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Review

A Comparative Impact Assessment of Hail Damage to Tile and Built-Up Roofing Systems: A Comprehensive Review

by
Gayatri Thakre
*,
Vinayak Kaushal
* and
Mohammad Najafi
Civil Engineering Department, The University of Texas at Arlington, Box 19308, Arlington, TX 76019, USA
*
Authors to whom correspondence should be addressed.
CivilEng 2025, 6(1), 6; https://doi.org/10.3390/civileng6010006
Submission received: 1 November 2024 / Revised: 9 December 2024 / Accepted: 22 January 2025 / Published: 30 January 2025
Browse Figures
Versions Notes

Abstract

:
Hail causes damage to property, including roofs, automobiles, and crops, with an average annual loss of USD 850 million. In residential structures in the southern U.S., tile roofing systems are common due to their resistance to the impact of hail and their long service life. Commercial low-slope roof systems are equally prone to hail-strike damages as steep residential roof systems. The objective of this paper is to present a literature review, inspection protocol, and case studies on a comparative assessment of the hail threshold for built-up roof (BUR) and tile roof (TR) systems. More than 90 published papers determining the hail impact assessment of different roofing systems from 1969 through 2024 were studied and analyzed. This study develops a comparative hail damage assessment study between BUR and TR systems and provides detailed statistical data and hail thresholds for various built-up roof composition systems. In addition, the different failure modes and their causes, the characteristics of hail impacts, and the variables influencing the impact resistance of these roofing systems were examined using field studies. To better understand the effects, it is recommended that an intelligent model be developed to predict the hail resistance threshold of various configurations of BUR and TR systems with critical variables.

Graphical Abstract

1. Introduction and Background

Two types of damaging hailstorms occur in the United States [1,2,3]. Frontal storms are more prevalent, where hailstones are formed during thunderstorms with strong, sustained updrafts. Initially, the formation of a hailstone begins with a hailstone “kernel” or “nucleus”, which is supercooled water freezing on contact with raindrops, dust, etc., and serves as a core for the hailstone’s initial growth [4]. The newly formed hailstones or frozen particles are lifted and cycled through the lower and upper regions of the thunderstorm at different temperatures, allowing more water to freeze on its surface. This juggling with the hailstone around the freezing level creates concentric layers of clear and white ice around the kernel [1,2,3,4].
The milky, opaque layer of ice is formed when water droplets freeze instantly upon impact with the forming hailstone and entrap air bubbles [5]. Similarly, transparent rings or wet growth forms lower in the cloud with temperatures near freezing levels, and warmer temperatures let the water droplets freeze slowly, allowing the air bubbles to escape. The cycle repeats until the hailstone grows too large to stay afloat and finally falls to earth, usually at a free-fall terminal velocity [6] (Figure 1). The reported hail densities range between 0.7 and 0.9 g m c m 3 , with the latter value being the density of pure ice [6,7]. A typical hailstorm swath contains the largest hailstones in only the central portion of the path, with marble- to pea-sized hailstones toward the outside of the swath [8,9]. There are generally fewer larger hailstones than small hailstones when present concurrently.
In residential structures, tile roofing systems are common in the southern U.S. due to their resistance to the impact of hail and their long service life. Additionally, commercial buildings and some portions of residential structures have low-sloped or flat roof systems. Commercial low-slope roof systems, like built-up roofing (BUR), and steep 7residential roof systems, like tile roofing (TR), are prone to hail-strike damages [8,9].
BUR consists of multiple plies or layers of roofing felt bonded together on-site with hot bitumen. The roof system consists of layers of bitumen and felts, hence the name (built-up). The heart of this roofing system lies in the roofing membrane, composed of bitumen and felts. The bitumen acts as a waterproofing agent and has adhesive properties. It arrives on-site as solid and is applied in one of the following ways: hot mopping, cold process asphalt, or self-adhesive materials. The bitumen is either a product of petroleum refining—asphalt—or a product of coal burning in power plants [8,9].
Tile roof systems include slate, clay, concrete, fiber cement, and asbestos [8,9]. Tile roof systems are known for their exceptional water-shedding capabilities and ability to withstand continuous age-related weathering. A typical service life of clay roof tiles is at least 70 years [8,9,10,11].
Hail varies in terms of several parameters, among which size, shape, density, and terminal velocity are the predominant factors affecting the overall impact or kinetic energy. The terminal velocity of hailstone, in turn, is influenced by various factors such as the drag coefficient, air density, and strong winds [8,9]. In the early 1960s, hail researchers calculated terminal velocities and the corresponding impact energy [11,12,13,14]. More recently, the earlier work was re-evaluated, and a new set of terminal velocities and corresponding impact energies has been found [15]. The following graph indicates that, for an increase in hailstone diameter from 1.0 inches (2.54 cm) to 3.0 inches (7.62 cm), the impact energy (measured in Joules) for hailstones increases by a factor of 113.6 [16].
Out of the whole world, the central parts of the United States (U.S.) are where large hail is most frequent [17,18]. According to a CoreLogic® report, in Texas alone, the residential property damage was estimated to be near USD 250 million, excluding vehicle or commercial property, after a hailstorm on 11 April 2016 [19,20,21,22].

2. Objectives and Methodology

The objectives of this paper are to present a literature review, inspection protocol for roof damage assessment, and field studies on a comparative assessment of hail thresholds for built-up (BUR) and tile roof (TR) systems. More than 90 published papers that directly or indirectly determine the hail impact assessment of different roofing systems from 1969 through 2024 were studied and analyzed. This study develops a comparative hail damage assessment table between BUR and TR systems and hail thresholds for various built-up roof composition systems. In addition, the different failure modes and their causes, the characteristics of hail impacts, and the variables influencing the impact resistance of these roofing systems were examined using field studies. As demonstrated in Figure 2, published papers were collected from various databases, such as ProQuest, Engineering Village, ASCE Database, Google Scholar, technical bulletins, books, and industry databases, that analyzed the impact of hail on BUR and TR systems from 1969 through 2023. All of these papers were reviewed, and essential data—the year of study, BUR system types, TR system types, hail thresholds, ASTM standards used, dependent and independent variables, and variables influencing impact resistance—were collected. The data were studied with the help of a case study, and the results demonstrated hail and non-strike damages to the roofing system.

3. Roofing Systems

3.1. Built-Up Roof Systems

By definition, a low-sloped roof system consists of roof categories that are installed on slopes that are 3:12 or less [22]. In particular, commercial buildings and some portions of residential structures have low-sloped or flat roof systems. The cost of removing and replacing commercial low-sloped roof systems is ten times greater (USD 100,000 vs. USD 10,000) than the replacement cost of a steep-sloped roof system [23]. Different types of finished surfaces for a low-sloped roof system include asphaltic bitumen built up in layers (BUR), bitumen modified with polymers (mod-bit), and single-ply synthetic materials.
The low-sloped roofing market accounted for about 63% of the total roofing market in 2001 [24]. The market decreased to 55% in 2023 and still dominates over half of the total roofing market [25] (Figure 3).
Built-up roofs have been around since 1844 and have dominated the marketplace for several years [26,27,28]. A built-up roof system comprises a roof deck, a vapor retarder, insulation, a membrane, and surfacing material (Figure 4).
Most felts used in current BUR systems are manufactured with a fiberglass-reinforced mat, though older felts can contain organic mats. Fiberglass felts are the latest in the roofing industry since they are more moisture-resistant than organic felts, which are moisture-resistant only in areas saturated with bitumen. The fiberglass mats are coated with bitumen at the plant, creating a strong bond with the felt. Built-up roofs commonly have a surface with gravel embedded in the top layer of hot bitumen to protect from damaging ultraviolet radiation and mechanical damage from hail and foot traffic. Another common surface coating is a granule-surfaced cap sheet or smooth surface, referred to as modified bituminous roofing. Above this, the aluminum or zinc coating increases reflectivity and provides UV protection to the roof surface (Figure 5).
BUR is laid down to conform to the roofing substrate and to seal all angles formed by projecting surfaces to create a single-unit waterproof membrane. The waterproofing properties of BUR depend on the existence of continuous bitumen films. The felt stabilizes and prevents the rupture or flow of the bituminous films and generally strengthens the roof covering. The simple principle for BUR is to turn the membrane up to make a skirting or base flashing on vertical surfaces, forming a large, watertight tray. The roof drainage system is the only outlet from this tray to remove water.
American Society for Testing and Materials (ASTM) D-312 provides standards for selecting the proper bitumen [29]. Bitumen is classified into four types (Table 1). The asphalt/bitumen is susceptible to damage from overheating. A drop in the softening point can crack or degrade the asphalt. As the softening point decreases, the “holding power” of the asphalt decreases, resulting in slippage [29]. If the overheating is gradual, the asphalt “ages” prematurely, losing the beneficial oils that help the asphalt to stay waterproof and unweathered. Depending on the type of bitumen, the owner may choose the kind of slope the roof will have [30].
Type I is “relatively susceptible” to flow at roof temperatures and is used from “dead level” to slopes not above ½-inch per foot [31,32,33,34,35]. It is common in the industry to limit Type I to ¼ inch, especially when used with glass fiber ply felts. It is a preferred “flood coat” or “top pour” on flat roofs, as it serves as a self-healing binder for the roofing gravel. Type II is “moderately susceptible” to flow and is used as a compromise on glass fiber roofs to slopes of ½ inch or higher. Type III has become very popular in the industry due to its high melting point, and it can be safely used on roof decks and joints without the concern of “drippage” into the structure. However, since bitumen has low tensile strength, it cannot withstand normal building stresses independently.
Hence, roofing felt, plies, or reinforcement fabric must be added to stabilize and strengthen the roofing membrane. Apart from strengthening the membrane, the roofing felt also resists the puncturing and tearing of the roof, eliminates bitumen flow, increases the resilience and pliability of the roofing membrane, protects bitumen from water degradation, and serves as a fire-retarding element in the membrane system.
Service Life: The roofing industry traditionally has assigned five years of anticipated service life to each felt ply; hence, a 25-year service life can be expected on a five-ply BUR [31,32,33]. The mean service life of an asphaltic-fiberglass-reinforced membrane is between 15 and 20 years [31,32,33,34,35].

3.2. Tile Roofing System (TR)

A common type of roofing system used for residential structures is the tile roofing system. Different types of common concrete and clay tiles include the following [36,37,38].
  • Barrel tiles: These are also known as S-tiles due to their semi-cylindrical shape. These tiles are heavy and are the most expensive design options. In clay barrel tiles, the wave pattern is formed by alternating concave dips with convex covers forming half-moon-shaped barrels.
  • Flat tiles: These are the most common concrete roofing tiles. For clay flat tiles, they have the durability of terracotta clay.
  • French tiles: These tiles have deep locks on all four sides, plus two prominent flutes on the surface. French clay tiles are a low-profile option with two protruding flutes per tile.
  • Double Roman tiles: The Double Roman is a standard profiled concrete roof. These Tiles have a small round roll and are also known as mission tiles.
  • Spanish tiles: These tiles provide a pattern of distinctive ripples across the roof and are popular in regions with heavy rain. These have an S-shaped design similar to mission tiles. These can be clay or concrete.
  • Scalloped tiles: These concrete tiles have a curved bottom edge, which gives them a fish-scale appearance.

3.3. Inspection Protocol for Roof Damage Assessment

The study of roofs begins by studying the collateral indicators, which provide tangible and clear clues about the direction of hailstorms, the density of hailstones, the diameter of the hailstones, and much more. The following is the summary of the inspection protocol (Figure 6). The collateral indicators include, but are not limited to, exterior metal appurtenances; exterior wood surfaces; windows; window screens; and heating, ventilating, and air conditioning (HVAC) condenser fins. On the roof, metal roof appurtenances such as box vents, powered attic vents, plumbing stack flashing, chimney caps, soft-metal flue caps, and flashings at wall–slope interfaces and valleys provide collateral evidence [39,40,41,42].
The inspection methodology for a tile roof system is unique and different from other roofing systems. The scope of the investigation or inspection for hail projects is typically clear: to determine whether the tile roofing system is damaged by hail. Further, the total number of tiles covering the roof slopes is estimated from the area measurements of the roof and the tiles (width and exposure). The amount of functional hail-strike damage to all roof slopes is reported as a percentage of the roof surface area for each elevation inspected.
If tiles are loose and can be removed without damage, the manufacturer’s information, size, and type of tile can be noted. Any evidence of fractures, punctures, or chips associated with the impact of hail or windborne debris will be documented. A fracture in the tile may provide evidence of the relative age of the break [42,43,44,45,46], for instance, bright-colored cracked edges or rounded edges filled with debris. Fractures also include right or left corner fractures, which must be studied for their general frequency and location throughout the roof slopes. The investigator should observe if corner breaks occur in relatively shaded areas like under trees.
Horizontal or vertical cracks in tile are typically not related to hail. Hail causes a radial-type fracture pattern. Horizontal and vertical cracks are typically from installation/handling or from foot traffic. It should be noted whether the cracked pieces are still in place, near the parent tile, or missing. For instance, fragments are often close to the originating tile if the hailstorm was recent. Any previous repairs, including different colored sealants, adhesive sheets, or other repair material if used, should also be considered if the damaged tiles are grouped closely along the travel paths, like a valley, which is unlikely to be the result of hail, since hailstones fall at random and are not localized in an area. Roof sketches are a good tool for annotating and indicating the location of the damage.

4. Comparative Impact Assessment and Discussion

A technical literature review is conducted using various academic and industry databases on the hail impact damages to BUR and TR systems. According to Greenfeld’s report, a number of non-bituminous roofing products were tested to determine the threshold of hail damage. Greenfeld reported that the smallest hailstone size causing damage to red clay tile was 2 inches for the center and 1 ¾ inches for the unsupported edge of the tile [46,47,48,49]. This study did not investigate concrete tiles.
According to [6], three concrete tile targets all exhibited fairly high degrees of hail resistance. Fracture/breakage occurred when the velocity was increased to 131 feet per second (40 m per second) or 89 mph, resulting in a kinetic energy of 71.49 feet per second. The flatter concrete tile shingle was the most hail-resistant concrete tile product tested. Multiple impacts with 2% inch (64 mm) hail were required before fracture/breakage occurred. Concrete tile systems appear to offer a very high degree of hail resistance. A lower-profile shingle— either flat or lower configurations—results in increased hail resistance.
According to [50,51], none of the concrete tiles tested were fractured by 1 in. diameter ice balls, even in their most sensitive locations. Four of the thirteen tiles were fractured at their corners with ice balls as small as 1.25 in. in diameter. Six of the thirteen tiles remained unbroken when impacted with 1.50 in. diameter ice balls. Ice balls 2.5 inches in diameter broke all tiles. These test results correlated well with their observations of concrete tile roofs after actual hailstorms.
A threshold is the smallest size of hail at which damage can occur. Based on testing and field experience, if a hard hailstone strikes perpendicular to a clay tile, the threshold size for the damage is 1 ½ inches in diameter. Similarly, for a concrete tile, the threshold size for damage is 1 ¾ inches in diameter [50,51,52]. Damage to roofing can be categorized as cosmetic or functional damage [53,54,55,56,57,58,59]. In the insurance and roofing market, a standard definition of functional damage to the roofing system is a structure defined as follows: (1) a reduction in or diminution of its water-shedding or weather resistance capabilities and (2) reduction in the expected long-term service life of a roofing material. On the other hand, cosmetic damage affects only the appearance or aesthetic appeal of the roof and does not fall under the category of functional damage [60,61,62,63,64].
To cause functional damage to a roofing tile system, a hailstone should have a minimum size to generate sufficient impact energy. Oftentimes, when hailstones are not of sufficient size to cause functional damage to tile roofing systems, burnish marks to the tiles’ exposures will be observed, while no associated penetrations, fractures, splits, chips, or impressions will be present. This is not functional damage and will not likely shorten the expected service life of the tiles [65,66,67,68].
“The smallest size where the hailstone can cause damage to a given material is known as the threshold size of hail for that material”. The hailstone size threshold for functional damage to any roofing material is defined as “The minimum or smallest size of natural hail at which functional damage typically begins to occur and refers to hailstones that strike perpendicular to the surface of the roofing material, which is in relatively good, mid-life conditions”.

4.1. Hail Threshold

4.1.1. Hail Threshold for BUR System

The threshold hail diameter needed to cause damage to the BUR roofing materials is summarized in Table 2.

4.1.2. Hail Threshold for TR System

The functional damage to roof tile caused by a hail impact must have one or all of the following conditions [50]:
  • A radiating fracture or multiple irregular fractures radiating out from the point of impact.
  • Complete shatter, penetration, or puncture through the tile.
  • Cracks or breaks in the tile’s exposure or fractures above the head lap, which are functional damage as they inhibit the tile’s ability to shed water.
  • Crescent-shaped breaks along the thinner edges or corners of S-shaped tiles, particularly where they interface with adjacent tiles.
  • Chips at the tile’s corners with the evidence of burnish marks.
  • Substrate damage or discernable impressions left that broke through the surface layers.
Experience has shown that some types of tiles are more prone to hail damage than others. For example, lightweight concrete and relatively thin tiles are more prone to hail damage than their heavier, thicker counterparts. The threshold hail diameter needed to cause damage to the concrete and clay tile roofing materials is summarized in Table 3.
The comparative assessment of BUR and TR systems is tabulated in Table 4.

4.2. Failure Modes

4.2.1. BUR System

Defects in BUR membranes are associated with installation anomalies, normal aging, and exposure to the elements [50,51,52,53,54,55]. The common defects in BUR are tabulated in Table 5.
Blistering. Blisters are spongy, dome-shaped areas caused by gases (air or water vapor) that expand beneath or within roof membrane plies [65]. They can also result from moisture trapped within a roof assembly, expanding to vapor (1400 × expansion). Blisters form from voids built into the roof, either between the plies or between the bottom of the membrane and the impermeable substrate [65]. Voids can form from asphalt that is too cold when poured so that it does not adhere to the felt, from skips in the bitumen mopping, from entrapped debris, from bitumen bubbling, from walking on freshly laid plies, or from curled felt edges. Blistering of built-up roofing takes two main forms: blistering between the roof membrane and substrate and blistering between the membrane plies.
A blister grows within a void from the expansion of gases under the sun’s intense heat. However, the mere expansion is not the sole reason for the large blisters [60]. A small void that grows into a characteristic bloated hump several inches high to a few feet across is a breathing action called thermal cycling. Pressure changes within the membrane’s voids cause blister growth. A blister cycles between positive and negative pressures daily and typically goes from overpressure during the day to underpressure at night. This also indicates that blisters are not air-tight but leak air through the microscopic cracks in the bitumen and microporous felts [63].
Blisters shorten the membrane’s life by decreasing its vulnerability to physical damage and weathering. The sloped sides of the blister will cause the aggregate to roll downhill and expose the flood coat and felt to increased embrittlement by ultraviolet rays. Puncturing the blisters by foot traffic or inadvertent human-made damage allows water/moisture to access the roof system directly.
Splitting/Ridging/Wrinkles. A long and narrow blister is a ridge [55,63]. It results from the movement of the roof assembly, the loss of the physical properties of the membrane as it ages, low fatigue resistance, and acceleration by vapor drive. Other causes include wrinkled felt, moisture through the substrate or insulation, blockage in holes of asphalt, curling edges of insulation and the slipping of felt plies, structural movement, the cracking of substrate, the differential movement of different decks, the lack of attachment to the substrate, the shrinkage in the concrete deck, and the lack of expansion joints in the roof membrane. Fiberglass felt has limited elongation properties and no recovery characteristics. Due to ultraviolet aging and water, it loses its minor elongation properties. After aging and the associated loss in physical properties, the movement induced by daily and seasonal thermal cycling results in the splitting and ridging of the membrane and, ultimately, fatigue failure. About 12% of roofs were reported to have these problems in 1988. It is pertinent to differentiate between blistering and ridging since these two distresses have different leak potentials. The severity levels of blisters are (1) low—the raised areas are noticeable by vision or feel, but the surfacing is still in place and felts are not exposed; (2) medium—the felts are exposed and show deterioration; and (3) high—the blisters are broken. At high severity levels, distinguishing between ridging and blistering is very important.
Deterioration from Ponding Water. The most probable cause of ponding water is improper design with inadequate slopes for drainage [63]. A sufficient slope must be provided to ensure proper drainage and avoid standing water ponds. Section R905 of the International Residential Code (IRC) and Section 1507 of the International Building Code (IBC) state that the minimum slope of BUR is ¼ inch per foot. However, roofs with built-up coal tar can have a minimum slope of ⅛ inch per foot. This code requirement is for new roofs, and existing roofs may have been designed under previous codes.
Bare Spots from Loss of Gravel. This is an area where the gravel is missing, the coating is lost, or the felt is deteriorated. The probable causes of bare spots include inadequate bonding of aggregate/gravel with the bitumen at the edges, corners, and through the field; gravels applied in adverse weather conditions; too thin or too fine gravel at the edges and corners; wind scour; erosion due to water flow; coating over dirty or poorly prepared felts; and traffic on the roof.

4.2.2. TR System

Corner Fractures. Corner fractures or chips along the tile edges can be mistakenly identified as hail damage. However, a close examination usually reveals algae or dirt with weathered edges or caulking, indicating that these have been historically present. A major cause of right corner fractures is shunting the tiles together with no room for expansion. Tiles expand typically due to thermal expansion/contraction and moisture. Hence, they need room to expand. With no room, the resulting strains fracture the thinner overlap region of the tile, which is the lower right corner (looking upslope). Hence, tiles with interlocking side joints should be installed with maximum “play” in order to accommodate lateral movement [64,65,66,67,68].
Corner fractures do not result in water infiltration, provided the crack remains below the head lap region. An installed tile’s required overlap portion (head lap region) is usually 3 inches. The secondary fractures extending above the head lap region are less common and qualify for replacement [68]. The other causes that promote right corner fractures include shrinkage cracks, which form as the tile dries unevenly, and a small nub that extends from the lower left corner on the adjacent tile.
Inadvertent Human-Made Damage to Tile Roofs. Due to the brittle nature of tiles, any activity by workers can damage the tiles. A person walking improperly on a tile can break the tiles across their widths. Typically, stepping on the bottom three inches of the installed tile is recommended, where the underlying lugs provide firm support and the weight is transferred through it to the deck below [65,66,67,68]. The center of the tile has little to no underlying support and, hence, is easier to break underfoot. Major footfall is common in high-traffic areas like valleys, and therefore, it is recommended to stay away from hips or valleys to avoid breaking cut tiles that would be more difficult to replace.
Voids in Tile. Often, irregular pop-outs on the surface of a concrete tile could be mistakenly marked as hail damage. These sporadic open voids in the face of tiles on several tiles are casting defects due to the air pockets in the concrete [63,64,65,66,67,68].

4.3. Case Studies on TR and BUR Systems

Normal-weight concrete roof tiles stamped with “Monier-Monray” on the underside were installed on a house built in 1980 in Denton County, Texas. The tiles were installed with nominal 1-inch-by 4-inch (1 × 4) battens with 12-inch spacing with no roof underlayment beneath the battens. The roof has a slope of 4:12. Based on the on-site study, collateral indicators included oblong dents in the downspouts, tears in the window screens, chips in the weathered wood fence, dents in the cooling fins, dents in the roof appurtenances, and a cracked skylight, indicating that hail up to 2 inches in diameter fell at this site.
A total of 12 field tiles have larger fractures extending along the tile exposure, punctures, chipping with crescent-shaped cracks, and a point of fracture with radiating cracks. These have associated spatter marks up to 1 ½ inches and expose a bright-colored concrete edge. Approximately 85 tiles have fractured or chipped lower-right corner edges throughout the roof on each elevation. The edges of the exposed tile are weathered, algae-covered, or covered with adhesive sheets or have caulking. Linear cracks or fractures along the length of the tile were commonly seen on the roof. The cracks are localized near the likely travel path, valleys, or near the roof penetrations. A few tiles have these open voids sporadically in the face of the tile on several elevations. Table 6 depicts the typical functional hail-strike damage for the roofs under consideration.
Built-up roofs with smooth and gravel surfacing were included in the study. The first two case studies included commercial building roofs with a smooth surface BUR for building 1 and a reflective coating as the top flood coat for building 2. The inspection of the metal surfaces indicated that the hail that struck the building and the roof surfaces was up to 2 ½ inches for building 1 and 2 ¼ inches for building 2. The top finished surface indicated circular indentations and penetrations consistent with hail impact. The hail stones caused damage to the entire top, and cracks or penetrations were present in the membrane when the test cuts were made.

5. Contribution to the Body of Knowledge and Conclusions

This paper presents a thorough technical review, inspection protocol for roof damage assessment, the authors’ experience, and case studies on a comparative assessment of hail thresholds for the BUR and TR systems.
  • This study developed a comparative hail damage assessment table between the BUR and TR systems and the hail threshold for various built-up roof composition systems. In addition, the different failure modes and their causes, the characteristics of hail impacts, and the variables influencing the impact resistance of these roofing systems were examined using field studies.
  • A methodology was presented with the help of field studies to determine whether the hail event damaged the TR and BUR systems.
  • It can be concluded that the house’s tile roof covering was damaged by hail impacts, with approximately 0.15% of the total roof area. The field tiles with larger fractures, punctures, splits, and crescent-shaped cracks have spatter marks and bright-colored concrete edges exposed, indicating the tiles were recently impacted by hail.
  • The corner cracks in tiles, most of the detached pieces near the parent tile, and exposed algae-covered edges indicate historical damage due to the thermal contraction and expansion of the tiles.
  • Numerous other defects were found on the roof, including inadvertent human-made damage, corner fractures, and open voids. A procedure of monitoring the service life and future repairs by homeowners of the studied roof coverings would provide data on the long-term effects of the impact of hail on the tile roofing system.

6. Recommendations for Future Research

In the future, there is a need to describe the effect of winds on the terminal velocity of the hail and the corresponding impact on the tile roofing system. More field campaigns should be conducted to measure hailstones on-site or shortly after they fall. More data can be collected by investigating a greater variety of roofing systems, and statistically critical roofs in a certain hail-prone area could be considered, which could further help us understand the performance of TR and BUR systems. To better understand the effect of the impact of hail on different compositions of BUR and TR systems, it is recommended that an intelligent model be developed to predict the hail resistance threshold of various configurations of BUR and TR systems with more critical input variables.

Author Contributions

Conceptualization, V.K. and M.N.; Methodology, G.T., V.K. and M.N.; Investigation, G.T.; Data curation, G.T.; Writing—original draft, G.T.; Writing—review and editing, V.K. and M.N.; Visualization, M.N.; Supervision, V.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No data, models, or codes were generated or used during the study.

Conflicts of Interest

The authors declare no conflict of interest.

Nomenclature

The following symbols are used in this paper:
E T Total energy of hailstone
E k Kinetic energy of hailstone
E p Potential energy of hailstone
AIArtificial intelligence
BURBuilt-up roofing system
CSurface cracked
DFoamboard delaminated
FFelts cracked
gGravitational constant (i.e., 9.8 m/s2, 32.2 feet/s2)
hHeight of hailstone above the ground
HVACHeating, ventilation, and air conditioning (HVAC)
mMass of hailstone
NNo visible indentation
PPenetrated roofing
NDNo damage
NRCANational Roofing Contractor Association
NTNot tested
SSurface shattered
THRThreshold for damage
TRTile roofing system
UTAUniversity of Texas at Arlington
vVelocity of hailstone at any given time

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Figure 1. Hailstone formation within a thunderstorm cloud. Source: [8].
Figure 1. Hailstone formation within a thunderstorm cloud. Source: [8].
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Figure 2. Research methodology.
Figure 2. Research methodology.
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Figure 3. Sales comparison of the product mix of low-sloped roofs in 2016 and 2023. Source: [25].
Figure 3. Sales comparison of the product mix of low-sloped roofs in 2016 and 2023. Source: [25].
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Figure 4. Built-up roof (BUR) composition. Source: [29].
Figure 4. Built-up roof (BUR) composition. Source: [29].
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Figure 5. BUR composition summary.
Figure 5. BUR composition summary.
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Figure 6. Inspection protocol for roof damage assessment.
Figure 6. Inspection protocol for roof damage assessment.
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Table 1. ASTM D-312 Classification.
Table 1. ASTM D-312 Classification.
ProductTypes of BitumenRoof GradeSoftening Point Min.Max.
140 °F (60 °C)Type IDead Level135 °F (57 °C)151 °F (66 °C)
170 °F (77 °C)Type IIFlat Grade158 °F (70 °C)176 °F (80 °C)
190 °F (88 °C)Type IIISteep Grade185 °F (85 °C)205 °F (96 °C)
220 °F (104 °C)Type IVSpecial Steep210 °F (99 °C)225 °F (107 °C)
Table 2. Hail resistance of BURs (modified from [26]).
Table 2. Hail resistance of BURs (modified from [26]).
Hail Damage Indentation Size (Mean Diameter of Indentation)
Hailstone Size, in (cm)1 ½
(3.8)		1 ¾
(4.5)		2
(5.1)		2 ½
(6.4)
1. Base sheet plus organic felt, asphalt flood coat on
a. ½-inch (1.3 cm) plywood 5/8 (1.6)5/8 (1.6)5/8 (1.6) C1 ¼ (1.6) C
b. 1-inch (2.5 cm) fiberboard on ½-inch (1.3 cm) plywood 5/8 (1.6)1 (2.5) C1 ¼ (3.2) C1 5/8 (4.1) C
c. 1-inch (2.5 cm) foamboard A on ½-inch (1.3 cm) plywood 5/8 (1.6)NT5/8 (1.6)2 ¼ (5.7) P
d. 1-inch (2.5 cm) Foamboard B on ½-inch (1.3 cm) plywood ¾ (1.9)NT1 ¼ (3.2) DNT
e. 1-inch (2.5 cm) asbestos cement 7/8 (2.2)NT1 (2.5) C1 ¼ (3.2) C
f. 1-inch (2.5 cm) fiberboard on 22 Ga. steel deck ¾ (1.9)7/8 (2.2)1 ¼ (3.2) C1 ¾ (4.5) C
g. 1-inch (2.5 cm) glass fiber insulation on 22 Ga. steel deckN1 (2.5) C1 ¼ (3.2) C2 ¼ (5.7) FP
2. Base sheet plus asbestos felt, asphalt flood coat on
a. ½-inch (1.3 cm) plywood NNTNN
b. 1-inch (2.5 cm) asbestos cement NN1 (2.5) N
c. 1-inch (2.5 cm) fiberboard on ½-inch (1.3 cm) plywood NN1 (2.5) CNT
3. Base sheet plus tarred felt, tar flood coat on
a. ½-inch (1.3 cm) plywood C½ (1.3) CCCS
b. 1-inch (2.5 cm) asbestos cement CNTNC
c. 1-inch (2.5 cm) fiberboard on ½ inch (1.3 cm) plywood CNTC2 (5) C
4. 2 glass felt + 1 glass cap sheet on
a. ½-inch (1.3 cm) plywood NNT½ (1.3)1 (2.5)
b. 1-inch (2.5 cm) asbestos cement NNTNN
c. 1-inch (2.5 cm) fiberboard on ½-inch (1.3 cm) plywood ¾ (1.9)NT1 (2.5)1 ½ (3.8) C
d. 1-inch (2.5 cm) fiberboard on 1-inch (2.5 cm) asbestos cement ½ (1.3)NTN1 ½ (3.8) C
e. ¾-inch (1.9 cm) glass fiber insulation on ½-inch (1.3 cm) plywood 5/8 (1.6)NT1 1/8 (2.8)1 ¾ (4.5) C
f. ¾-inch (1.9 cm) glass fiber insulation on 1-inch (2.5 cm) asbestos cement ½ (1.3)NT7/8 (2.2)1 ½ (3.8) C
5. 2 base sheets, asphalt flood coat on
a. ½-inch (1.3 cm) plywood ½ (1.3) CNT7/8 (2.2) C1 ¼ (3.2) C
b. 1-inch (2.5 cm) asbestos cement NNTNN
c. 1-inch (2.5 cm) fiberboard on ½ inch (1.3 cm) plywood ¾ (1.9) C¾ (1.9) C1 1/8 (2.8) CNT
d. 1-inch (2.5 cm) fiberboard on 1-inch (2.5 cm) asbestos cement 5/8 (1.6) C7/8 (2.2) C1 (2.5) CNT
6. 2 base sheets, asphalt flood coat + slag on
a. ½ inch (1.3 cm) plywood NNTNN
b. 1-inch (2.5 cm) asbestos cement NNTNN
c. 1-inch (2.5 cm) fiberboard on ½ inch (1.3 cm) plywood NNTNN
d. 1-inch (2.5 cm) fiberboard on 1-inch (2.5 cm) asbestos cement NNTNN
Note: C—surface cracked; D—foamboard delaminated; F—felts cracked; N—no visible indentation; P—penetrated roofing; S—surface shattered; NT—not tested.
Table 3. Threshold hail size for concrete and clay tile roofing.
Table 3. Threshold hail size for concrete and clay tile roofing.
ReferencesHail Size (Inches)Damage Classification
Marshall et al. [48]
Concrete Tile1.0ND
1.254 of the 13 tiles had corners damaged
1.57 of 13 tiles are damaged
2.5all the tiles are broken
Clay S-Tile1.0ND
1.25ND
1.5All tile corners broke
Marshall et al. [47]
Flat Concrete Tile1.2520 percent (%) tiles are damaged
1.550% of tiles are damaged
1.7550% of tiles are damaged
2100% of tiles are damaged
S-Shaped Conc. Tile1–1.75ND
2.080% of tiles are damaged
Koontz J.D. [5]
Concrete Tile2.5Fractures with multiple impacts
Haag [9]
Clay1.5THR
Concrete Tile1.75THR
Greenfeld [29]
Red Clay Tile1.5–1.75Unsupported edges
2Center
Note: N.D.—no damage; THR—threshold for damage.
Table 4. Comparative assessment of the impact of hail on BUR and TR systems.
Table 4. Comparative assessment of the impact of hail on BUR and TR systems.
      Hail Diameter
Literature		Hail Threshold for Roofing Configuration Systems (Inches)
Built-UpConcrete and Clay Tile
11 ¼1 ½1 5/8 1 ¾ 22 ¼ 2 ½11 ¼1 ½1 5/8 1 ¾ 22 ¼ 2 ½
Greenfeld [29]☐☐☐☐☐☐☐☐☐☐☒☒☒☒☐☐
Haag [51]☐☐☐☒☒☒☐☐☐☐☐☐☐☐☐☐
Koontz [5]☐☐☐☐☐☐☐☐☐☐☐☐☐☐☐☒
Cullen [14]☐☐☒☐☐☒☒☐☐☐☐☐☐☐☐☒
Crenshaw and Koontz [6]☐☐☐☐☐☐☐☐☐☐☐☐☐☐☐☐
Noon [7]☐☐☒☒☒☒☐☐☐☐☐☐☐☐☐☒
Marshall et al. [46]☐☐☐☐☐☒☐☐☐☐☐☐☐☐☐☐
Marshall et al. [48]☐☐☐☐☐☐☐☐☐☒☒☐☐☐☐☒
Marshall et al. [47]☐☐☐☐☐☐☐☐☐☒☒☐☒☒☐☒
Haag [9]☐☐☐☐☐☐☐☐☐☐☒☐☒☐☐☐
RICOWI [10]☐☐☐☐☐☐☐☐☐☐☐☐☐☐☐☐
Petty [8]☐☐☒☒☒☒☐☐☐☐☐☐☐☐☐☐
RICOWI [59]☐☐☒☒☒☒☒☒☐☐☐☐☐☐☐☐
Table 5. Built-up roofing distress.
Table 5. Built-up roofing distress.
FlashingMembrane
Base FlashingBlisters
Metal Cap FlashingRidges
Flashed PenetrationsSplits
Alligator Cracking
Surface Deterioration
Bare Spots on Gravel
Ponding
Fish mouths
Slippage
Table 6. Typical damage to roofing systems.
Table 6. Typical damage to roofing systems.
DefectsImage
Hail-Strike Types of Failures
A crescent-shaped crack in a concrete field tile with a central impact pointCivileng 06 00006 i001
A crescent-shaped crack in a concrete field tile with an associated spatter markCivileng 06 00006 i002
Circular cracks in a BUR membrane with associated dull interiors exposedCivileng 06 00006 i003
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Thakre, G.; Kaushal, V.; Najafi, M. A Comparative Impact Assessment of Hail Damage to Tile and Built-Up Roofing Systems: A Comprehensive Review. CivilEng 2025, 6, 6. https://doi.org/10.3390/civileng6010006

AMA Style

Thakre G, Kaushal V, Najafi M. A Comparative Impact Assessment of Hail Damage to Tile and Built-Up Roofing Systems: A Comprehensive Review. CivilEng. 2025; 6(1):6. https://doi.org/10.3390/civileng6010006

Chicago/Turabian Style

Thakre, Gayatri, Vinayak Kaushal, and Mohammad Najafi. 2025. "A Comparative Impact Assessment of Hail Damage to Tile and Built-Up Roofing Systems: A Comprehensive Review" CivilEng 6, no. 1: 6. https://doi.org/10.3390/civileng6010006

APA Style

Thakre, G., Kaushal, V., & Najafi, M. (2025). A Comparative Impact Assessment of Hail Damage to Tile and Built-Up Roofing Systems: A Comprehensive Review. CivilEng, 6(1), 6. https://doi.org/10.3390/civileng6010006

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