Thermal Splitting Versus Impact Damage on Shingles

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Shingles crack for more than one reason, and the two most commonly confused causes rarely leave identical evidence once examined closely. Thermal splitting develops over time as a shingle repeatedly expands in summer heat and contracts in winter cold, a cycle that Minnesota roofs experience with particular intensity given the wide seasonal temperature swings common in the region. Over years of this cycling, the asphalt and mat can develop a straight or gently curving split, typically running from the butt edge of the shingle up toward the top, often following the same general line across multiple shingles in a course because the underlying stress is consistent across the roof rather than localized to one point.

Impact damage from hail forms differently. A hailstone striking a shingle bruises the mat at the point of impact, compressing the asphalt and often displacing or removing granules in a roughly circular pattern centered on the strike point. Beneath the surface, that compression can fracture the fiberglass or organic mat in a localized area rather than along a long straight line, and the randomness of hail impact typically produces damage scattered across a roof slope in a pattern that reflects the storm’s path rather than a pattern tied to the shingle’s own internal stress lines.

Distinguishing the two on inspection generally involves examining both the surface pattern and the underlying mat condition. A thermal split tends to be a single continuous line with relatively little granule disturbance immediately around it, while hail bruising typically shows a rounded or irregular depression with visible mat fracture directly beneath displaced granules, often accompanied by similar marks on adjacent soft metal components such as gutters, downspouts, and roof vents that would not be affected by thermal cycling. Checking those metal components is a standard step, since hail leaves comparable dents on soft metals regardless of the roofing material, giving an independent point of comparison that thermal splitting cannot produce.

Roof age and prior weather history are also part of this evaluation, since a shingle nearing the end of its expected service life is more prone to thermal splitting generally, independent of any specific storm event, while a newer shingle with clustered impact marks and matching metal damage points more clearly toward a discrete hail event. Careful photographic documentation of both the crack pattern and the surrounding roof and metal components, gathered during a single inspection, gives whoever reviews the file the clearest basis for understanding which mechanism, or combination of mechanisms, produced the damage observed.

Test squares, a method where a defined section of roof is marked off and examined in detail, sometimes lifting a sample of shingles to inspect the underside and mat, are commonly used when the overall extent of hail damage across a roof needs to be estimated from a representative sample rather than a shingle-by-shingle count. The same method can be applied when evaluating whether thermal splitting is present broadly across a roof’s service life rather than isolated to a few shingles, since a consistent pattern found across multiple test squares supports a conclusion about the roof as a whole in a way that a single observed crack cannot.

This article offers general education on the appraisal process. Nothing here is legal advice, and state law and policy language govern each claim.

Russ Lis is an independent property insurance appraiser and umpire in Minnesota who serves clients nationwide. Contact Appraisal Resolution.