Resisting hail with SPF roofing

October 1, 2015

Photo © BigStockPhoto/trendobjects

By Mason Knowles
In the words of renowned expert Richard Fricklas, former director of the Roofing Industry’s Educational Institute (RIEI), “There seems to be a mindset among some roofing contractors, as well as building owners and designers, that foam roofs are not suitable for hail regions at all.” According to Fricklas, however, the material has an excellent story to tell when it comes to wind and hail resistance.

The National Roofing Foundation’s (NRFs) report on sprayed polyurethane foam (SPF) roofing systems by Rene Dupuis shows sprayfoam’s potential in hail-prone locales.

“With the continued trends toward ‘sustainable construction,’ it is within the best interests of the roofing industry to repair rather than replace whenever possible,” said Dupuis. His research indicated the damage caused by wind-driven missiles typically does not cause the roof to leak unless the penetration extends through the foam, meaning unlike other roofing systems, SPF roofs may only need repairing when others require replacement.

Damage to SPF roofs
Hailstorms are frequently accompanied by high winds, which can create airborne debris that can damage the coating and SPF roofing system. The type of repairs required for this damage depends on the size and severity of the damage.

Missile damage refers to cuts, gouges, dents, and abrasions to the coating and SPF caused by materials such as tree branches, signs, parts of other buildings (e.g. shingles, metal panels, flashings, doors, and windows), and many other non-secured items hitting the roof during a windstorm. Consequently, damage from wind-driven missiles is likely to be quite varied, depending on the items that strike the roof.

Small cuts or gouges (less than 76 mm [3 in.] in diameter) in the SPF can be repaired by caulking the holes after the damaged material is removed. Larger damaged areas should be repaired by removing the impacted sprayfoam, applying new material and coating to the void.

Wind damage may be isolated to small areas of the roof or cover large areas. Thomas Smith, AIA, noted in his observations of SPF roofs struck by Hurricane Andrew, “It appears a thickness of 50 mm (2 in.) [of foam] is sufficient to prevent penetration of most missiles.”

Damage most likely to occur to SPF roofing systems during a hail event include cracks, punctures, and dents to the roof surface. Both the protective covering/coating and the SPF can be damaged. When hail strikes an SPF roof, cracks shaped like crow’s feet or semi-circles may appear on the coating surface.

The diameter of the cracks can be used to determine the hailstone size. Depending on the size, weight, and shape of the hail, the SPF may be dented as well. The depression will typically range from 3 to 19 mm (1/8 to 3/4 in.) in depth. Hail damage can be isolated to small areas or cover the whole roof. Determining short or long-term repairs depends on identifying the severity of the damage. It is important to note both the size and quantity of hail dents and cracks. For example, 15 dents with a 76 mm (3 in.) diameter on a 9-m2 (100-sf) roof may be less problematic than hundreds of 19-mm (3/4-in.) diameter dents.

Sometimes, cases of mechanical damage are not discovered for months or even years after the damage occurred. In these circumstances, repair procedures differ depending on the extent of ultraviolet (UV) degradation of exposed foam and moisture absorption of the roof. Any UV-degraded or moisture-laden SPF in the cuts, cracks, and dents should be removed and caulked. If the cuts, cracks, or dents are too numerous to remove and caulk, the affected areas should be scarified, refoamed, and coated.

Dents on HVAC equipment signal a hail inspection should be conducted. Photos courtesy Mason Knowles

Designing specifications
There are some key factors and considerations when designing an SPF roofing system that determine whether the roof will resist damage.

First, the physical characteristics of the SPF roofing system must be considered. The most common system begins with a layer of at least 25 mm (1 in.)—and more often 50 to 75 mm (2 to 3 in.)—of a high-density sprayed polyurethane foam covered with a minimum of 635 to 762 µm (25 to 30 mils) of a protective coating of either acrylic, silicone, or polyurethane.

SPF used in roofing ranges in density from 40 to 48 kg/m3 (2.5 to 3.2 lb/cf) with compressive strength from 275 to 413 kPa (40 to 60 psi). The typical polyisocyanurate (polyiso) or expanded polystyrene (EPS) roof insulation averages 137 kPa (20 psi), with the highest compressive strength available at 172 kPa (25 psi). The much higher compressive strength of the SPF, combined with the thousands of tiny cells, tends to act as a shock absorber against impact. The top of the foam can get crushed, but the material below remains intact.

SPF’s high compressive strength also provides a sturdier substrate than other roof insulations for the coatings applied to it. The coatings used over the foam add additional strength to the system, depending on their tensile strength, flexibility, and thickness. These systems installed over softer substrates are more susceptible to hail than those installed to stiffer substrates, which offer additional resistance to hail.

In single-ply roof systems, hail can cause major, widespread damage to the insulation underlying the roofing membrane. When this insulation is crushed, dented, or separated from the facer, it can cause the membrane to become unsupported—leading to a decrease in puncture resistance and the possibility of seams opening up.

The basic SPF roofing system can usually withstand hail impacts of 13- to 25-mm (1/2- to 1-in.) hailstones leaving only a small depression 3 to 6 mm (1/4 to 1/8 in.) deep in the foam and no cracks in the coating. More than a dozen SPF roofing systems passed Factory Mutual Global’s Class I Roofing Covering severe hail test using this basic system.

As hailstones increase in size, the depressions in the SPF get larger, and cracks in the coating become more frequent. However, even severe hail damage can usually be repaired, as long as repairs are performed within a reasonable period. This is an important factor since most other roofing systems require tear-off and replacement.

Repairing SPF roofs
With many roofing systems, the exterior membrane is the waterproofing or water-shedding medium. When hail or other missiles impact, crack, or bruise a roof system such as single ply, shingles, or built-up roof (BUR), leaks are apt to follow. However, the damage to SPF roofing systems is typically a cracked coating and indentation to the foam itself.

The damage typically occurs in the top 3 to 6 mm (1/8 to 3/4 in.) of the foam, depending on the size of the hail and the physical properties of the SPF and coating. Since the foam below the dent is closed cell, it continues to provide water resistance so leaks do not occur. This allows a SPF roof to be repaired in the areas of damage rather than having to remove and replace the whole system.

As reported in the Roofing Industry Committee on Weather Issues (RICOWI) hailstorm Investigations in Oklahoma City in 2004:

Immediate leaks into the buildings would not be expected from the hail-caused fractures in the coating as the foam is closed-cell and would not allow liquid water to pass through.

Five SPF roofs were inspected with three of the roofs (all with hail reported ranging in size from over 25 to 50 mm [1 to 2 in.]) having fractures in the coating. The two undamaged roofs were struck by hail from 12.5 to 25 mm (1/2 to 1 in.) in diameter.

Other roofing systems and their hail resistance
By evaluating results of hail investigations over the last 30 years, it is apparent some roofing systems perform better than others.

Asphalt shingles
Asphalt-shingled roofs have shown a predilection of damage after hailstorms. For example, in the RICOWI hail investigations of the Dallas/Ft Worth, Texas storm of 2011, with a total of 63 asphalt and modified-bitumen (mod-bit) shingle roofs inspected:

40 showed some form of damage (categories two or higher) and 28 having moderate (category three or higher) or greater damage reported. Maximum hail sizes on the asphalt shingle roofs inspected ranged from 6 to 82 mm (0.25 to 3.25 in.) in diameter…. Roofs with damage category two or higher had been struck with hailstones 25 mm (1 in.) or larger. Of the 25 asphalt shingle roofs rated with damage categories three or higher (moderate to severe) 92 percent had been struck with hailstones 32 mm (1.25 in.) diameter or larger. Shingles judged to be 9 years and newer had an average damage rating of 2.1 while the shingles older than 9 years had an average damage rating 4.2.

Slit samples can show the severity of damage from hail impact. The crack in this sample is less than 6 mm (1/4 in.), indicating light damage.

According to RICOWI, aggregate-covered BUR assemblies appeared to perform well in the Dallas/Ft. Worth storm:

Five of the six roofs inspected were impacted by hail 57 mm (2.25 in.) or larger and one roof was impacted with 44 mm (1.75 in.) hail. All were rated with damage levels one or two, indicating little observable damage and general good performance.

In the Oklahoma investigation, 13 BUR roofs were investigated. Some of the aggregate-covered BURs performed well, enduring “hail impacts of 38 to 50 mm (1.5 to 2 in.) without apparent damage in the field of the roof.” However, damage was found in the field (and flashings) of five roofs with hail size ranging from 25 to 63 mm (1 to 2.5 in.) in diameter.

Single-ply membrane
There are many types of single-ply membranes that are divided into two main types; thermoplastic and thermoset.

Ethylene propylene diene monomer (EPDM) is the most common thermoset single-ply membrane. It has a very high tensile strength and a high resistance to punctures. EPDM can either be installed loose-laid with ballast (e.g. crushed aggregate or river rock) or fully adhered with a variety of adhesives.

However, the Whole Building Design Guide (WBDG) on Roofing Systems[4] published by National Institute of Building Sciences (NIBS) in 2015, cautioned:

Generally loose-laid, ballasted roofing systems are not recommended, or at least discouraged, for three reasons:

It stands to reason a hail event would increase the potential for punctures with the ballasted system under these circumstances, and a fully adhered EPDM membrane would have more hail resistance than the ballasted system. While the ballast itself may help absorb impact in the field of the roof, the stretched and exposed membrane at the parapets, copings, and penetrations would be more at risk.

Thermoplastic systems
Polyvinyl chloride (PVC) roofing membranes are among the oldest single plies. These systems have undergone significant formula changes over the last 30 years to minimize a problem with the membranes becoming brittle over time. As reported in the WBDG in 2015:

PVC is a naturally brittle material and must be modified with plasticizers to be suitable for roofing. Some early formulations of PVC suffered from plasticizer leaching out over time and experienced catastrophic failures. 

One should verify the PVC membrane’s stability by selecting those that have been manufactured for many years. PVC providers have added reinforcement to the systems in the ‘90s, but still had problems with the systems becoming brittle over time. A 2000 study by Frank Foley, Jim Koontz and Joseph Vilaitis, “Aging and Hail Research of PVC Membranes” sampled and tested 87 membranes of various ages for plasticizer migration and hail simulation testing. Their conclusions were as follows:

The older a PVC membrane, the more vulnerable the membrane is to impact damage. While this is true of all roof membranes eroding or becoming more rigid with exposed PVC membranes, this vulnerability is a particular concern. Not all PVC membranes are of the same quality. The data on plasticizer loss and impact resistance demonstrates one of the four manufacturers provides a superior product to the U.S. low-slope roofing market.

TPO membranes
Thermoplastic polyolefin (TPO) membranes were introduced as an alternative to the PVC membranes. They do not require plasticizers for flexibility and were deemed an improvement. However, it is important to keep in mind TPO membranes are fairly new to the market, so long-term performance data is unavailable.

Metal roof systems
The Oklahoma study reported that the effects of hail on metal roofs was mostly cosmetic, with
a few cases of distorted seams or spalled granule surface, but only when the hailstone size was extremely large.

The Dallas/Ft. Worth RICOWI investigation report concluded:

Almost no damage was found in areas where the maximum hailstone size was less than 25 mm (1 in.) in diameter with the exception of badly deteriorated and unsupported material. When maximum hailstone size was between 25 and 50 mm (1 and 2 in.) in diameter, the level of damage ranged from none to considerable depending on the material, age, condition, roof slope and support conditions. When maximum hailstone size was greater than 50 mm (2 in.) in diameter, most roofing material sustained damage or denting of metal.

However, the WBDG reports:

One should be cautious about using continuous sheet metal in a flat roof situation. Sheet metal is prone to wider, more extreme temperature swings because of its dense nature as a material, especially in the sunlight on a roof. This will cause significant expansion/contraction movements in the sheet metal surface. The movements themselves are difficult to manage, but combined with necessary roof penetrations for vents, drains, curbs, and wall corners, which bind the inevitable movements, tears or seam breaks in the sheet metal are highly likely. It is more difficult to achieve a reliable, long-lasting watertight system on a low-slope roof with metal than it is with the other low-slope membrane materials.

This crack extends 19 mm (3/4 in.) into the foam, showing that the hail damage is more severe. This is likely due to larger hailstones.

It should be noted most hailstorms are associated with large temperature swings, thereby increasing the thermal shock expansion and contraction within a metal roof system. In other words, even if the metal is not structurally damaged, there can be leaks from movement tears and seam breaks.

With the exception of metal roofing, even light to moderate hail damage to roofing systems such as single ply-membranes, shingles, and BURs can lead to roof leaks. Consequently, tear-off and replacement is the most common recommendation by roof consultants for hail-damaged roofs in category two and above.

Increasing the hail impact resistance of the typical SPF roof
While the basic SPF roof system has good hail impact resistance, additional design elements can be used to significantly increase its hail resistance. The following strategies can be used to develop an SPF roof that can be resistant to cracks and dents from fairly large hail in the 38 to 50-mm (11/2 to 2-in.) category.

Use higher-density foam
SPF roofing foam ranges in density from 40 to 48 kg/m3 (21/2 to 3 lb/cf). As the density increases, so does the compressive strength of the foam and its resistance to impact. To provide the best hail impact protection, one should use an SPF that has a core density greater than 48 kg/m3 and under 64 kg/m3 (3 lb/cf [under 4 lb/cf]) and a compressive strength of 413 kPa (60 psi) or more.

Add a third layer of coating
Most SPF roofing systems consist of two layers of 254 to 381 µm (10 to 15 mils) of coating. Adding another layer (thereby increasing the thickness) significantly amplifies its tensile strength, providing greater hail impact resistance.

Combine tensile strength with high flexibility
Impact resistance of a coating system is a combination of both tensile strength and flexibility. For example, a coating system with 2756-kPa (400-psi) tensile strength and 400 percent elongation is less likely to crack from hail impact than a coating system with 10,335 kPa (1500 psi) tensile strength and 75 percent elongation. A combination of high tensile strength and high long-term flexibility provides the greatest protection.

Top coating system with granules or crushed aggregate
Field observations show adding granules or crushed aggregate helps reduce hail impact damage more than just coating alone.

Weathering characteristic and field history of the coating
It does little good to use a coating that starts with great tensile strength and flexibility, but loses its flexibility after a few years. It is important to use a coating that has demonstrated good impact resistance in the field over time, particularly against moderate to heavy hail.

Inspection procedures
The first step in making an evaluation and repair recommendation of a damaged SPF roofing system begins with an inspection including visual observations, and destructive sampling and testing. The following inspection procedures that are listed in the Spray Polyurethane Foam Alliance’s (SPFA’s) technical document AY 122 of an existing roofing system can be helpful.

Visual inspection
Some visual cues can assist in determining whether the roof has been damaged. While inspecting, one should pay special attention to:

Physical inspection
The following steps are essential to a physical inspection:

Analyze inspection
Core and slit samples should be examined for the following characteristics:

Roof sketch
Indicate the following on a roof sketch:


Repair recommendations
After obtaining information from the roof inspection, recommendations can be developed specific to the damage sustained. As discussed, repairs will vary depending on the severity and the frequency of the damage.

Figure 1, obtained from SPFA’s technical document AY 139, can help classify the mechanical damage and provide repair recommendations. Other factors affecting repair recommendations include age and condition of SPF and coating, service life expected of the roof system, cost of repair, and amount covered by insurance.

Climactic factors
The recommendations listed in Figure 1 are not specific to regions or varying climates. While the recommendations remain consistent in varying climates, consequences of untreated wind and hail damage to SPF roofs vary in different climates.

Hot arid climates
Assemblies in Phoenix or Las Vegas are more forgiving regarding light to moderate hail damage. However, cracks in the coating can allow UV degradation over time. This degradation may take months to occur, but typically does not affect the roof’s water-resistant characteristics since the low humidity and exceptional drying characteristics of the climate do not allow the SPF to saturate over time.

The main concern of light to moderate hail damage in this climate is to maintain the coating’s capacity for re-coat/renewal. Eventually, UV degradation of the SPF under the coating will affect the adhesion of the coating to the foam. It is recommended the damaged areas be removed and caulked or refoamed. It should be noted there are cases of lightly hail-damaged roofs in these areas being successfully recoated years later without any specific hail damage repairs occurring. Still, it is prudent to make hail damage repairs as soon as practically possible.

Hot humid climates
The climates found in locations such as South Texas and Florida have more complex factors affecting hail-damaged SPF roofs. High temperatures tend to dry out the moisture that has seeped into the cracks and crushed foam cells very quickly. On the other hand, the high humidity creates a higher potential for SPF saturation particularly during cooler times of the year. Lower-perm-rated coatings/coverings also increase the potential for moisture saturation of the hail-damaged roofs because they do not allow drying to occur as efficiently as higher perm-rated products. As in the hot arid climates, UV degradation that can affect coating adhesion typically occurs within a few months.

Cool climates
In cooler temperatures, the greater concern of hail-damaged roofs is moisture saturating into the SPF. There may be long periods where drying conditions do not occur. In many areas snow may stay on the roof for extended periods increasing the possibility of moisture saturation. It is important to repair crushed foam and coating cracks as soon as possible to prevent moisture saturation of the damaged areas.

Sprayed polyurethane foam roofing systems have unique characteristics that allow the repair rather than the replacement of the system after hail and wind damage. These damage repairs to SPF roofing systems vary according to size, severity, and the length of time after the initial defacing. It is important to inspect and evaluate the damage in order to make the correct repair recommendations. However, with the correct repair, SPF roofing systems can perform for many years after a significant wind or hailstorm.

Mason Knowles is president of Mason Knowles Consulting LLC, specializing in providing educational/training, troubleshooting problem applications, technical services and articles, and presentations specific for the sprayed polyurethane foam (SPF) industry. He has 42 years of experience in the sprayfoam industry as a contractor, manufacturer, and trade association executive. Knowles chairs the ASTM Subcommittee on Sprayfoam Roofing and the ASTM Task Group responsible for ASTM C 1029, Spray-applied Polyurethane Foam Specification. He is a Sprayed Polyurethane Foam Association (SPFA)-accredited building and roofing inspector and an instructor for SPFA courses for applicators and inspectors. Knowles is a member of the Roofing Industry Committee on Weather Issues’ (RICOWI’s) Hurricane and Hail Investigation Teams. He can be contacted at[7].

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  4. Whole Building Design Guide (WBDG) on Roofing Systems:
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