Roof coverings—like all building materials—pass standardized tests in their brand-new condition. However, these materials experience everyday weather on top of homes often for years before experiencing an extreme event that tests their limits.  

The IBHS Roof Aging Farm project provides unique insight into the impact of everyday weather on roofs. Roofs experience relentless exposure to ultraviolet radiation from the sun every day. The ultraviolet wavelength is known to degrade asphalt, including the asphalt found in asphalt shingles. 

Sunshine can drive roof temperatures over 200°F. Asphalt shingles rely on heat from the sun to self-seal. The sealant softens when heated and relies on gravity to gain its grip on the shingle below.  

As air temperatures vary diurnally, so do roof temperatures, and these variations can be as large as 80° to 90°F in one day for a roof. Annual maximum and minimum roof temperatures often occur in different months depending on where geographically the roof is located.  

Precipitation can drop roof temperatures as much as 60°F in 5 minutes. Rain is also known to dislodge granules from asphalt shingles. Even a passing cloud can drop temperatures by 10°F in 5 minutes. The 2017 Solar Eclipse demonstrated the impact of sun exposure on asphalt shingle roof temperatures on the Richburg, SC Aging Farm. Richburg, SC experienced 99.6% totality around 2:30 p.m. local time which dropped air temperatures 7°F but dropped roof temperatures nearly 50°F. 

Roof orientation drives the amount of solar radiation hitting the roof slope which impacts roof temperatures. South-facing roof slopes receive more direct radiation and reach higher maximum temperatures than north-facing slopes. Because south-facing slopes reach higher temperatures, the change in temperature due to passing clouds or rain showers is often more extreme for south-facing slopes.  

Asphalt shingles experiencing the same weather on different IBHS Roof Aging Farms react differently. Across the IBHS Roof Aging Farms, the number of times the shingle temperature changed 25°F in 5 minutes demonstrates some of the product-to-product variability of asphalt shingles. In one year, the number of times a south facing roof slope in Richburg, SC observed a temperature change of at least 25°F in 5 minutes ranges from 74 times to 623 times! Comparing north and south slopes, one roof hut on the Richburg, SC farm observed a 25°F temperature change 623 times in a year on the south slope but only 39 times on the north slope.  

Frequent, rapid temperature fluctuations impact materials like asphalt shingles. As a thermoplastic material, asphalt softens when heated and hardens when cooled. The material properties of metal as well as clay and concrete tile are not known to be impacted by these weather conditions. 

• • Granule Loss & Blistering: Asphalt shingles are typically manufactured with more granules than needed. These extra granules are known as hitchhikers. While shedding these hitchhiker granules is expected, some products shed more and more granules over the years on the aging farm until the underlying asphalt is exposed. 

• • Unsealing & Curling: Asphalt shingles may become unsealed over time as the self-sealing strip losses its grip on the tab below. The edges of the asphalt shingle tabs may even begin to peel up and curl with time.

• • Exposed Fasteners: As the roof deck expands and contracts with temperature changes, the nails may begin to back out of the roof. This can affect the shingle surrounding the nail and above the fastener. 

• • Buckling: The expansion and contraction of the wood roof deck can also lead to buckling over time where the lumps and ridges appear on the roof. 

• • Staining: Streaks of discoloration can appear over time on the asphalt shingles.  

These distresses show how an asphalt shingle roof changes with time as it experiences everyday weather. These distresses are indicators of age. Active IBHS research is exploring how this everyday weather impacts the asphalt shingle roof’s ability to perform in severe weather.  

Asphalt Shingle Maintenance

Keeping an asphalt shingle roof clear of debris helps limit trapped moisture on the roof. Pruning trees away from the roof reduces the likelihood of damage to the roof cover surface. When an asphalt shingle roof becomes unsealed, a roofer may be able to reseal the shingle with additional sealant.  

Tile Roofs

Clay, concrete, and slate change little with time. It is best practice to clear debris from a tile roof. Inspections and maintenance must be done with caution as improperly walking on a tile roof can damage the roof. If care is not taken, walking on a tile roof can crack and damage tiles. However, tile roofs offer more opportunities for repairability than other roof covers because individual tiles can be swapped out in the event of damage. 

Metal Roofs

While the metal roof cover changes little with time, fasteners—particularly exposed fasteners—should be inspected. The rubber gaskets around exposed fasteners can deteriorate with age. Fasteners can back out of metal roofs and need to be reinstalled. When adding gutters to a metal roof, maintain a consistent choice of metal to reduce the likelihood of rust and corrosion.  

Wind exploits vulnerabilities in roof covers which often begins a cascade of damage because of the subsequent water entry potential.  

When wind blows across a roof, the wind acts to try and suck the roof up and off the building. These suction pressures are acting on the roof cover and trying to lift it off the roof. The roof cover must also resist pressure from the wind flowing up the roof.  

As the shingles fail, the underlayment is exposed to wind loads it is typically not designed to withstand. Once this cascade begins, a typical asphalt shingle roof is no longer able to perform its water-shedding function and is vulnerable to rain. 

Because an asphalt shingle’s sealant is heat-activated, it needs time in the sun to heat up and seal after installation before a new roof is ready to withstand these pressures to the best of its ability. Once sealed, a typical asphalt shingle roof is best able to withstand the wind, and its resistance immediately starts to decline from there. After 7-10 years, the sealant weakens enough to significantly reduce the shingle’s ability to resist these pressures. Some shingle tabs may even become unsealed, offering no resistance against the wind. As sealants weaken across the roof, the roof’s vulnerability increases and the probability of damage from high winds increases.   

For a tile roof, the weight of a tile means it requires greater suction pressures to lift a tile off the roof. Tile roofs installed to modern building codes are mechanically attached to the roof deck further allowing roofing tiles to resist suction forces of the wind.  

For a metal roof, failure typically requires the wind to lift large sheets of roof cover. If the wind succeeds in removing a metal panel, large areas of the roof deck may be exposed and vulnerable to water entry. However, modern-code compliant metal roofs are mechanically fastened to the roof structure allowing the metal roof cover to resist stronger suction forces from the wind.  

The behavior of a hailstone on impact drives the damage it will cause to a roof cover. When hail hits a roof, it produces one of three impact modes: 

1. It bounces off the roof, remaining a fully intact stone. 
2. It shatters into many pieces. 
3. It turns to slush on the surface of the roof. 

These impact modes occur based on the compressive strength of the hail where harder hailstones are more likely to bounce and shatter whereas softer hail is more likely to turn to slush.  

The IBHS Hail Field Study measures the compressive strength of hailstones. By creating a database of hailstone measurements, laboratory testing can replicate the impact modes across the spectrum of hailstone strengths.  

The mass, diameter, and velocity of a falling hailstone determine the kinetic energy it has on impact. The IBHS Hail Field Study and its fleet of impact disdrometers have revolutionized understanding of these parameters. Using this revolutionary data, the IBHS Impact-Resistance Test Protocol for Asphalt Shingles (IRTP) tests shingles to a lower but more realistic kinetic energy than existing test methods.  

Compressive strength drives impact mode; mass and diameter of the hailstone drives kinetic energy; kinetic energy drives damage mode.

Impact Angle 

A hailstone transfers the most kinetic energy to a surface per unit of surface area in a perpendicular impact. Perpendicular impacts are typically assumed in standardized testing.  

Damage Modes by Roof Cover 

Hail damage to roof covers may hinder the material’s ability to perform its water shedding function. How hail damages a roof varies by roof cover material. 

Asphalt Shingles 

When hail impacts an asphalt shingle, it can: 

1. Dent the shingle. 
2. Breach – or tear – the shingle. 
3. Displace granules on the shingle. 

These damage modes are consistent across different types of asphalt shingles, although the damage severity varies by product and hail size. Severe damage can create the potential for water entry. The IBHS Roof Shingle Hail Impact Ratings use these damage modes to assess the performance of popular impact resistant-labeled shingles against 2-inch hail. 

Tile  

When large hail hits a tile roof, the tile may crack or shatter. These damages vary by tile thickness, material, and location of the impact.  

Metal  

When hail hits a metal roof, metal is observed to dent. While dents change the aesthetic of a roof, there is currently no evidence that dents impact the functionality of a roof. Particularly on discontinuous metal roofs, denting can be exacerbated in preformed shapes of the metal, making those areas of discontinuous metal roofs more vulnerable to large hail impacts. Stone-coated metal roofs have a textured stone coating to mask this appearance.