When Hail Strikes: Assessing and Identifying Building Damage

By Carla Ladner, P.Eng.

Every year, hailstorms cause damage throughout Canada. While the most significant storms are often encountered in Southern Alberta, hail can cause damage in many other regions, including but not limited to Saskatchewan, Manitoba and Ontario. Hail typically forms when rain droplets are pushed up into the atmosphere by a large updraft. If the rain is pushed high enough into a colder layer of the atmosphere, it will freeze and become a hailstone. It will then fall to the ground when the updraft can no longer support the hailstone. If a stone starts to fall but is pushed up again, it can collect additional moisture. This moisture can freeze, adding layers and creating a larger stone (Figure 1).  The larger a stone, generally, the larger the wind forces, the combination of which increases the extent of damage. 

Figure 1 – Hailstone with layers

The speed of a hailstone fall depends on the size of the hailstone, local wind conditions, temperature as it falls, and friction between the stone and the atmosphere. Its speed is often directly correlated with its force of impact and resulting damage.

Hail typically strikes uniformly across a surface and therefore, very rarely would you see a single impact mark from a hailstorm. However, this is not always the case. Some impacts cause more damage than others, as the size of hailstones within a single hailstorm can vary. Hail damage would typically not be concentrated in a valley or at the edge of a roof. It is common for hail to affect one entire slope of a roof or one face of a structure more than others, depending on the direction of the storm.

To properly assess the damage from a hailstorm, a thorough review of all available information is required. This includes a review of available photographs taken during or shortly after the storm, contractor notes, homeowner reports, historical Google images, media reports and any other available information including historical climatic or storm-specific data. However, hailstorm data available in Canada is often very limited compared to the United States. Some limited information including storm notes may be available from Environment Canada. Other sources of information include other government agencies or private weather websites but the size and velocity of hailstones are generally not well recorded from Canadian sources. As such, we also rely on other sources of climatic information as indicated above.

Hailstorms can cause significant damage to residential cladding (Figure 2). Siding will fracture, crack or split. Severe hail impacts can even affect sheathing membranes or the wall sheathing. Larger hailstones could affect stucco in a similar manner. However, it would take an extremely large hail impact to affect masonry veneers. It is not uncommon for uneven features of these masonry elements to be mistaken for hail damage (Figure 3).

Freshly poured concrete may be prone to dents or cracks due to hail, if not protected before it cures. It is extremely unlikely that cured concrete surfaces would see hail damage. Note that some protective or aesthetic coatings applied to concrete may be affected by hail or heavy rains, and previously deteriorated concrete surfaces may deteriorate further due to hail impacts.

For hail assessments, roofing materials are considered “damaged” when the service life of the roofing material is compromised. Different roofing materials respond in different ways to hail impacts (cracks, indentations, bruises, etc.). Hail will fall randomly across an entire affected roof surface. Clues as to whether a roof surface has been impacted by hail can be found on metal chimneys, fences, flashings, and other elements of a property that are prone to denting. However, building materials are an indication of all hail that impacted a property, and may not necessarily be from a single, recent storm. Historical storm events must also be considered.

While size is not the only factor influencing the force of a hailstone impact, the following list shows typical hail sizes that represent the minimum at which damage to common roofing materials is likely to occur:

• Asphalt shingles: 1–1.5 inches in diameter (quarter- to golf-ball-sized hailstones)
• Cedar shakes or shingles – 1.5 inches in diameter (golf ball-sized hailstones)
• Clay or concrete tiles: 75 inches in diameter (golf ball size or larger)
• Metal panel dent or damage to the functionality of metal roofing: 2.5 inches in diameter (baseball-sized)
  • Metal roofing panels will dent at a smaller diameter of hailstones / impact. Note that dents only affect the function of a metal roofing product if a coating is damaged, which could allow for future corrosion.

It should be noted that the size of visible damage on a roof surface is not necessarily the size of the hail that impacted the roof.

To evaluate the extent of hail damage over an entire roof surface, a 10’ by 10’ square can be established to examine the impacts in detail. This evaluation method is generally done when widespread hail-related damage is identified. The amount of damage can then be extrapolated across the entire roof and can be used to determine if the roof covering can be repaired or should be replaced. If damage is generally not determined to be hail related, or it is obvious that the damage requires wholesale roof covering replacement, a 10’ by 10’ square may be deemed unnecessary.

Signs of damage from hail common to different roof covering types include:

• Asphalt shingles: loss of granules, bruises, splitting or punctures. If hail impacts are allowed to age, the shingle will deteriorate prematurely in areas where granules are missing, and the asphalt is exposed (Figure 4).   

Figure 4 – Hail-damaged asphalt shingles

• Metal roofing: dents or punctures. If the dent does not fracture the roofing panel or damage a protective coating, hail impacts are considered cosmetic in nature and would not necessarily pose a functional issue to the roof (Figure 5).
• Clay or concrete tiles: chips, cracks and fractures (Figure 6).
• Cedar shakes or shingles: chips, splits and indentations (Figure 7).
• Modified bitumen (SBS) membranes: Hail impacts can displace granules, cause indentations (circular marks on the membrane), fracture the membrane and delaminate plies of the roofing assembly (Figure 8). However, depending on the system, a modified bitumen roof is generally more resistant to severe hail damage.
• Other flat roofing products (i.e., PVC, TPO and EPDM) will see impact damage (generally evidenced by either indentations, cracking or round bullseye type marks), fractures, delamination, etc. A CEP forensic investigator can provide specific information about these products.

The damage described herein is general in nature. For comprehensive analysis, CEP Forensic can assist in determining the presence and extent of damage from hail impacts and whether hail-related damage will affect the function of a roofing system or cladding assembly.