ibs/sq of slag

(14.7 kg/m2) (6), no damage was done to the

roof membrane by any of the hailstones. (The

slag was retained by cheese cloth when the

decks were tested in vertical positions). The

hailstone energy was dissipated in scattering

7
Table 3. Hail, resistance of built-up roofs visual inspection

Haiktone size, in (cm)

Hail damage indentation size ^

(3.8) IH (4.5) (5.1) 2yi (6.4)

Roof construction

(1) Base sheet +3 plies of 15 lb. organic felt+a 60 Ib/sq (2.9 kg/m 2) asphalt flood coat (20-25 Ib/sq (1.0-1.2 kg/m')

interply asphalt] on

(la) ^-m (1.3 cm) Plywood %

(lb) 1-in (2.5 cm) Fiberboard on }4-io (1.3 cm) plywood %

(Ic) 1-in (2.5 cm) Foamboard A on (1.3 cm) plywood ^

(Id) 1-in (2.5 cm) Foamboard B on 34-in (1.3 cm) plywood %

(le) 1-in (2.5 cm) Asbestos cement %

(If) 1-in (2.5 cm) Fiberboard on 22 Ga. steel decking ^

(Ig) 1-in (2.5 cm) Glass fiber insulation on 22 Ga. steel decking. N

(2) Base sheet -|-3 asbestos felts +60 Ibs/sq (2.9 kg/m^)

asphalt flood coat [20-25 Ibs/sq (1.0-1.2 kg/m^) interply

asphalt] on

(2a) l4-m (1.3 cm) Plywood N

(2b) 1-in (2.5 cm) Asbestos cement N

(2c) 1-in (2.5 cm) Fiberboard on yi-in (1.3 cm) plywood N

(3) Base sheet +3 tarred felts +75 Ibs/sq (3.7 kg/m') tar flood coat [25 Ib/sq (1.2 kg/m=) interply tar] on

(3a) M-in (1.3 cm) Plywood C

(3b) 1-in (2 5 cm) Asbestos cement C

(3c) 1-in (2.5 cm) Fiberboard on J^in (1.3 cm) plywood C

(4) 2 Glass felts+1 glass cap sheet (20-25 Ib/sq (1.0-1.2 kg/m') interply asphalt] on

(4a) H-in (1.3 cm) Plywood N

(4b) 1-in (2.5 cm) Asbestos cement N

(4c) 1-in (2.5 cm) Fiberboard on H-in (1-3 cm) plywood M

(4d) 1-in (2.5 cm) Fiberboard on 1-in (2.5 cm) asbestos cement- }4

(4e) %-in (1.9 cm) Glass fiber insulation on K-in (1-3 cm) plywood 5^

(4f) %-iB (1.9 cm) Glass fiber insulation on 1-in (2.5 cm)

asbestos cement }4

(5) 2 Base sheets+60 Ibs/sq (2.9 kg/m') asphalt flood coat

[20-25 Ibs/sq (1.0-1.2 kg/m') interply asphalt] on

(5a) Vi-in (1.3 cm) Plywood )^ C

(5b) 1-in (2.5 cm) Asbestos cement N

(5c) 1-in (2.5 cm) Fiberboard on 3^in (1.3 cm) plywood H C

(5d) 1-in (2.5 cm) Fiberboard on 1-in (2.5 cm) asbestos cement- ^ C

(6) 2 Base sheets+60 Ib/sq (2.9 kg/m') flood coat +300 Ib/sq

(14.7 kg/m') slag [20-25 Ib/sq (1.0-1.2 kg/m') interply

asphalt] on

(6a) }4-in (1.3 cm) Plywood N

(Gb) 1-in (2.5 cm) Asbestos cement N

(6c) 1-in (2.5 cm) Fiberboard on }^in (1.3 cm) plywood N

(6d) 1-in (2.5 cm) Fiberboard on 1-in (2.5 cm) asbestos cement- N

(1.6) (1.6) (1.6)

(1.9)

(2.2)

(1.9)

(1.3)

(1.9) (1.6)

1 C

1 c

(1.6)

(2.5)

(2.2)

(2.5)

N

N

(1.3)

(1.9) —

(1.3) —

(1.6) —

(1.3) —

H C H C

(1.9)

(2.2)

Cb (1.6) IM c IK C (3.2) 1% C

(1.6) 2H. P IH D (3.2)

1 G (2.5) IM c IM C (3.2) iJi C (3.2) 2H FP

N N

1 (2.5) N

C (2.8)

C CS N c C 2C

(1.3) 1 N N

1 (2.5) N

1J4 (2.8) Wi c

Va (2.2)

% C (2.2) IMC

N N

IH C (2.8)

1 C (2.5)

N N

N N

N N

N N

(3.2)

(4.1)

(5.7)

(3.2)

(4.5) (5.7)

(2.5)

(3.8)

(3.8)

(4.5)

(3.8)

(3.2)

» Mean diameter of indentation.

•> C, Surface cracked. D, Foamboard delaminated.

shattered. —, Not tested. F, Felts cracked. N, No visible indentation. P, penetrated roofing. S, Coatlne

the slag; "nests" of various sizes were left in

the slag. In summary, each roofing membrane performed better on the denser substrates than

on the lighter substrates, the roofings made

with inorganic felts performed better than

those made with organic felts and the slag sur- faced roofing was not damaged by hailstones

up to and including 2i/2-in (6.4 cm) in diameter.

4.3. Nonbituminous Roofing

A number of nonbituminous roofings were

tested for comparison purposes. These were

applied in accordance with their supplier's

recommendations. The levels of failure used in

these evaluations were cracking for brittle roofings and objectionable indentations for

metal roofing. Table 4 is a summary of the

results of these tests.

8
Table 4. Threshold of hail damage for nonbituminoua roofing

Description Diameter of smallest hailstone causing damage

Edge Center Unsupported

' 8-in (0-3 cm) Asbestos cement shingles 1^

14-in (0.6 cm) Asbestos cement shingles 2

12 in X 18 in X J4 in (31 cm x 46 cm x 0.6 cm) Green slate, 7-in (18 cm) exposure 1%

12 in X 18 in X }| in (31 cm x 46 cm x 0.6 cm) Grey slate, 7-in (18 cm) exposure —

>i-in (1.3 cm) Cedar shingles—dry —

}^-in (1.3 cm) Cedar shingles—wet —

?4-in (1.9 cm) Red clay tile —

Standing seam terne metal •

cm

(3.8)

(5.1)

(4.5) 22

2IM

2

em

(4.5)

(5.1)

(5.1) (5.1)

(3.8)

(3.8)

(5.1)

21%

IH

1%

em

(4.5)

(4.5) 5.1)

(3.8)

(4.5)

(3.8) (4.5)

• Dents proportional to hail size—visible for all hailstone sizes. The plywood deck cracked below the dents with hailstones larger than 2H in (6.4 cm). —, Note tested. All roofings tested were vulnerable to hail damage. As with the asphalt shingles, these other products contained areas of different vulnerability.

5. Conclusions

(a) All roofing materials have some resist- ance 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

area of II/2 to 2-in (3.8-5.1 cm) stones for most

prepared roofings.

(b) Because of the ways in which prepared

roofings are applied, most products iiave areas

of different vulnerability.

(c) Heavier shingles tend to be more hail- resistance than Type 235 shingles.

(d) Weathering tends to lower the hail re- sistance of asphalt shingles.

(e) The solidly supported areas of roofing

tend to be the most resistant to hail damage.

(f) Built-up roofs on hard 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 in- crease the hail resistance of roofing.

(i) The slate, asbestos cement and tile roofings tested contained areas of different vulnerability and cracked under the impact of

11/2- to 2-in (3.8-5.1 cm) hailstones.

(j) The sheet metal roofing was dented by

all sizes of hailstones used. Deck cracking oc- curred when 2 1/4 -in (6.4 cm) hailstones were

used.

This work was sponsored by the Research

Committee of the Asphalt Roofing Manufacturers Association, under the Research Associate Plan, at the National Bureau of Standards.

The author wishes to thank Royce Stine and

Thomas Crowe for their help in constructing

the apparatus and collecting the data upon

which this article is based. He also thanks the

Materials Durability and Analvsis Section, and

particularly its Chief, W. C. Cullen, for their

help and encouragement that made this work

possible.

6. References

[1] Storm Data, Environmental Science Services Administration, U.S. Department of Commerce,

Asheville, N.C.

[2] Douglas, R. H., Recent hail research: A review,

Meteorol, Monographs 5, 157-167 (1963).

[3] Werckmann, H., The Language of Hailstorms and

Hailstones, USASRDL Rep. 2277, U.S. Army

Research & Development Laboratory, Ft. Monmouth, N.J. (1962).

[4] Schleusener, R. A., Hailstorm characterization and

the crystal structure of hail, Meteorol. Monographs 5, 173-176 (1963).

[5] Texas Hailstorms, Texas State Board of Insurance,

Austin, Texas.

[6] Schumann, T. E., The theory of hailstone formation. Quart. J. Roy. Meteorol. Soc. 64, 3-4 (1938).

[7] Flora, S. D., The Serious Toll of Hail Storms,

Rough Notes 82, 14, 1939.

[8] Fuchs, E. R., Hailstorm, American Roofer 10 (Nov.

1955).

[9] Laurie, J. A. P., Hail and Its Effects on Buildings,

C.S.I.R. Research Report No. 176, Bull. 21, 1-12,

National Building Research Institute, Pretoria,

South Africa (1960).

[10] Bi'owning, K. A., F. H. Ludlam, and W. C. Lacklin. The density and structure of hailstones,

Quart. J. R. Meteorol. Soc. 89, 75-84 (1963)

Roy. meteorol.

[11] Bilham, E. G., and E. F. Relf, The dynamics of large hailstones. Quart. J. Roy Meteorol. Soc.

63, 149-162 (1937).

[12] Manufacture, Selection and Application of Asphalt

Roofing and Siding Products, . JNinth edition,

1966, Asphalt Roofing Industry Bureau, New

York, N.Y.

T^^U.S. Government Printing Office: 1969 0—348-822

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